A comprehensive review on charger technologies, types, and charging stations models for electric vehicles

Abstract

Electric vehicles (EVs) are popular now due to zero carbon emissions. Hence, with the advancement of EVs, charging station (CS) design also plays a vital role. CS is generally called a charge or power supply point and delivers power to the EVs. Usually, CSs are either of the direct current (DC) type, as the EVs need a DC supply or in some cases of the alternating current (AC) type, as the traditional power grid delivers AC power. Usually, on-board chargers (on-BCs) and off-board chargers (off-BCs) are used to charge the EV batteries. Due to heavy loads, size, and budget constraints, many on-BC facilities have power limits, which can be overcome by designing the on-BC with an electrical motor. In different types of off- and on-BCs, the power flow can be in one or two directions. Uni-directional power flow reduces hardware needs and makes connecting problems easier, whereas bi-directional power flow allows battery energy to be injected back into the grid. The primary issue with EVs is the charging time as well as the need for charging infrastructure. The infrastructure for fast charging makes on-board energy storage less expensive and more essential. This paper details various charging technologies, including wired and wireless methods. Also, numerous on-board and off-board charging topologies are summarized in the literature. Different EV battery charging standards and levels are also discussed. The paper also delineates several alternative CS topologies based on architecture, energy storage, and renewable energy sources. Considering the present scenario, having a sophisticated quick EV charging network is crucial to ensure maximum EV charging with renewable power and reduce grid strain.

Nomenclature

EVs	–	Electric Vehicles
on-BC	–	on-board charger
off-BC	–	off-board charger
V2G	–	Vehicle-to-Grid
G2V	–	Grid-to-Vehicle
PEV	–	Plug-in -Electric Vehicle
PHEV	–	Plug-in Hybrid Electric Vehicle
BEV	–	Battery -Electric Vehicle
LLC	–	Inductance, Inductance, Capacitance
EVSE	–	Electric Vehicle Supply Equipment
CS	–	Charging Station
PFC	–	Power Factor Correction
ESS	–	Energy Storage System
BESS	–	Battery Energy Storage System
DCFCs	–	DC Fast Charging stations
RES	–	Renewable Energy Sources
UFCS	–	Ultra-Fast Charging Station
LFT	–	Low Frequency Transformer
HFT	–	High Frequency Transformer
SST	–	Solid State Transformer
PFC	–	Power Factor Correction
FCS	–	Fast Charging Station

1. Introduction

Road transportation accounts for 16 % of global emissions, and electric vehicles (EVs) are the most promising technology for reducing the above digit. An EV's life cycle emissions are influenced by the source of the electricity that charges it, which varies by region. EVs frequently provide a life cycle emissions advantage over comparable conventional vehicles powered by gasoline or diesel in regions where the production of electricity utilizes relatively low-polluting energy sources. The sale of EVs have skyrocketed in recent years, and above trend is expected to continue as the vehicles become more practical, more widely available in a variety of models, and more powerful. It is predict that by 2025, 15 % of all new cars sold will be electric, and that if this rate of growth is maintained, transportation-related CO₂ emissions can be brought into line with the net zero emissions by 2050. Numerous countries throughout the world are currently striving to meet a specific target in the field of the renewable energy environment. To mitigate the effects of rising fuel costs and to implement higher-standard environmental laws, the EV offers an alternative that fulfills the desire for an environmentally friendly form of transportation with lower emissions and improved fuel economy [1]. One of the largest expenses associated with owning a vehicle is the cost of fuel and maintenance. It has become common knowledge that EV consumers save a lot of money in above aspects. EVs are notable for their simplicity and low maintenance requirements. As a result, replacing the engine oil is not a concern, making it simpler and less expensive to maintain electric cars. However, electric cars are still a niche market product. High retail prices and a dearth of charging stations (CSs) have hindered sales in underdeveloped and emerging economies.

CS must be able to inject reactive power into the grid in order to stabilize the point of common coupling voltage and minimize the effects of the megawatt power demand of fast charging. By utilizing renewable energy sources (RES) in part, CS allows the power system to draw less power and alleviates EV charging issues. Due to the prevalence of the electric grid in close proximity to numerous parking areas, EVs and plug-in hybrid EVs (PHEVs) can be charged in a wide range of accessible environments. The term “electric vehicle supply equipment (EVSE)” refers to the CS that is used to safely transfer energy from the electrical grid to a car's battery. For the widespread adoption of EV usage, charging infrastructure is crucial. In fact, there is a demand for EV CSs next to petrol stations, especially on highways where the impact on the distribution network for electric utilities is small [2].

In urban areas with high land costs, a compact design for CS is extremely important. Analyzing and investigating EVs and how they interact with their chargers and CSs is facilitated by a complete knowledge of EV batteries. Moreover, charging systems can use communications via power line devices to interact with the power grid and adjust charging according to electrical grid conditions [3]. To maximize energy usage, prolong battery life, and prevent charging and discharging, a battery management system should monitor the battery pack during operation. Because of various factors the transition from internal combustion engine (ICE) vehicles to EVs has been slow. The three most significant obstacles to EV adoption are high retail costs, range anxiety, and the absence of a charging infrastructure. These challenges have been standardized by improvements in battery technology, power electronics, magnetics, and incentives. In addition to select the correct charging technology, selecting the proper charging method is an important aspect to consider while charging. The majority of EV charging can be accomplished overnight in residential using a Level 1 charging outlet. Level 2 charging, which necessitates a 240 V outlet, is usually described as the preferred method for both residential and public facilities. For Levels 1 and 2, 1-Ø solutions are typically employed. Level 3 direct current (DC) fast charging is designed for commercial as well as public applications similar to a fuel station and this require 3-Ø solutions [4].

The low-voltage network's charging facilities will increase the distribution system's complexity, dynamics, and operational capabilities, requiring large-scale upgrades to meet rising charging demands. To ensure smooth operation and to enable a smooth transition to EVs, an ultra-fast charge station (UFCS) network is highly needed that resembles the petrol refueling network [5]. The automobile market in India is currently the world's fourth largest and the country has chosen to support EVs as a signatory to the Paris agreement. But the lack of charging points is one of the most important things keeping EVs from taking over the market. Infrastructure planning in India is difficult due to poor roads, power distribution, and traffic [6]. [7] provides an example of an EV rapid charging station that is connected to the grid. With low harmonic currents, this type ensures power transfer reliability. Integration of solar PV producing systems with charging stations is proposed as a method of energy management predicated on the most efficient flow of power, which would mitigate the negative effects of fast charging on the grid. The voltage that corresponds to the maximum power is assessed and provided to the controller as a reference signal while the DC-link voltage is adjusted to remain constant in order to evacuate the maximum amount of solar power available [8].

Significant effort is being put into improving the economics and effectiveness of hybrid and plug-in EV (PEV) charging infrastructure that utilizes RESs [9]. Hence dependency on RESs is gradually growing due to the need of prolonged periods of surplus electricity and to decrease the demand. The intermittent nature of the above resources necessitates the installation of a storage system for their electricity generation. In order to provide improved dynamic performance even in the face of dynamic variations in the motor speed, machine parameters, and disturbances, the drive system needs more effective control systems [10]. The primary advantage of storage is its ability to store excess energy and make it available when it's most needed. An electric vehicle's performance, cost, and dependability are all affected by the battery. Various battery technologies are utilized in electric vehicle applications, including lithium-ion (Li-Ion), nickel-metal hybrid (NiMH), lead acid, aluminum-ion (AI-Ion), solid-state batteries, metal-air batteries, and ultra-capacitors [11]. For renewable energy-based standalone hybrid EVCS, storage is required and can account for as much as 51 % of the total plant capital cost. The charging infrastructure guidelines for India are stated below.

1.1. India's charging infrastructure guidelines

The Ministry of Electricity issued the charging station strategy in 2018 [6]. Its department's main guidelines are.

• •Private charging in residential zones is allowed. Distribution companies should supply home charging infrastructure.
• •Centralized charging stations are relicensed.
• •A public charging station's basic requirements include a unique transformer, 33/11 kV lines, civil works, and liquid-cooled cables. Before use, authorities must test public charging stations.
• •The Central Electricity Authority manages all CS databases.
• •EVCS’ electricity supply tariffs cannot exceed 15 % of the average supply tariff.
• •The first places to have charging stations should be megacities, state capitals, and union territory headquarters.

Modern trends present significant difficulties for the EVCS. The range of currently available EVs is sufficient for most driving scenarios. A UFCS infrastructure that can recharge an EV's battery at a rate similar to filling a fuel tank is crucial to achieving net zero carbon emission and encouraging widespread adoption of EVs. Smart charging can lower the grid's electrical load and increase power transmission efficiency. Sustainable EV chargers can be developed using RESs like solar and wind. EV owners can also adjust the frequency with intelligent charging. Smart-charging EVs with ancillary services will reduce energy costs by 60 %. This paper comprehensively overviews the available EV chargers, charging technologies, CS topologies, and UFCS applications. The below sections constitute the structures of the review paper.

• •Section-1 provides an overview of the paper's Introduction.
• •Classification of EV charging technologies with operations, advantages and disadvantages are addressed in Section-2.
• •Section-3 describes the different types of EV battery chargers, and their advantages and disadvantages.
• •The many types of charging power levels, connectors and standards are covered in Section-4.
• •The details of the various charging levels, relevant charging connectors, and standards are in Section-5.
• •Section-6 outlines the various CS topologies.
• •The tendencies for the future EVCS is in Section-7.
• •Section-8 describes conclusion and future purview.

2. Classification of charging technologies

According to how chargers are tied to the EV, charging technologies are categorized in three ways: wired, wireless and battery exchange. The above section details the classification of several charging technologies employed in BEVs. The various charging technologies for EVs are shown in Fig. 1 [12].

Fig. 1.

Ev charging technologies.

2.1. Wired (conductive) charging technology

Different charging facilities are provided through conductive charging, which involves an electrical contact between the vehicle and the charging port. In wired charging, two main charging categories are commonly used - AC (alternating current) and DC (direct current) charging systems. The AC charging system delivers power from the grid to the EV, which is then converted to DC by using an on-board battery charger (on-BC) available in the EV. With the provision of on-BC, the current and voltage are automatically managed as needed by the EV, and hence, the AC charging system has the advantage of removing the need for a CS. A DC charging system evades the on-BC to charge the EV battery, and the battery is directly charged from the off-BC, which can convert the AC grid voltage to DC voltage. Wired charging also provisions vehicle-to-grid (V2G) facility to reduce grid loss, regulate voltage, boost active power, and reduce reactive power. Fig. 2 illustrates visual representations of the wired charging system [152].

Fig. 2.

Wired charging Illustrations.

The operations of both Uni-directional and bi-directional chargers are as follows.

• (i)Operation of an Uni-Directional Battery Charger:

Diode bridge rectifiers (DBR), filter stages, and DC-DC converters are all included in uni-directional chargers. The DBR can be single-phase or three-phase to boost charging power [2]. Additionally, isolation is ensured during EV charging by using a high-frequency isolation transformer. The above converter topology cannot inject energy from an EV battery into the utility grid. Providing reactive power to or from the primary electrical grid without draining EV batteries enables auxiliary services, particularly voltage regulation. Uni-directional battery chargers offer a simple, cost-effective, and safe method for managing EV deployments. As a result, the power grid needs topologies that allow EVs to operate as a form of distributed energy storage. The 1- Ø uni-directional on-BC is depicted in Fig. 3.

• (ii)Operation of a Bi-Directional Battery Charger

Fig. 3.

1- Ø Uni-directional on-BC.

The two main parts of a bi-directional EV charger are a DC-DC converter and an active front end (AFE) that can be single or three phases. AFE is a bi-directional AC-DC converter that regulates DC bus voltage, controls quasi-sinusoidal grid currents, and maintains a unity power factor reactive power exchange with the grid. During the second stage, it is possible to regulate the battery's charging current. Depending on the circuit arrangement, the DC-DC converter might be isolated. The two main parts of a bi-directional EV charger are a DC-DC converter and an active front end (AFE) that can be single or three phases. Fig. 4 illustrates the 3- Ø bi-directional on-BC.

Fig. 4.

3- Ø Bi-directional off-BC.

Following are the steps a typical wired battery charger takes to charge a battery [13].

• •A typical battery charger rectifies and conditions the grid power drawn for charging batteries.
• •Depending on the need, a group of power converters, including an AC-to-DC (rectifier), DC-to-AC (inverter), or DC-to-DC converter (chopper), transform the charger's input power. On the basis of conversion stages, chargers are further classified as single stage, two stage, etc.
• •Battery chargers cannot be accomplished in a single stage since battery chargers for automotive use require high power, fast charging techniques, and grid power factor restrictions. Rectification of alternating current (AC), power factor correction (PFC), conversion of direct current (DC) to alternating current (AC) (high frequency), galvanic isolation, rectification, and finally, DC-DC conversion to fulfill charging strategy are the numerous steps of a charger.

2.1.1. AC charging

In AC charging, on-BC in the EV is further categorized into 1-Ø slow charging and 3-Ø fast charging.

2.1.1.1. 1-Ø on-BC slow charging

Fig. 5 shows a 1-Ø on-BC slow charging technology. A power factor correction (PFC) circuit is fed by the 1- Ø grid voltage, which is converted into DC using the AC-DC converter. Then, the PFC circuit's output voltage is sent to the intermediate DC-link connection, where it is regulated to the desired DC output voltage by an isolated DC-DC converter.1-Ø slow charging is further divided into two types: unidirectional and bidirectional chargers based on the direction of power flow. The unidirectional charger allows the power grid to keep supplying electricity to many BEVs, which is a considerable load [12]. Since the bidirectional charger can be used with G2V and V2G systems, it is more efficient than the traditional unidirectional charger. However, battery life and charging system costs will be decreased and increased due to bidirectional charging's frequent charging and discharging. 1- Ø on-BC slow charging offers a power output of up to 2 kW and a minimum charging period of 6 h.

Fig. 5.

1-Ø on-BC Slow Charging Technology.

2.1.1.2. 3-Ø on-BC fast charging

Due to an intermediate power rating of up to 20 kW, 3-Ø on-BC fast charging technologies can offer a quicker charging capacity. The most prevalent topology for such charging systems is a dual-active bridge. Due to its simplicity, the above technique is preferred for most EVs. Fig. 6 depicts fast charging technologies for on-BC.

Fig. 6.

3-Ø on-BC Fast Charging Technology.

2.1.2. DC charging

There are two DC charging solutions for EVs: off-BC fast and rapid charging. With a rectifying unit already built into the charging station, DC charging can supply EV power without additional wiring. Consequently, the driving system's overall weight and size are decreased. The capacities and compositions of EV charging batteries from multiple manufacturers are given in Table 1 [14].

Table 1.

Different manufacturers’ EV charging battery capacities and compositions.

Manufacturer	EV Model	Size of Battery (in kWh)	Battery Used	Energy Capacity (in kWh)	Type of EV
Mitsubishi	i-MiEV	16	Li-ion	12.8	EV
Toyota	Rav4	42	NiMh	33.6	EV
Tesla	Roadster	53	Li-ion	42.4	EV
Frisker	karma	22	Li-ion	17.6	PHEV
Toyota	Prius	5.2	NCA	4.1	PHEV
Sabaru	Stella	7.3	Li-ion	9.2	EV
General motors	Chervolet spark	21.3	LFP	17.04	EV
General motors	Chervolet volt	17.1	Spun LMO	13.7	PHEV
BMW	Cooper	28	Li-ion	22.4	EV

2.1.2.1. Off-BC fast charging

DC fast charging is a popular technology widely recognized for reducing charging time to less than an hour. Major car manufacturers such as BMW, Nissan, Tesla, and Hyundai have developed DC fast charging stations capable of fully recharging a battery within an hour. Companies like BMW, Nissan, Tesla, and Hyundai have introduced DC fast charging stations, which can fully recharge a battery within 1 h.

The power level of the DC quick charging can range from 20 to 120 kW, the charging time can be less than 1 h, and the battery voltage can vary from DC 320–450 V [12]. Fig. 7 represents the off-BC fast charging technology for 3-Ø with the inclusion of a charging rating. Fast-charging off-BCs have battery voltages of 300–450V.

Fig. 7.

3-Ø off-BC Fast Charging Technology.

2.1.2.2. Off-BC rapid charging

Rapid DC charging requires more current and power compared to fast charging technology. The EV can be charged in 15 min, and the battery voltage ranges from 320 to 500V. Tesla offers one of the most well-known rapid chargers, powered by DC 480 V and 250 kW. The off-BC fast charging technology is illustrated in Fig. 8.

Fig. 8.

3-Ø off-BC Rapid Charging Technology.

2.2. Wireless charging technology

According to Faraday's law of electromagnetic induction of transformers, a wireless power transfer system operates. EVs are charged inductively using magnetic, contactless power transfer [4]. Inductive chargers are wireless charging systems that could be dynamic or stationary, only used when the car is parked or in static modes, such as parking lots, garages, or traffic lights [12]. In collaboration with Evatran, Nissan and Chevrolet have created wireless charging systems for their EV models, the Nissan LEAF and Chevrolet Volt [15]. Using Massachusetts Institute of Technology (MIT) developed technologies; Delphi and WiTricity are working with Audi, Toyota, and Mitsubishi to incorporate magnetic resonance WPT technology into their EV models. Fig. 9 illustrates visual representations of the wireless charging system.

Fig. 9.

Wireless charging Illustrations.

2.2.1. Operation of wireless charging

To supply electricity to portable devices, electromagnetic induction (wireless) has been used. Automobiles, electric toothbrushes, power tools, and healthcare products use inductive charging. Inductive charging gets its name because it distributes energy via inductive coupling. An inductive coil inside the charging station conducts alternating currents.

The magnetic field results from the flow of electric charge, and variations in the strength of the electric current develop in changes to the magnetic field's strength. The rotating magnetic field makes the electric current in the portable device's induction coil go back and forth. The electric current is turned into a DC current used to charge the battery. Fig. 10 illustrates the structure of a wireless power transmission system. High-frequency alternating current was created by converting a 220V, 50Hz AC supply into alternating current, which is then supplied to the transmitter coil. The transmitter coil then produces a rotating magnetic field that cuts the receiver coil and generates AC power output at the receiver coil. For wireless charging to be effective, it is crucial to maintain the resonance frequency between the transmitter and receiver. Also, compensating networks are included at the two ends of the receiver and the transmitter coils to keep the resonant frequencies constant. At the receiver side, alternating current is rectified into direct current and delivered to the battery via the battery management system.

Fig. 10.

Structure of a wireless power transfer system.

2.2.2. Advantages and disadvantages of wireless charging

Table 2 represents the advantages and disadvantages of the wireless charging technology.

Table 2.

Wireless charging technology's benefits and drawbacks.

Advantages	Disadvantages
•Safe because no connected connection is present.	•The charging system should be standardized.
•Free from servicing	•Wireless charging systems are complicated to set up.
•Protected from dust and moisture	•The car must be parked precisely where the charger coils are mounted to charge the battery.
•The need for a cumbersome automobile charging cord is eliminated.	–
•The charging process is quick and straightforward.	–

Based on the transmitted range, wireless charging methods can be categorized into three categories: charging in the vicinity, wide-area, and long-distance charging.

2.2.3. Near field charging technology

Inductive, magneto-resonant, and capacitive charging are examples of near-field charging systems for battery EVs.

2.2.3.1. Inductive Charging Technology

Power is delivered from a transmitter plate to a receiver plate via an electromagnetic field, making inductive charging one of the most recent and cost-effective nearby charging solutions for modern transportation, as can be seen in Fig. 11. To build a dependable and effective wireless power transfer system to charge the batteries of BEVs, one of the most crucial steps is designing an appropriate power pad. However, wireless power transfer systems still face several obstacles, such as the power pad and coil designs, electromagnetic field protection, and more [16].

Fig. 11.

Inductive charging technology.

2.2.3.2. Magneto-resonant charging technology

Magneto-resonant charging is superior to inductive charging due to the resonant frequency, which may be amplified by adding compensating capacitors, allowing for large transmission distances. These charging technologies are being deployed in phases: Phase 1- simple residential systems; Phase 2 - parking spaces; Phase 3 - on-street parking; and Phase 4 - dynamic charging systems or future highway technology. Fig. 12 depicts the magneto-resonant charging technology addressed in the literature.

Fig. 12.

Magneto-resonant charging technology.

2.2.3.3. Capacitive charging technology

A capacitive charging can be established by applying an electric field between two metallic plates equipped with transmitter and receiver pads, providing a direct connection between the plates and the power supply or load. Fig. 13 is a representation of capacitive charging technology. To establish an electric field between the plates and induce a current, as in a receiver pad, they are connected in parallel, functioning as two capacitors. The rate of change of the electric field between the receiver and transmitter pads is equivalent to this induced current.

Fig. 13.

Capacitive charging technology.

2.2.4. Wide-area charging technology

Charging technology based on magnetic gears is another name for wide area charging technology. This technological principle depends heavily on mechanical force to transmit energy. They have a lower power range of 1.5 kW–3 kW, making them appropriate for charging applications. The mechanical contact between two side-by-side synchronized permanent magnets determines the system's operation. The technology for wide-area charging is depicted in Fig. 14.

Fig. 14.

Wide - area charging technology.

2.2.5. Long-distance charging

Technologies that utilize electromagnetic radiation to charge across long distances include laser charging, microwave charging, and radio wave charging. The energy transmission in laser charging technology is accomplished via a resonant beam with a frequency of up to 3.59 ∗1014 Hz, generated by a distributed laser charging (DLC) transmitter and received by a DLC receiver [12].

Microwave charging systems have been tested where power supply over a long distance, like 100 km, is required, such as helicopters, platforms based on balloons, helicopters, experimental aero planes, experimental vehicles, and so on [12]. The issue with long distance charging technique is that if the transmitter and receiver pads stop communicating, the charging stops.

Rectennas with a high-pass filter for frequencies up to 30 GHz, a rectifier for converting the high-frequency sine wave into a lower-frequency one, and a low-pass filter for the low-frequency components are used for the above charging technology.

2.3. Battery exchange (battery swap station)

Battery swap stations have been operated on the principle that the owners rent batteries monthly. A battery swapping system permits EVs to swap out their battery packs instead of charging them at a charging point. There are two ways to swap the batteries. One is that the owner might later return and get the battery pack fully charged after a battery swap. The second option would be to continue using the replacement battery while keeping the original one and receiving or paying the price difference. Battery swapping is primarily used in electric forklifts. Instead of plugging in an EV, wireless charging systems require drivers to park their cars over a coil embedded within the ground or lying on its surface to receive power. Wireless charging eliminates the need for physical connections with the vehicle, making charging more convenient and straightforward. Fig. 15 illustrates the configuration of the battery-swapping system.

Fig. 15.

Block diagram of the Battery Swapping System.

Batteries, AC-DC chargers, and distribution transformers comprise most of the electrical parts of battery swapping systems. Due to the high power requirements of the battery swapping technology, the utility supplies AC power at distribution voltage levels. The power rating of the AC-DC charger is determined by charging profiles. To effectively charge the batteries, the AC-DC charger and the AC transformer transform AC power into DC power.

To swap one or more battery units, the battery swap system contains several changing bays, one of which can integrate an EV [17]. The battery supply system can also be exported to the grid to help out the grid. The battery swap system can sometimes be done [18] by planning the battery's charge-discharge cycle based on how much electricity costs. When modeling a photovoltaic-based battery swapping station, service accessibility and photovoltaic self-consumption are considered [19]. If there are problems with the initial cost of EVs, how fast they can be charged, or if there are problems with the electricity supply, it might be better to have CSs. In Ref. [20], a framework for two-stage resource optimization was made. The above framework includes investing in battery purchases during the planning stage and deciding about battery allocation during the operating location.

An ideal battery charging mechanism in battery swapping stations was developed in Ref. [21], where EVs act as both battery consumers and providers. Here, the switching station gathers data on EV arrival rates as well as the structure of the power cost. At each time, it decides whether to charge or not charge the batteries within the storage based on such information. In Ref. [22], a new centralized way to charge EVs that considers charging priorities and locations has been created. In this strategy, energy loss and voltage changes in power systems and overall charge costs are kept to a minimum by using an empirical method based on a population.

A closed loop supply chain-based swapping proposal was made [23] to realize the cumulative functionality of battery charging as well as swapping stations, whereas this quality of battery swapping service at the swapping system is ensured with a system numerical methods subscription model to fulfill the optimal operation of the battery swapping charging system. The problems with battery swapping technology are that there aren't enough infrastructures for charging or swapping stations and that the capital costs for battery swapping stations are high. Furthermore, the battery of an EV must be precisely designed to allow battery switching.

3. Different types of EV battery chargers

Battery chargers are essential for the advancement of EVs. The parameters of the battery charger influence the charging time and battery life. The efficiency and reliability of a battery charger are crucial, and it should have high energy density, low cost, and be compact and lightweight. How components are controlled, and the switching strategies used play a significant role in determining the charger's performance. The battery charger usually comprises a bi-directional DC-DC converter, where boost and buck operations are carried out by adequately giving gate pulses to the switches in the inverter. The typical function of an EV charger is to regulate and process electric current to allow energy to enter the battery. An EV charger generally integrates and analyses electric current to improve energy flow into an EV's battery, and its demand evolved as a result of the availability of AC power from the electric grid, but EVs require DC power.

Lithium-ion batteries are used in many EVs because they meet the higher performance standards that EVs need. So, the lithium-ion battery has better power and energy density, a higher net voltage, and a longer lifespan than the other biochemical types of batteries [24]. Liquid electrolytes are used in lithium-ion batteries. Lithium-ion was diffused across the liquid electrolyte in lithium-ion batteries to transport ions. It controls 2/3rd of the worldwide battery market today [25]. Due to their higher power and energy densities, superior thermal stability, and long cycle lives, the newly developed all-solid-state batteries have gained much attention [26]. It uses solid electrolytes. The capacity of EV batteries is increased by using solid-state batteries. Lithium-ion diffuses through the solid electrolyte in solid-state batteries.

The two significant issues for EV owners are long charging times and a need for charging facilities. Plug-in hybrids and battery electric vehicles (BEV) could produce more enormous travelling capabilities by boosting battery capacity and electric component energy efficiency. The battery management system monitors the battery voltage, state of charge, and temperature and evaluates the battery when an EV is charged from the grid. Two different power flows can occur between the EV and the power grid. Uni-directional chargers enable EVs to charge but not to add energy to the grid. Because of the simplicity with which uni-directional chargers are controlled [27], it is relatively easy for utilities to manage heavily loaded feeders caused by several EVs. The two features of a traditional bi-directional charger are an active grid-connected bi-directional AC-DC converter that regulates the power factor and a bidirectional DC-DC converter that controls battery current [4]. The bi-directional chargers allow for V2G functionality and power stabilization [28]. The bi-directional power flow improves both the G2V and the V2G modes [29].

In Fig. 16, an EV connected to the power grid and different types of chargers, such as on-board, off-board, and wireless chargers, are shown. The on-BC device is built into the car, letting the high-voltage battery charge from the AC grid while the vehicle is parked [30]. Generally, the on-BC has been considered conductive or inductive based on uni-directional or bi-directional power flow [31]. Metal-to-metal contact is used in conductive chargers, most appliances, and electronic devices. Conductive charging systems are created by making real-time contact between the connector and the charge inlet. The EV could be charged for 8–16 h using the on-BC.

Fig. 16.

Electric vehicle power system with various chargers.

Compared to on-BC, off-BC could charge the EV in 30 min with an output voltage of up to 600V and a current of up to 400 A. Off-board chargers are often called “DC fast chargers” or “superchargers”. The key benefit of this off-BC is the quick charging time. The charging technology, as well as the charging method, must be considered during the charging procedure. Lithium-ion batteries’ three most common charging strategies are constant-current, constant-voltage, and pulse-current charging methods [32]. The development of an EVCS and its optimal location is essential for the extensive adoption of EVs and the use of cheap and clean electrical energy from the grid and RESs [33]. Fig. 17 presents the classification of EV battery chargers. Like all energy storage systems [ESS], EV batteries can improve distribution network power quality, stability, and cost with V2G.

Fig. 17.

Ev battery charger classification.

3.1. Benefits and drawbacks of uni-directional and Bi-directional EV battery chargers

The benefits and drawbacks of Uni-directional vs. Bi-directional charges are listed in Table 3 [34].

Table 3.

Uni-directional vs. Bi-directional battery chargers: Benefits and Drawbacks.

Charger type &Topology	Uni-Directional (1- Ø and/or 3- Ø)		Bi-Directional (1- Ø and/or 3- Ø)
	Benefits	Drawbacks	Benefits	Drawbacks
Converter used	1.Buck: •Basic step-down converter •Requires fewer components •Energy waste is minimal •Input current with pulsations •Driving while floating	1.Buck: •Losses due to switching and conduction •Utilizing half of the B-H curve for the inductor core •Lacking enough power factor correction •Essential to use an oversized filter •Safeguards against power surges and short circuits	1.Half-Bridge: •To be used as a step-up or step-down voltage regulator •Indispensable optical coupling •Low cost, few parts, and tremendous stress on those parts. •Similar to buck-boost 2- quadrant operation, the passive and active parts are the same. •Reduced losses in conduction and switching	1.Half-Bridge: •The boost mode causes the output current to be intermittent.
	2.Boost: •Voltage booster •Intensive power factor correction •Needed a compact filter	2.Boost: •A minimum of one set of diodes and one capacitor or other energy storage element with a transistor are required. •Input current that is not pulsing	2.Full-Bridge: •Converter for step-up or step-down •The reduction of switching and conduction losses •Strengthening in power	2.Full-Bridge: •Inputs with more excellent pulse width modulation and a more significant number of components •Increased expense, complicated control •Stress on individual parts is minimal. •Significantly greater conversion rates
	3.Buck-Boost: •Stepping up or down the voltage is possible. •An inverter with a negative output polarity •Low device count •Intermittent current input	3.Buck-Boost: •Input voltage is highly prone to ripples. •Electricity stresses increase. •The Necessity of optical coupling	3.Multi-level: •Reduced load on individual parts •Reduced losses, high efficiency •Reductions are made to switching frequency and dimensions. •Afilter is inexpensive and tiny.	3.Multi-level: •Due to additional components, cost and complexity have also increased. •Sophisticated control and additional wiring. •Elevated-frequency elements
	4.Fly back: •Inexpensive and simple converter •The fewest number of parts, just 4 •Alternating current input •Output filter inductors are not necessary. •Step up or down as needed. •Switch between inverting and non-inverting modes •It is possible to construct power supply with several outputs.	4.Fly back: •Maintain DC isolation •stress caused by high current and voltage •Efficiency is poor because of the leaking inductance.	4.Matrix: •There are few higher order harmonics. •All waveforms, both input and output are sinusoidal. •Lack of sub harmonics. •The input power factor is fully regulated. •Energy flow in both directions exists by default. •Omitting large energy-storing capacitors •The highest voltage transformation ratio possible •Devices with semiconductors are more advanced.	4.Matrix: •A system that is susceptible to input voltage perturbations
	5.Forward: •Raise or lower as needed. •Current input that pulses •Reduced price	5.Forward: •Inadequate use of the magnetic core	–	–

3.2. Comparison between pros and cons of on-BC and off-BC

A possible solution is to use a unidirectional charger, which has fewer hardware needs, fewer problems with connections, and a tendency to slow down battery wear. With the relevant power transition stages, a bi-directional charging system makes it possible to charge from the grid, supply the battery energy into the grid, and regulate the energy.

The benefits and drawbacks of on- and off-BC is listed in Table 4 [34]. The chargers have been put into two groups based on where they are located [4] as discussed in the following sub-section.

Table 4.

A comparison of pros and cons of on-BC and off-BC.

Charger Type	Pros	Cons
On-BC	•No battery heating issue •A low-power charger •Recharge anywhere there is an electrical outlet	•A lower transmission of energy (kW) •Operates on pilot signal J1172 •Weight of EVs •Low charging speed •Control of the battery management system (BMS) is achieved via on-board rectification. •Incorporating additional charging components in the design of an EV can lead to a decrease in portability. •The communication protocols are not up to industry requirements for charger interface. •The supply voltage, maximum current capacity, and charging station phase configuration are items that the on-board BMS requires.
Off-BC	•Greater power output (kW) •Eliminating EV's load •Enhanced BMS systems •Batteries' capacity to accept charge significantly limits their power output. •Charge quickly •Increased complexity and cost	•Problem with battery heating •Lack of ability to recharge at many locations •High-power charge •BMS integration with charging stations is extremely expensive and difficult. •Charging stations are accountable for faulty charging operations. •Charging BMSs that are fewer sophisticated. •For off-board BMS, it is impossible to identify faulty battery pack cells.

3.3. On-board chargers

The on-BCs comprise the front-end AC-DC and back-end DC-DC stages, such as a power factor correction pre-regulation circuit and an isolated DC-DC converter connected to the EV battery. A large DC-link capacitor was generally installed between these two stages to decouple the low-frequency current ripples. The typical block diagram of the on-BC is displayed in Fig. 18. Both output and input capacitors are frequently used for filter capacitors, also termed electromagnetic interference filters [29]. The above input capacitors are employed to eliminate unwanted signals and line noise. Boost power factor correction converters are usually incorporated in the front-end rectifier to achieve a high power factor and minimize harmonic distortion. Because of the possibility to simultaneously achieve a higher switching frequency and reduced switching losses, resonant power converters are frequently utilized for DC-DC converters [35], especially the LLC (inductance-inductance-capacitance) design due to its capacity to perform at zero voltage switching, excellent voltage control, and reduced voltage stress at the output diodes. Fig. 19 illustrates the internal structure of an on-BC [36].

Fig. 18.

Typical Block Diagram of an on-BC.

Fig. 19.

Internal architecture of an on-BC.

3.3.1. Two-stage on-BC

The two stages that make up an on-BC are the AC-DC (or primary) stage and the DC-DC (or secondary) stage. A boost PFC converter is generally included in the front-end rectifier to help it achieve a high power factor and low harmonic distortions. A half-bridge, full-bridge, or multilevel diode bridge can function as a rectifier circuit [29]. All semiconductor power devices in Ref. [37] have an on-board fixed-frequency zero-voltage switching system. Both the phase-shift full bridge power converter and the front-end full-bridge rectifier have used an additional resonant section to ensure zero voltage switching. The constant frequency zero voltage switching on-BC with two stages is depicted in Fig. 20.

Fig. 20.

1-Ø Two-Stage on-BC with zero-voltage switching at a fixed frequency.

Utilizing the silicon-carbide bi-directional LLC charger protocol has enabled high efficiency and energy density [38]. It has the reconfigurable bridge-less totem pole PFC, used to attain unity power factor. Also, a 300-kHz LLC exploits electromagnetic inclusion and has a negligible voltage switching frequency. PFC with interleaved bridgeless totem poles is used in the first stage because it can handle high power and produce high-quality grid current. LLC converters are employed in the second stage due to their high power density and efficiency. The 3-Ø, two-stage, bi-directional LLC on-BC is depicted in Fig. 21.

Fig. 21.

3-Ø Two-Stage Bi-Directional LLC on-BC.

3.3.2. Single-stage on-BC

It is possible to produce a one-stage battery charger by combining the AC-DC rectifier and the DC-DC converter. The single-stage type of battery charger is used when cost and size are essential. A single-stage battery charger lets users eliminate bulky and expensive parts like inductors and DC-link capacitors.

A bi-directional DAB converter is utilized in a single-stage configuration to enable it to function as a single-phase AC-DC module. Researchers have suggested using a material with a wide-band-gap that exhibits a significant energy gap between the valence and conduction bands [39] for improving the performance of power semiconductor devices by using an advanced material obtained by substituting ‘Si’ material with ‘wide band gap’ material. Totem-pole power factor correction is efficient due to its quick or null reverse recovery features. With excellent energy efficiency and power density, wide-band-gap devices can utilize the on-BC. Planar magnetics were proposed to achieve perfect power density. An innovative soft-switching two-directional current source and rectifier based on on-BC topology was designed [40] for EVs with multiple battery packs. The series structure of DC choppers was used to implement the higher DC-link voltage utilizing several low-voltage battery sets. In addition, the capacitor in the equivalent circuit assisted in lowering the high dv/dt value caused by fast-switching silicon carbide systems.

An isolated single-phase on-BC is addressed without a DC link and any external snubbing or clamping circuits [41]. It includes two magnetic parts used for soft switching and stable switching frequencies. The regulation of voltage stability and improvement of power factor correction has been achieved by reducing the circulating current by implementing primary phase-shift modulation. Also, the DC link capacitor is the main part that stores energy and decreases the ripple in the output voltage of the PFC circuit. The DC-DC converter controls the voltage and current of the primary battery charging system. The 1-Ø single-stage on-BC with DAB is illustrated in Fig. 22.

Fig. 22.

1-Ø Single-Stage on-BC with Dual Active Bridge Converter.

3.3.3. Integrated on-BC

Many integrated designs are suggested to minimize the number of components and lower the battery charger's size, weight, and price. While the period of charging, the motor coil and traction inverter function as a grid filter and an active front-end power factor correction component. The PEV propulsion system and its traction converter were used to explain a new single-phase integrated charger. The charger architecture regulates battery voltage, current, and PFC without using many more significant add-on components. A single inverter/rectifier is used for all EV operation modes by the integrated on-board, a bi-directional EV charger. By using the same power switches for EV traction/propulsion, braking, and battery recharging, the integrated design reduces the amount of power electrical components.

The integrated on-board EV chargers face a primary issue caused by the current flow through the motor windings, which is the torque they produce. Utilizing a mechanical brake while the battery is charging is one method of reducing torque [2]. The design and control strategies for a single-phase bi-directional on-BC in a hybrid electric vehicle (HEV) employing a hybrid starter generator (HSG) and an inverter are provided in Ref. [42]. By adding some extra power relays, the HSG system can now charge batteries instead of a regular on-BC. Based on the grid's instruction, a single-phase bi-directional on-BC offered a control approach, allowing for battery charge and reactive power supply. The integrated on-BC requires no unique stator winding design and uses a 2-leg converter, auxiliary inductors, and relays [43]. A control approach is suggested to govern inverter currents for output voltage regulation, power factor unity, and zero magnetic torque during operation. Regardless of where the synchronous motor's rotor with a permanent magnet starts out, zero torque and flux are obtained. The described integrated switching reluctance motor (SRM) powertrain topology in Ref. [44] for PHEVs offers numerous driving and battery-charging functionalities with reduced power consumption compared to previous topologies. Depending on load conditions, the driving mode offers four operation modes: battery driving, generator driving, generator and traction battery hybrid driving, and regenerative braking. To charge the traction battery, the rectifier, SRM windings, and power converter form a 3-phase interleaved boost PFC converter, reducing input current ripple and improving charging power quality. Three different charging modes, namely grid to traction battery (G2T), generator to auxiliary battery (G2A), and traction battery to auxiliary battery (T2A), are attained to increase the charging versatility of the PHEV.

The dual inverter drive-based 3-phase EV charger is integrated in Ref. [45]. By using drive-train parts like power electronics and cooling equipment, integrated charging lowers the cost of the CS. The dual inverter drive reduces the most significant current ripples throughout the charger. During the charging period, a balanced DC flow has been provided through the dual inverters’ windings to prevent torque generation. In Ref. [46], a contemporary single-phase, 2-stage on-board integrated charger was described to leverage the advantages of 5-phase hybrid excitation flux switching motors. The suggested architecture overcomes the existing challenges by employing the multi-level topology, choosing the proper winding options, and redesigning the field coils. The 3-Ø integrated the on-BC excluding capacitor is shown in Fig. 23.

Fig. 23.

3-Ø Integrated capacitor less on-BC.

A generalized concept of charging power output was created and examined to keep the vehicles stable throughout the charging procedure. When an EV is charged, the unity power factor and zero-order current control are discussed. It was better to use an integrated full-bridge DC-DC converter with a diode clamping circuit and modified pulse width modulation [42]. The 3-phase induction machine stator winding designs are described and split into two equal parts at zero electrical degrees for the on-BC. When the rotor is stable, either in parallel or in series, one of two ways to wind the three-phase coils was looked into to make a good filter [47]. Fig. 24 represents the 9-Phase Integrated on-BC[ 52]

Fig. 24.

9-Phase Integrated on-BC.

A 9-Ø fast integrated on-BC representation can be viewed in Fig. 24. In Ref. [48], to minimize the complexity of integration, the on-BC was described with a newly developed rotatory displacement and charging velocity during the charging process. The charging power system was studied to rule out displacement and noise. In Ref. [49], a generic single-phase on-BC without electrolytic capacitors was made for single-phase and three-phase grids. Single-phase on-BC without electrolytic capacitors was created to accomplish zero-voltage switching with a grid and a wide range of battery voltages using only one control variable, the phase shift angle and a new integrated charger consisting of a bi-directional DC-DC converter for PEVs [50]. The given bi-directional DC-DC converter with a diode rectifier can increase or decrease the voltage in all vehicle modes. A bi-directional DC-DC converter with a diode rectifier enables the selection of a large variety of battery voltages and the charging of batteries with a universal voltage level range (100–260 V) and effective brake energy regulation. In Ref. [43], an asymmetric nine-phase machine and a charging inverter with a built-in on-BC were made. The three neutral terminals were combined using nine-phase equipment throughout the charging procedure. As a result, the field in the integrated charger includes a three-phase LC filter, a three-phase current source converter, a dual-inverter drive, an open-wound permanent magnet synchronous motor (PMSM), and two energy storage modules. Due to the utilization of a dual-inverter drive, the above mentioned architecture exceeds the capabilities of current source converter-based integrated chargers. Multiple energy storage media, high-voltage drive function, and the ability to switch to single-inverter operation in the event of a component failure are all made possible by dual-inverter drives. The above approach has been optimized for power factor unity and V2G efficiency.

3.3.4. Multi-functional on-BC

A silicon carbide bi-directional portable charger for EVs with several uses is discussed in Ref. [51]. Here, a single battery charger has performed various tasks, including connectivity between EVs and DC micro-grids in vehicle-to-vehicle (V2V) mode and outdoor DC load power supply. The above charger achieved high efficiency while using two levels of direct DC--DC conversion. A new non-isolated multipurpose on-BC [52], which combines an on-BC and a low-voltage charger, has been made available. It was made with an active power decoupling design that didn't need extra switching parts and was used to lower the DC link capacitance. The bar chart in Fig. 25 displays the slow EV charger stock over the past ten years [53].

Fig. 25.

Slow EV chargers stock in last decade.

Fig. 26 provides an overview of the configuration of the multi-functional on-BC. This charger serves two purposes. One is that the low-voltage charging circuit acts as an active power decoupling circuit when the vehicle is linked to the grid to charge or discharge the high-voltage battery, using the second-order oscillatory power in the DC connection. The second is that, while converting high-voltage batteries to low-voltage batteries, the low-voltage charging circuit performs the function of an LLC resonant converter. The above discussion addresses the several on-board chargers demonstrated in existing studies and tabulated in Table 5.

Fig. 26.

Layout of Multi-Functional on-BC.

Table 5.

Comparison of various on-BC based on different parameters.

Ref. No.	Configuration	Converter Used	Control	Switching Frequency	Grid Voltage, frequency	Output Voltage	Output Power	Charging method	Machine Used	Efficiency
[54]	Non-conventional On-BC	Cascade of buck converter with full-bridge	Constant current control	50 kHz	110V, 60Hz	180V	6 kW	–	Axial flux permanent motor	–
[55]	Non-isolated 1-stage high efficient on-BC	Buck-boost	2-phase interleaved control	–	–	–	3.7 kW	–	–	97.6 %
[56]	Isolated on-BC with Sic devices	∗bridgeless boost ac-dc ∗ Isolated full-bridge phase shift dc-dc	–	200 kHz	–	–	6.1 kW	–	–	95 %
[35]	1-kW PEV charger	∗Interleaved boost ∗LLC based full-bridge multi resonant dc-dc	–	200 kHz	1-Ø 110V, 60Hz		1-kW	CC-CV	–	–
[57]	Integrated ZVS	Full-bridge converter	–	100 kHz	400V	–	2 kW	–	–	–
[58]	Soft switching high frequency link PWM	Bridgeless boost rectifier	–	50 kHz	230Vrms,60Hz	(250–450)V	2 kW	–	–	98 %
[59]	Dual active bridge based on-BC	∗Full-bridge ac-dc ∗Dual active bridge dc-dc	Rotating frame based closed loop	∗Si- 50 kHz ∗GaN-500kHz	∗Si-240V ∗ GaN-150V	∗Si-366V ∗ GaN-250V	∗Si-1kW ∗GaN-1kW	–	–	–
[50]	Multi-functional	–	–	10 kHz	110V, 60Hz	200V	3.3 kW	–	–	–
[43]	Integrated On-BC	–	Phase lock loop	10 kHz	240 Vrms,50Hz	–	–	CC	9-Ø machine	–
[60]	9-switch converter based Integrated motor drive	–	Field oriented control	10 kHz	150V, 50Hz	380V	2.2 kW	–	Symmetrical 6-Ø machine	–
[61]	1-Ø integrated on-BC	–	∗Grid current control ∗Current balancing control ∗Interleaving control strategy	10 kHz	240V,50Hz	600V	–	–	∗Asymmetrical-9-Ø,6-Ø ∗Symmetrical-5-Ø,6-Ø machine	∗G2V-(79–86)% ∗V2G-(81–89)%
[62]	1-Ø integrated on-BC	Diode bridge rectifier	∗Input current inner loop ∗Output voltage/current outer loop	15 kHz	240V,60Hz	420V	3 kW	CC-CV	3-Ø,8-pole round rotor PMSM	93 %
[63]	High power density & efficiency integrated on-BC	Cascaded buck-boost	∗on-BC-Asynchronous control ∗LDC-Duty frequency& hybrid control	150 kHz	(85–265)V		6.6 kW	–	–	97.3 %
[41]	LDC system and 800V battery based noel on-BC	∗Full-bridge LLC converter ∗LDC converter	–	185 kHz	9360-675)V	–	∗For On-BC-3.3 kW ∗For LDC-1.8 kW	–	–	–
[64]	An isolated bidirectional PWM on-BC	PWM resonant converter	Fixed frequency control	50 kHz	415V	(250–415)V	6.6 kW	–	–	97.2 %
[65]	Non-isolated 1-stage	–	Phase shift control	–	–	350V	3.7 kW	CC-CV	–	97.29 %
[66]	9-Ø PM EDROC	∗PWM voltage source rectifier ∗Buck converter	Phase locked loop	10 kHz	–	288V	3.3 kW	CC-CV	9-Ø PM machine	–
[67]	3-Ø integrated On-BC	–	–	20 kHz	120Vrms	(280–420)V	3.3 kW	–	3-Ø 8-pole rotor round PMSM	92.6 %
[68]	1-stage soft-switching on-BC	∗Full-bridge diode rectifier ∗High frequency dc-dc converter with active clamp circuit	dsPIC33EP32MC202 micro chip	50 kHz	220 Vrms	360V	3.3 kW	CC-CV	–	96.0 %
[69]	1-Ø,2-switch,non-isolated universal on-BC	Buck &boost cells cascaded combination	Average current mode control	30 kHz	(85–265)V, 60Hz	(150–450)V	1 kW	–	–	96.0 %
[46]	2-stage,1-Ø Integrated on-BC	Buck converter	∗second order generalized integrator control ∗bang-bang control ∗current vector control	10 kHz	220V,50Hz	(144–288)V	5 kW	–	5-Ø hybrid excitation flux switching machine	At 144V-91.47 % At 288V-95.56 %
[42]	Clamping diode circuit based on-BC	Full-bridge dc-dc	–	80 kHz	–	180V	540W	–	–	92.10 %
[70]	Reduced switching stress based 1-Ø integrated	∗Interleaved boost converter ∗3-level dc-dc converter	∗Interleaved control algorithm ∗Control algorithm for 3-level dc-dc converter	–	230V,50Hz	400V	8 kW	CC-CV	–	–
[71]	1-Ø integrated on-BC with V2G	–	∗Resonant current controller ∗Energy balance controller	20 kHz	480 Vrms	400V	∗G2V-1.92 kW ∗V2G-19.2 kW	–	–	–
[72]	Electric-Drive-Reconstructed on-BC	∗ Six-phase voltage-source-inverter	∗ Phase-locked loop ∗Outer loop, Inner loop ∗ PWM generator ∗ fault diagnosis	–	45 Vrms	150V	1.25 kW	CC-CV	asymmetrical six-phase permanent magnet synchronous machine	∗For pre-fault operation-90 % ∗ For post-fault operation-89 %
[51]	Bi-directional Sic multi-functional on-BC	∗Full-bridge LLC ∗Totem pole bridgeless power factor correction converter	∗DC link voltage control ∗Grid current control	66.7 kHz	(90–265)V	400V	6.6 kW	–	–	∗V2V-96.1 % ∗G2V->95.7 %
[40]	Current source rectifier soft-switching on-BC	∗2 dc choppers in cascade connection	Voltage oriented control scheme	40 kHz	100 Vrms,50Hz	100V	–	–	–	–
[73]	A novel multi-functional Sic based on-BC	∗HV charger – Bi-directional buck-boost ∗LV charger- Resonant LLC	–	100 kHz	110 Vrms,60Hz	∗HV charger-(250–430)V ∗LV charger-(12–13)V	2 kW	–	–	∗On-BC-96.1 % ∗LV charger-95.3 %
[37]	Constant frequency ZVS on-BC	∗Front end full-bridge rectifier ∗Phase shift full-bridge dc-dc ∗Active clamp resonance branch	–	75 kHz	–	–	3.3 kW	CC-CV	–	97.2 %
[45]	3-Ø dual inverter drive based integrated on-BC	∗Current source converter ∗Dual inverter	∗CSC modulation control ∗Nested control ∗ESU charge balance control	10 kHz	208 Vrms, 60Hz	–	10 kW	CC-CV	–	94.0 %
[66]	Solar powered EV based EDROC	6-Ø inverter	∗Fixed voltage/current control ∗MPPT control	–	100V	–	2 kW		6-Ø PMSM	93 %
[51]	Bi-directional Sic multi-functional on-BC	∗Full-bridge LLC ∗Totem pole bridgeless power factor correction converter	∗DC link voltage control ∗Grid current control	66.7 kHz	(90–265)V	400V	6.6 kW	–	–	∗V2V-96.1 % ∗G2V->95.7 %
[40]	Current source rectifier soft-switching on-BC	∗2 dc choppers in cascade connection	Voltage oriented control scheme	40 kHz	100 Vrms,50Hz	100V	–	–	–	–
[73]	A novel multi-functional Sic based on-BC	∗HV charger – Bi-directional buck-boost ∗LV charger- Resonant LLC	–	100 kHz	110 Vrms,60Hz	∗HV charger-(250–430)V ∗LV charger-(12–13)V	2 kW	–	–	∗On-BC-96.1 % ∗LV charger-95.3 %
[37]	Constant frequency ZVS on-BC	∗Front end full-bridge rectifier ∗Phase shift full-bridge dc-dc ∗Active clamp resonance branch	–	75 kHz	–	–	3.3 kW	CC-CV	–	97.2 %
[45]	3-Ø dual inverter drive based integrated on-BC	∗Current source converter ∗Dual inverter	∗CSC modulation control ∗Nested control ∗ESU charge balance control	10 kHz	208 Vrms, 60Hz	–	10 kW	CC-CV	–	94.0 %
[74]	Two-stage, standard, single controlled PWM on-BC	∗Boost rectifier PFC stage ∗Half-bridge LLC resonant converter	Single controlled PWM	20 kHz	220V	24V	3.3 kW	CC-CV	–	97.6 %
[75]	On-BC that efficiently charging multiple EVs	∗ boost ∗SEPIC ∗fly-back	∗Current controller ∗Voltage controller	5 kHz	230V 50/60Hz	48V	3.2 kW	CC-CV	–	95 %
[2]	Universal battery charger	Boost-Buck circuit	∗A dual-loop distinct current controller ∗Active harmonic minimization method	40 kHz	85–264 VRMS, 50/60Hz	450V	1.5 kW	CC-CV	–	95.6 %
[76]	A single-stage topology based on the LCLC resonant structure	∗PFC circuit ∗LCLC resonant circuit	∗An LCLC-based DC-DC converter a ∗Rectifier with boost stage.	∗Series resonant frequency- 265 kHz ∗ Parallel resonant frequency- 473 kHz	230V,50Hz	48V	1500W	CC-CV	–	–

3.4. Off-board chargers

The size of the battery has the most significant impact on charging time. Off-BC is the most effective way to reduce charging time. Charging systems not built into the vehicle are called “off-board”. A schematic diagram of an off-BC has been displayed in Fig. 27. An AC-DC converter has been placed externally on the EV in an off-BC configuration. Incoming AC grid power has been converted into the DC power required to charge the battery pack via an off-BC. The numerous off-BCs established in earlier research are described in the following sub-section.

Fig. 27.

Configuration of an off-board charger.

3.4.1. Two-stage off-BC

A new design for a two-stage converter-based quick battery charger for 3-Ø EVs is presented in Ref. [77]. The first stage (AC-DC) consists of two full-bridge voltage source converters that operate simultaneously and serve as a connection between both the grid system and the DC-Link, which is shown in Fig. 28. As the resulting frequency is twice as high as each converter's switching frequency, both converters may operate in interleaved mode. The second stage (DC-DC), which comprises a bi-directional, 3-level asymmetrical voltage-source converter, acts as a connection between the DC-link and the batteries. The DC-DC converter operates similarly to an interleaved converter because it's managed by a single controller at a single switching frequency, with the coupling filter's output frequency approximately equal to double switching frequency.

Fig. 28.

Two-full bridge voltage source converter based two-stage off-BC.

3.4.2. Single-stage off-BC

In [78], EV charger topology is implemented with a DC–DC converter connected between the battery and the DC bus and a DC–AC converter connected between the grid and the DC bus. The filters have LCL setups on both sides of a phase leg output. Combining the common point of three-phase capacitors on the DC-AC side to the DC bus negative terminal creates a bypass channel for zero-sequence voltage control. The 3-phase off-BC with a single stage is depicted in Fig. 29.

Fig. 29.

3-Ø Single-Stage off-BC.

3.4.3. Integrated off-BC

A bi-directional, non-isolated, integrated off-BC design that reduces load voltage and current to extend the motor's lifespan and durability is described [79] and is accomplished by adding the following parameters to an EV's drive.

• •A standard mode voltage control LC filter was used in each phase.
• •A traditional mode inductor was used to improve leakage current filtration.
• •Switches were used to switch in both traction and charging modes.

An integrated off-BC employs contactors to switch between traction and charging modes, as shown in Fig. 30.

Fig. 30.

Integrated off-BC with contactors.

A typical EV drivetrain includes semi-conductive devices and a driving system-a linear 3-level power converter-based rapid charger with an integrated inductor as described in Ref. [80]. The mass, loss, and dimensions of individual and unified inductors have been compared, and inductor design has also been explored. The propagated and balanced currents, as well as the quasi- and 180-interlaced control strategies, were introduced. The ripples in the current were eliminated for the two different operation modes. Fig. 31 depicts a DC fast charger that is integrated into the drivetrain and has both boost and buck functionality and the ability to drive and charge simultaneously.

Fig. 31.

Integrated Dual Inverter based off-BC.

3.4.4. Multifunctional off-BC

For the G2V, V2G, and V2H functions, it is suggested in Ref. [81] to use a bi-directional EV charger. To avoid electrolytic capacitors, the proposed charger uses sinusoidal charging. In order to ensure that the charger can turn on and off with zero current under different loads while using sinusoidal charging, a non-regulating series resonant converter has been selected as the isolated DC-DC converter for the suggested charger. The control of the AC-DC converter allows it to operate in G2V, V2G, or V2H in addition to power factor correction, facilitating the control of each configuration and the corresponding changes. Comparing all connector types, it is clear that CHAdeMO is a good choice for DC rapid chargers [82]. The fast charging facilities established across different nations like Europe, the United States, and China are shown in Fig. 32 [83]. Table 7 is a list of the different off-BCs that have been talked about in previous research. Table 6 lists the maximum power, current, and power availability for different types of connectors in a DC fast-charging system.

Fig. 32.

Fast EV chargers stock in last decade.

Table 7.

Comparison of various off-board chargers based on different parameters.

Ref. No.	Configuration	Converter Used	Control	Switching Frequency	Grid Voltage, frequency	Output Voltage	Output Power	Charging method	Machine Used	Efficiency
[84]	2-stage controlled AC-DC	∗Full-bridge diode rectifier ∗Active power filter ∗Interleaved buck converter	∗DC-DC stage control ∗Dual loop analog control	17 kHz	3-Ø,(380–415)Vrms, 50Hz	430V	50 kW	CC-CV-CP	–	95 %
[85]	V2Gbi-directional off-BC with reactive power operation	∗3-Ø AC-DC boost rectifier	Off-board charging control strategy	18 kHz	208V,60Hz	370V	12.5 kW	CC-CV	–	–
[86]	Integrated off-BC	–	∗Voltage oriented control	10 kHz	3-Ø 415V, 50Hz	–	–	CV	∗5- Ø machine ∗5- Ø inverter	–
[87]	Dual-inverter	–	∗Constant current control ∗voltage balancing control	7.5 kHz	Input power-50kW DC	–	–	CC	Open end winding PMSM	–
[81]	An isolated bi-directional charger without capacitor	∗Half bridge series resonant converter ∗Full-bridge inverter	∗Constant power control ∗Constant voltage control	–	220V,60Hz	–	3.3 kW	–	–	∗Charging-95.7 % ∗Discharging-95.4 %
[77]	Dual-stage converter based off-BC	∗AC-DC front end converter DC-DC back end converter	∗Grid-current control ∗DC-link voltage control ∗Battery charging current control	20 kHz	400 ± 10V, 50Hz	–	–	–	–	–
[88]	Grid voltage control based off-BC	∗Front-end AC-DC ∗Back end DC-DC	Vehicle charging control ∗Grid voltage regulation control	10 kHz	3-Ø, 400V,50Hz	–	2kVA	CC, reduced CC	–	–
[89]	3-Ø grid-tied interleaved based ripple free off-BC	∗3-Ø interleaved active rectifier ∗Interleaved 3-Ø DC-DC chopper	∗Ripple free output current control ∗Voltage control	–	–	–	150 kW	CC-CV	–	–
[90]	Reconfigurable multi-objective off-BC	∗Voltage source converter ∗Bi-directional DC-DC Boost	∗Standalone mode control ∗Grid connected control ∗Adaptive DC link voltage control	–	230V,50Hz	–	3.7 kW	CC-CV	–	–
[79]	Non-isolated bi-directional Integrated off-BC	–	∗Common mode voltage control	80 kHz	3-Ø,400V,50Hz	–	11 kW	CC-CV	PMSM	98.4 %
[91]	Dual inverter based 3-Ø off-BC	∗3-Ø Current source converter ∗Dual inverter	∗Grid current control ∗CC-CV charging control ∗Motor winding current balance control	∗Current source converter-60kHz ∗dual inverter-10kHz	208Vrms,60Hz	–	10 kW	CC-CV	PMSM motor	94.3 %
[92]	3-Ø Quasi 1-stage converter based off-BC without filter	∗3-Ø Vienna rectifier based unfolder ∗DC-DC current source converter	∗Vienna rectifier control	–	400V ± 10 %	(300–470)V	5 kW	–	–	94.8 %
[93]	Multi-functional PV-integrated off-BC	∗Bi-directional DC-DC	∗Cascaded H bridge AC-DC control ∗Energy storage bi-directional DC-DC control ∗ EV bi-directional DC-DC control	20 kHz	–	–	12.6kVA	CC-CV	–
[78]	Non-isolated off-BC without transformer	∗DC-DC buck ∗DC-AC	DC-AC control: ∗phase locked loop control ∗Active power control ∗Reactive power control ∗Zero sequence voltage control DC-DC control: ∗CC control ∗CV control ∗Adaptive damping control	∗V1-20 kHz ∗V2-80 kHz	480V	(200–650)Vdc	22 kW	∗CC-CV Charging ∗CC discharging	–	∗V1-99 % ∗V2-98.5 %
[34]	bridge-less modified Cuk-based off-BC	∗ front-end PFC converter ∗ back-end isolated stage	Voltage control loop	10 kHz	400V	48V	1000W	–	–	91.19 %
[94]	An off-BC with a three-phase, six-pulse voltage rectifier	∗3- Ø rectifier circuit ∗Pulse generation circuit	∗Voltage controlled oscillator ∗PID controller	–	11 kV,50Hz	450V	90 kW	CC-CV Charging	–	–
[95]	Solar-Fuel-Cell, Switched Capacitor Z-Source Converter basedoff-BC	–	multi-output-based adaptive neuro fuzzy inference system (ANFIS) controller	–	–	–	30.2 kW	CC-CV Charging	–	–

Table 6.

Different parameters for DC fast charging systems.

Criteria	CHAdeMo IEEE 2030.1.1 IEC 62196-3	GB/T GB/T 20234.3 IEC 62196-3	Tesla Model	CCS Type 1 SAE J1772 IEC 62196-3	CCS Type 2 IEC 62196-3
Max Voltage	100V	100V	410V	600V	100V
Max Current	400A	250A	330A	200A	200A
Available Power	400 kW	120 kW	135 kW	150 kW	175 kW

4. Classification of charging power levels, connectors and standards of an EV

EVs can be charged via grid-powered AC or DC chargers.

4.1. Different charging levels

Three different levels can be used to categorize conductive CSs. The output power of a given EVSE differentiates CS from a Level 1 charger, a Level 2 charger, and a DC Fast Charger. All EVs are compatible with Level 1 and Level 2 chargers. Some cars cannot be charged at level 3 due to manufacturer-set constraints, such as battery charging rate [96].

4.1.1. Level 1 on-BC

Level 1 on-BC employs a standard 120 V, 15 A, 1-grounding plug, such as a NEMA 5–15R [4]. The minimum power needed to charge an EV was 3.7 kW [97]. A typical J1772 adapter has been used to plug into the EV AC port when using Level 1 in the United States. No additional equipment is required for a home or business location.

4.1.2. Level 2 on-BC

On-BC level 2 chargers typically charge up to 240 V and 60 A [98]. These are getting more popular in the global EV community. Level-2 chargers have a maximum output power of 22 kW [97]. Level-2 on-BC in the 6.6 kW–7.4 kW range is employed in prominent EVs globally. In the United States, level-2 charging has been accomplished using an SAE J1772 connector or a Tesla-model connector. For level-2 charging, the IEC62196-2 Type-2 connector was employed in Europe, while the GB/T 20234 AC connector was used in China.

4.1.3. Level 3 off-BC

DC Level-3 chargers offer a direct power supply ranging from 50 kW to over 150 kW, with a voltage range of 208–600 V. Because of its high power output and thus increased weight, Level 3 DC adapters are installed off-board the EV [99]. The DC Level 3 plug is CHAdeMO and Tesla supercharger compatible. Subsequently, China uses a GB/T 20234 DC plug for fast charging. Typically, only public CSs offer level-3 charging.

4.1.4. Level 4 off-BC

Level 4, commonly called superfast charging, has a power range of 120–350 kW. It has a maximum power output of 350 kW and can complete a 15-min 80 % vehicle charge with a 24 kWh battery. However, the infrastructure needed for charge level is specialized and has yet to be extensively used [100]. The categorization of various power levels and standards in multiple countries is depicted in Table 8 [46].

Table 8.

Classification of different power levels and standards in various countries.

Power Level	Power Rating(kW)	Plug Type
		Europe	United States	China
AC Level1	3.7	–	SAE J1772 Type 1	–
AC Level2	3.7–22	IEC 62196-2 Type 2	SAE J1772 Type 1	GB/T 20234 AC
	≤22	–	Tesla	–
AC Level3	22–43.5	IEC62196-2 Type 2	SAE J3068	GB/T 20234 AC
DC Level3	≤200	CCS Combo - 2 IEC 62196-3	CCS Combo - 1 SAE J1772 OR IEC 62196-3	GB/T 20234 DC

The various charging levels, power ratings, and charging times for on-board and off-board chargers are illustrated in Table 9 [4]. Fig. 33 illustrates different charging levels for EV on-board and off-board chargers.

Table 9.

Different levels of EV charging power ratings.

Power level	Model of charger	Usage type	Interface for energy supply	Power to be expected	Duration for Charging	Automobile technology
Level – 1 120V AC	On-BC 1-Ø	home (or) office	Convenient supply	1.4 KW/12A 1.9 KW/20A	4-11 Hours 11-36 Hours	5-15 KWh PHEV 16-50 KWh EV
Level – 2 240V AC 400V AC	On-BC 1-Ø or 3-Ø	Public (or) Private places	Customized EVSE	4 KW/17A 8 KW/32A 19.2 KW/80A	1-4 Hours 2-6 Hours 2-3 Hours	5-15 KWh PHEV 16-30 KWh EV 3-50 KWh EV
Level – 3 208–600V DC	Off-BC 3-Ø	corporate, comparable to a petrol station	Customized EVSE	50 KW 100 KW	0.4–1 Hour 0.2–0.5 Hour	20-50 KWh EV

Fig. 33.

Different charging levels for EVs with on- and off-board chargers.

By 2040, the International Energy Agency (IEA) aims to have 548 million EVs on the road [157]. Residential and non-residential CS is the two categories under which CS are categorized. The charger can be fitted with DC fast charging technology as well as Level 1, Level 2, and Level 3 fast charging capabilities. A large percentage of EV charging happens at home, utilizing slow-charging ports. However, in the future, commercial establishments will be furnished with CSs that can cater to various charging connectors, thereby promoting the use of EVs [101]. EV charging infrastructure is included in several international standards. Manufacturers in the US utilize SAE and IEEE, while IEC is commonly used in Europe. The Japanese invented the CHAdeMO EV charging protocol. Guobiao (GB/T) standards, equal to IEC standards for AC charging, are used in China for both AC and DC charging and were created by the ISO/IEC Chinese National Committee [102].

4.2. Guidelines and standards for charging infrastructure

The Ministry of Power released a set of revised guidelines and standards for EV charging infrastructure on January 14th, 2022 [103].

• •Owners of EVs have the convenience of charging their cars at home or work using their already-existing electrical connections.
• •Distinguishing between different pricing use cases:

The indication allows for identifying domestic, public, and captive charging.

• •EV charging will continue to be a de-licensed activity:

The technical, safety, and performance standards and protocols established by the Ministry of Power, Bureau of Energy Efficiency (BEE), and Central Electricity Authority (CEA) are the criteria for obtaining a license to set up public CSs.

• •Revenue sharing for land use to make CSs profitable:

A revenue-sharing scheme for CS land has been implemented to make them financially sustainable during the expansion of EVs. Public CSs will be established on a revenue-sharing basis, with the Government/Public entity providing land. A fixed rate of ₹ 1/kWh (used for charging) will be paid quarterly to the Land-Owning Agency.

• •Central EV Public Charging Infrastructure Nodal Agency:

The Central Nodal Agency for the launch of EV Public Charging Facilities will be the Bureau of Energy Efficiency (BEE). The Central Nodal Agency should get the required support from all pertinent organizations, notably the Central Electricity Authority (CEA). Each State Government must choose a Nodal Agency to build up the necessary infrastructure for charging.

• •Fixed service charge cap by state governments:

The State Government shall set the maximum Service Charges to be levied by such Charging Stations because electricity is provided at reduced rates, and, in many cases, the Central/State Governments provide subsidies for establishing Public Charging Stations.

• •Tariff for power supply to public EV charging stations:

Up to March 31, 2025, the “Average Cost of Supply” cannot be exceeded by the single-part price for power supply to public EV charging stations. The exact fee will apply to Battery CS (BCS).

• •Schedules for establishing a connection to the Public CSs (PCS) are outlined as follows:

The Electrical Power (Rights of Users) Act establishes time limits for various actions. In light of this, PCS connectivity must be made available within seven days in major cities, 15 days in smaller municipal areas, and one month in remote regions. The distribution licensees must supply a new connection or adjust an existing connection within these periods.

• •Public EV CS Database:

In collaboration with State Nodal Agencies (SNAs), the Bureau of Energy Efficiency (BEE) must set up and preserve a national online database of all Public CSs. The Bureau of Energy Efficiency will construct a Web-Portal/Software/Mobile App for the federal public charging station database.

• •Web Service Providers:

Public Charging Stations must partner with at least one Internet Network Service Provider (NSP) to allow EV owners to reserve charging spaces remotely. The above online information for EV owners should also include details on the location, the kinds and numbers of installed or available chargers, service fees for EV charging, etc.

• •Uniform Pricing Principles Regardless of Technology:

The guidelines have been made even more technology-neutral by incorporating the current international and new Indian charging standards.

• •Access to all:

Open access allows any generation firm to supply electricity to any public CS/chain. Once the complete application is received, Open Access will be delivered within 15 days. They must pay the surcharge, the current cross-subsidy (maximum 20 %, under Tariff Policy Guidelines), transmission, and wheeling charges.

• •Charge Points:

In order to address the concern of EV users running out of charge, it is mandated to install at least one CS within a 3 km × 3 km grid as per the standards. On the two sides of highways/roads, one CS will be placed every 25 km. Every 100 km must have at least one Fast CS meeting Charging Infrastructure Specifications for long-range and heavy-duty EVs like Buses, Lorries, etc.

• •
  Guidelines for Implementing Public Infrastructure for EV Charging:

  • ✓
    As per the results of a survey conducted in 2011, the initial phase of the infrastructure development strategy in metropolitan areas with a population of more than four million might involve expanding the coverage of major expressways and significant highways within 1–3 years.
  • ✓
    State capitals and UT headquarters might be covered for diffused and demonstrative effect in Phase II (3–5 Years).
• •Distinctive elements of PHEVs, EVs, and their supporting infrastructure are presented in Table 10 [104].

Table 10.

Infrastructure differences between PHEVs and EVs.

PHEVs and EVs	Battery(Li-ion) kWh	Geographical Range (Miles)	Connector Type	Level 1 Charger		Level 2 Charger		Level 3 Charger
				Demand (kW)	Time to Charge Probably (Hrs.)	Demand (kW)	Time to Charge Probably (Hrs.)	Demand (kW)	Time to Charge Probably (Hrs.)
Nissan Leaf	24	160	SAE J1772	1.8	11–15	6.6	2.5–3	Up to 50	0.3–0.4
Tesla Roadster	54	340	SAE J1772	1.8	>20	9.7–10.7	1.5–2	100	0.37
Toyota RAV4	42	160	SAE J1772	1.9	17	9.6	3.4	50	0.34
Mitsubishi i-MiEV	16	145	TEPCO/JARI SAE J1772	1.5	Up to 10	3.6	Up to 4	Up to 50	0.26
Toyota Prius	5.20	25	TEPCO/JARI SAE J1772	1.5–1.8	3	3.2	Up to 1.6	–	–
Chevrolet Volt	17	64	SAE J1772	1.4	7–10	3.7	Up to 4	–	–

4.3. Various charging connectors

The AC and DC charging connectors are available, each explained below.

4.3.1. Plugs for AC charging

The connector's size, shape, and pin-out vary depending on the type of EV, the nation where it was made, and the charger's degree of power. This is because different regions have different AC mains voltage levels and frequencies. A standard AC connector comprises a couple of tiny pins for communication and two or more main pins, depending on the voltage. Currently, four AC connector types are used globally, as mentioned below.

4.3.1.1. Connector of type 1

This connector is designed for AC single-phase charging and features a circular layout with five pins, including two AC lines, two signal lines, and one protected earth path. The maximum rating for voltage is 120 V or 240 V, with current capacities of up to 80 A [156].

4.3.1.2. Connector of type 2

This connector can accept both AC and DC power sources for charging. It is additionally compatible with 3-phase AC. Single-phase maximum ratings range from 230 V to 80 A at current, whereas three-phase top ratings range from 400 V to 63 A.

4.3.1.3. Connector for Tesla US

Tesla designed this US-specific connection. The Tesla connector is suitable for use with both single-phase AC and DC power. 17.2 kW at 240 VAC is the highest rating for the charging power.

4.3.2. Plugs for DC charging

It is intended that DC fast chargers would eventually replace level 1 and level 2 chargers. According to the supplier, their ratings range from 50 to 500 kW. Power conversion and control stage size and cost increase as power capability increases. The above detail is a primary reason for off-board DC rapid chargers. The other factor is a security concern. Passenger safety is paramount when high-power converters and more significant components for power handling are used [156].

4.3.2.1. Combined Charging System combos 1 and 2

The governing body for combo-type connections is called CharIN. The main attraction is that the Combined Charging System (CCS) connector works with both AC and DC charging. IEC 62196-1, IEC 62196-2, and IEC 62196-3 standards include these connections. The highest power that these connectors can handle is 350 kW, and they can operate 350 A at voltages between 200 V and 1 kV.

4.3.2.2. The CHAdeMO protocol

The companies like Toyota Motor Corporation, Nissan Motor Co. Ltd., Mitsubishi Motors Corporation, Fuji Heavy Industries Ltd., and Tokyo Electric Power Company, Inc. started the “CHAdeMO Association” in 2010. The IEC (61851-23, −24, and 62196-3) and IEEE (IEEE Standard 2030.1.1TM-2015) standards include CHAdeMO. The CHAdeMO DC standard, capable of handling 200–400 kW, is the first to provide V2X (vehicle-to-x) communication using the 1.1 protocol.

4.3.2.3. DC connector for Tesla

The US Tesla superchargers employ an exclusive connector. The Tesla connector is distinguished because it utilizes the same connector and pins for AC and DC charging. They also provide an adaptor for compatibility with CHAdeMO charging stations. The maximum power rating for these plugs is 120 kW.

4.3.2.4. Chinese GB/T plug

DC charging connectors in China use the 20234.3–2015 standard. A Controller Access Network (CAN) interface allows this connector to talk to the vehicle's electrical management system. This connector is particularly appealing since it can simultaneously charge the primary high-voltage and the secondary low-voltage auxiliary batteries. It can supply currents of up to 250 A over a nominal voltage that ranges from 750 V to 1 kV.

Fig. 34 displays commonly used EV charging standards, including IEC and SAE. The public charging station to battery-electric light-duty vehicle proportion is depicted in Fig. 35 [105].

Fig. 34.

Charging Standards: (a) charging ports, (b) connectors.

Fig. 35.

Public charging station to battery-electric light duty vehicle proportion.

Equipment manufacturers’ descriptions of DC-fast chargers are shown in Table 11 [106]. Numerous charging connectors and their respective standards are shown in Table 12 [107].

Table 11.

Details provided by equipment manufacturers regarding DC-fast chargers.

Type of Fast Charger	Voltage Input (3-phase AC) (V)	Power Factor at Full Load	Efficiency (%)	Voltage Output (DC) (V)	Output Current and Power (A),(kW)	Plug-In Power Adapter	Size (H × W × D) (mm)	The mass (kg)
Tesla Terra HP	380V	>0.99	95	150–920	375/500, 150	CHAdeMO 1.2	2103 × 1170 × 770	350
EFAECE-QC45	380V	0.98	93	50–500	120,50	ChadeMO/Combo-1	1800 × 600 × 600	600
Tritium Veefil PK	380V	0.95	98.5	920	500,475	CHAdeMO/CCS	1998 × 980 × 525	700
Delta Ultra-Fast	380V	0.99	94	170–550	300,150	CHAdeMO/CCS	2079 × 998 × 852	400
EVTec Espresso	380V	0.93	93	170–500	300,120	CHAdeMO/CCS	2000 × 930 × 850	400

Table 12.

Standards for charging connectors.

Type of Connector	Mode of Charging	Symbolic expressions	Nation	Ports	Voltage, Current, Power	Protocols
CHAdeMO (JEV G105-1993)	AC		Japan	7 control lines (comm. CAN) and 3 power pins (DC+, DC-, and PE).	200–500 V, ≤400 A, 200 kW CHAdeMO 2.0: 1000V, ≤400 A, 400 kW	IEC 61851-23, 24 IEC 62196-3 IEEE2030.1.1TM-2015
Type1/j1772	AC		America and Japan	3 power the ports (L, N, PE).	1-Ø 120 V, ≤16 A,1.9 kW 1F 240 V, ≤80 A, 19.2 kW	SAE j1772 IEC 62196
Type 2/Mennekes	AC		EU China	5 power pins (L1, L2, L3, N, PE) and Two control signals: CP and PP (PWM for CP).	1-Ø 230 V, ≤32 A,7.4 kW 1-Ø 400 V, ≤63 A,43 kW	IEC 62196 GB/T 20234.2–2015
	DC		EU-based Tesla	Power pins: DC+, DC-, and PE. Control signals: 7 (CAN)	400 V, ≤140 A, 56 kW	IEC 62196
CSS and Combination	DC		USA	3 AC pins, 2 DC pins, and 2control signals (PLC, PP).	350 kW, ≤350 A, and 200–1000 V DC	IEC 62196-1/2/3 IEC 61851-1/22 IEC 61851-1/23 ISO/IEC 15118 DIN SPEC 70121 SAE J2847/2
			EU	5 AC pins, 2 DC pins, and 2 control signals (CP (PLC), PP)
GB/T	DC		China	5 power pins (DC+, DC, PE, and 2 additional power pins) BT 4 control pins: 2 PP, 2 CAN.	750/1000 V, ≤250 A, 237.5 kW	GB/T 20234
Tesla	AC		Tesla (excluding EU)	3 power pins (DC+, DC, and E) and 2 control pins (CP and PP).	1-Ø 240 V, ≤72 A, 17.2 kW	IEC 62196
	DC			2 control signals (CP, PP) and 3 power pins (L1, N, and E).	400 V, ≤650 A, 250 kW

5. EV charging techniques

Lithium-ion batteries are highly suitable for powering EVs due to their superior power density, minimal memory effect, and reduced capacity loss compared to other energy storage devices [108]. The charging process for a lithium-ion battery is particularly delicate since it impacts the rate at which electrochemical side reactions occur within the battery and, hence, the battery's cycle life [109]. The comparison of the different methods for charging and discharging batteries is presented in Table 13. The Li-ion battery charging methods are described in the following sub-sections.

Table 13.

Comparison of various battery charging and discharge systems.

Type of charging	Facilities	Charge and discharge capability	Price, Size, and Capacity	Charge/Discharge Effectiveness	Problems in the Real World
on-BC	Low	Moderate	High	High	Low Price, Size, and Capacity
off-BC	High	High	Low	High	High hazard
Wireless	Low	High	Low	Moderate	Generating flux.
Integrated on-BC (or) off-BC	High	Low	High	Low	Reduced power transfer ability

5.1. Trickle-Constant Current-Constant Voltage technique

It has four steps. The trickle charge is the first step; it only starts when the battery is completely depleted, meaning that the voltage is below 3.0V and the current steadily lowers until the voltage reaches 4.2V. When the current flow reaches a specific level, the end of the charging stage is initiated. In this stage, the charging gets done slowly, extending the charging duration while reducing the battery's cycle life [108]. The charging process starts with a steady current until a particular voltage level (the cut-off voltage) is reached. The typical cut-off voltage for Li-ion using the conventional carbon materials of cobalt, nickel, manganese, and aluminium is 4.20 V/cell. 50 mV per cell is the tolerance. Battery charging continues at a constant voltage, the same as the cut-off value. When the battery's current drops to approximately 3–5 % of its original rating, it is considered fully charged. However, it is not recommended to use trickle or float charging methods on a fully charged Li-ion battery. When the voltage falls below a predetermined level, a topping charge can be used instead of a trickle charge [29]. The constant-current, constant-voltage mode of charging is shown in Fig. 36.

Fig. 36.

Constant Current-Constant Voltage method.

5.2. Method of Charging in Five Stages

The five-step charging strategy is a multi-stage (five-stage) constant-current charging method that divides the charging time into five steps. The charging current is adjusted to a fixed threshold value at each stage. The battery's voltage rises as it charges. When it reaches the limit voltage that has been predetermined, the stage number increases and a new recharging current set value is effectively implemented. The procedure will be repeated until stage number five is reached, [110]. The five stages of the charging procedure are depicted in Fig. 37.

Fig. 37.

Method of charging in five stages.

5.3. Technique of charging using pulses

This charging method injects the charging current into the battery in pulses, allowing the ions to spread and neutralize during the recovery time. The average current regulates the charging rate, which you can change by changing the pulse width. Using this method, charging times can be reduced, the effects of polarization can be diminished, and cycle lifetimes can be made longer. Duty-fixed and duty-varied pulse-charge strategies are two distinct pulse-charging techniques. Compared to the traditional duty-fixed method, the duty-variable strategy may improve charging efficiency and speed [111]. AC impedance is suggested to find the best pulse charging frequency based on the lowest AC impedance frequency [110].

5.4. Technique of boost charging

With the traditional charging method, such as CC-CV systems, charging periods are prolonged because of protective conditions and cycle life requirements. “Boost charging” is a way to charge Li-ion batteries quickly. Almost empty batteries can be charged quickly with a high current. Boost charging means that almost entirely dead batteries can be rapidly charged at very high currents without any harm [53]. The boost charging method is the best way to get a completely dead battery to 1/3rd of its total capacity in 5 min without damaging it.

6. Charging station topologies

The primary function of the CS is to deliver a high level of power to the battery while offering a sufficient amount of charging voltage and current, which could vary widely. The batteries are charged using DC electricity, while the grid provides most AC power. The structure of the CS for EVs is shown in Fig. 38.

Fig. 38.

Layout of the charging station.

6.1. Charging station based on configuration

An FCS with uni-directional DC takes a three-phase AC input and converts it to DC. The DC output is then regulated using a DC-DC converter to a level best suited for charging the EV battery. It is possible to remotely replace the power converter modules in a bi-directional DC fast CS. The power flows between a quick CS and the power grid can be controlled in both directions using the control topology [112]. An EVCS that provides electricity to EVs is commonly called a CS, also known as a charge point or electric vehicle supply equipment (EVSE).

6.1.1. Back-to-back AC-DC-DC charging station

The front-end AC-DC converter, a harmonic filter, and a DC-link that supplies power to DC-DC converters linked to it constitute this topology. It is connected to the network via a distribution transformer. Fig. 39 provides a schematic representation of a back-to-back AC-DC-DC CS. With the utilization of bidirectional converters, it becomes effortless to establish V2G and S2G (site-to-grid) capabilities. In addition to the DC chargers, back-to-back topology can also use other subsystems, such as ESSs and RESs. V2G and S2G make the topology feasible for employing battery energy storage and RESs to minimize the fluctuation of the energy supply from renewable sources and mitigate the possible negative impacts of quick charging on the distribution grid. The control technique does not require interaction between the front-end converter and specific EV chargers [113]. Similarly, fluctuations in both AC and DC voltage levels on the DC link and grid voltage are used to determine how much and in which way power is being exchanged.

Fig. 39.

Charging station with back-to-back AC-DC-DC topology.

6.1.1.1. Bi-directional back-to-back AC-DC-DC charging Station

The CS comprises a two-way front-end rectifier, a DC-link capacitor, and two-way EV chargers that can work with V2G.In Ref. [112], a control system is set up for a DC quick charging system that lets real and reactive power transfers happen in both directions. As demonstrated in Fig. 40, a CS with a bi-directional AC-DC-DC topology is a comparatively simple and compact design. The bi-directional DC fast CS's control topology can be applied to any AC-DC or DC-DC converter. The DC-link voltage at the standard DC bus is kept constant by regulating the reactive power injection from the universal AC-DC converter for voltage stability and power factor correction. At the same time, DC-DC converters are managed to enable the charging and discharging of the EVs.

Fig. 40.

Bi-directional charging station topology.

A level-3 DC CSs dual active bridge (DAB) converter architecture is described [114]. Three-phase DAB consists of converters that operate in the conduction mode. The main side bridge is treated as a primary bridge when the DAB is in operation, and the secondary side bridge is considered a secondary bridge. The bridges are named according to the mode of operation of the isolating transformer. The primary bridge functions as a DC-AC converter in all modes of operation, supplying stepped AC output to the secondary bridge through an isolated transformer. For DC output, the secondary bridge serves as a rectifier. The design and control of a novel bi-directional AC-DC rectifier for use in 1-Ø EV charger applications are shown in Ref. [115]. The above control illustrates a seamless active and reactive power transition. The dimensions of an input filter are also decreased since the harmonics are moved to double the switching frequency.

6.1.1.2. Unidirectional back-to-back AC-DC-DC charging station

Battery storage integration is possible by using batteries in the DC-link of buck-boost converters rather than capacitors. The uni-directional architecture only works in one direction, and there are no grid support features because the frontend converter was made to work as a diode rectifier to save money. Fig. 41 illustrates the overall structure of an uni-directional CS. A topology-based comparison of available CSs is depicted in Table 14 [113].

Fig. 41.

Uni-directional Charging station topology.

Table 14.

Topological comparisons of different charge stations.

Type of CS Topology	Grid Compatibility	Bi-directional	Incorporation of BES	Incorporation of RES	Consistency		Intensity of power
					Total Components used	Intricacy of Control
AC-DC-DC bidirectional	ALL	YES	YES	YES	LESS	LESS	NORMAL
AC-DC-DC unidirectional	APF	NO	YES	NO	LESS	LESS	NORMAL
Centralized AC-linked multi-port CS	V2G	YES	NO	NO	MORE	LESS	LESS
Transformer less CS with DC link	ALL	YES	YES	YES	MORE	MORE	MORE
Transformer less CS with DC link	ALL	YES	YES	YES	MORE	MORE	MORE

6.1.2. Centralized AC-linked multi-port charging stations

A centralized DC-DC converter that combines the ideas of a multiport converter and a partial power processing converter is addressed in Ref. [116]. The above can be done to keep the number of conversion steps minimum and galvanic isolation between converter ports. For quick CSs with PV, hybrid ESS, and micro-grid systems, a multiport DC-DC converter with a modified nonlinear power-driven DC-DC converter stabilizes power fluctuations. To ensure continuous bi-directional power supply from the utility grid to EV CSs, a space vector pulse width modulation system with limited space has been developed for the multiport DC-DC converter, as mentioned in Ref. [117].

The multiport converters may control the bulk of real power from the micro-grid, hybrid ESS, and PV. The electrical grid's reliability is increased through photovoltaic (PV) generation, hybrid ESSs, and EV charging. For FCS for EVs, a new type of DC-DC converter called a ring-connected dual active bridge configuration has been suggested [111]. A new closed-loop control has been made to find the best way to run each DAB so that the total root mean square current is as low as possible. In the event that any DC ports are left unconnected and in an inactive state, bypass switches are provided to each DAB to avoid any redundant power processing stages. Fig. 42 gives a depiction of the structure of a multiport CS.

Fig. 42.

Multi-port CS topology with centralized AC-Link.

A medium-frequency AC-link transformer isolates the source from the batteries of an EV. One modular multi-level converter leg is placed on the primary side of the ideal transformer, and two modular multi-level converter legs are located on the secondary side, creating a modular design with two conversion stages (DC-AC-DC). In this system, the flow of power to EVs connected through MMC is controlled by a vector control scheme. A second-order generalized integrator-frequency locked loop with a pre-filter provides DC-offset, harmonic, and sub-harmonic rejection-capable reference frames.

6.1.3. Transformer less charging station

The transformer used in traditional partial power converters has been replaced with an impedance network in a new transformer-less partial power converter. The layout of a transformer-free CS is depicted in Fig. 43. Using a battery storage system as an alternative to capacitors is an added feature of transformer-less topology [118]. The architecture is primarily designed for quick charging of a single EV; however, it is possible to use several CSs simultaneously to create a multiple-slot CS.

Fig. 43.

Transformer less CS topology with partial power converter.

6.2. ENERGY-STORAGE based fast charging stations

By using energy storage systems, quick CSs can have less of an effect on the distribution of the MV grid. These systems can reduce peak power consumption and do other things for the network. Furthermore, ESS can enhance the voltage level if there is an excessive voltage drop along the lines; however, such a function requires a voltage control [119].The Mega Watt CSs flowchart for EV charging loads is shown in Fig. 44 [120].

Fig. 44.

Flowchart for estimating charging loads with Mega-Watt Charging Stations.

6.2.1. Incentive for energy-storage-based fast-charging stations

By retaining electricity during times of low demand and releasing it when EV CSs are in use, battery ESS can assist lower demand charges through peak shaving and can drastically lower EV charging costs, especially with DC fast charging facilities. The reasons for essential ESS based CSs are as follows.

6.2.1.1. Infrastructure improvements to the grid

An ESS is a better option for DCFCSs to eliminate the amount of power needed from the Grid. The ESS can be charged when electricity demand is high, and prices are low, and EVs could be charged with more electricity from the ESS without overloading the Grid at a fixed flat rate. In rural places with low grid capacity, building an ESS-based charging point may be cheaper and more straightforward than improving the grid infrastructure [121].

6.2.1.2. Running cost of DCFC stations

An ESS could lower the monthly fee associated with demand charges. It is feasible to deploy a battery ESS capable of meeting the entire energy demand required to charge three EVs at a CS in a row. The initial investment in a BESS consists of the battery and the cost of the power electronic converters. High-power DC-DC and AC-DC converters are needed for a DC-FC station lacking battery storage. A low-power AC-DC rectifier and a high-power DC-DC converter will do for a DCFC system with power storage because the AC connections will be rated for less power. As a result, the costs of the electric power converters for the two types of DCFCSs can be roughly compared, with the DCFC system, including power storage, having lower prices.

6.2.1.3. Grid integration of renewable energy sources

By storing the energy produced by sizable PV plants, an ESS can aid in lowering frequency and voltage instability as well as peak removal [122]. Hokkaido, Japan, has built one of the largest battery storage installations in the world, with a capacity of 60 MWh, so that photovoltaic energy can be added to the grid. In Southern California, a 32-MWh lithium-ion BESS is being developed to offer voltage level and frequency control close to a 4500-MW wind farm [123].

6.2.2. Topology of energy-storage-based fast-charging stations

Independent or rural hybrid PEV CSs are placed where the users can charge their EV that are far from the nearest utility substation. An EVCS's infrastructure can be classified into four primary groups: (1) CSs lacking ESS, (2) CSs having ESS, (3) CSs with REs, ESS, and grid connection, and (4) CSs using REs, ESS, but without grid supply [124]. Fig. 45 illustrates the fundamental structure of a PEV CS that integrates a hybrid vehicle's battery. In a stand-alone CS, there are different ways it can work, such as “generation to vehicle (G2V)”, “generation to storage (G 2 ESS)”, “storage to vehicle (ESS 2 V)”, “vehicle to storage (V2ESS)”, and “vehicle to vehicle (V2V)”. The entire above are referred to as independent or rural hybrid PEV CSs. Three main ESS layouts are used in independent hybrid PEV CSs. The various storage options available for EVCSs are.

Fig. 45.

Stand-alone charging station powered by Renewable Energy Sources.

6.2.2.1. Topology of One System Energy Storage

The one-system energy storage structure supplies electricity utilizing locally sourced renewable energy resources and a single ESS. The most popular and powerful option for storage is the lithium-ion battery [125], Most of the time, these systems are easier to design and cost less because they only use one type of storage. Fig. 46 depicts the topology of an individual ESS. The different storage options for EVCS are addressed in Table 15 [121]. Table 16 represents the comparison of different one-system energy storage topologies.

Fig. 46.

Topology of one system energy storage.

Table 15.

EV charging systems’ storage options.

Availability of Storage	Kind of storage
Mechanical	Compressed air energy storage, flywheel storage, and pumped hydropower generation storage.
Electrical	Powerful Capacitors and Superior Magnetic Storage
Electro Chemical	Chargeable Battery (Li-ion, NiCad, Lead Acid, NaS)
Chemical	Hydrogen storage, fuel cells, and renewables
Thermal	Molten Salt, Latent Heat, and Hot and Cold Water Storage

Table 16.

Comparison of different one-system energy storage topologies.

Ref.	The kind of storage	RES	Benefits	Drawbacks
[126]	Li-ion battery	Photovoltaic and wind	•Going to consider vehicle arrival and charge. •High stored energy density.	•Li-ion batteries have a high cost of capital.
[127]	Lead Acid battery	Photovoltaic and wind	•The low cost of capital to store.	•Low storage life and expensive replacement.
[128]	Lead Acid battery	Solar and bioenergy	•Reductions in CO2 emissions of 34.68 % when solar energy is combined with biogas and lead-acid batteries	•Lead-acid batteries harm the environment.
[129]	Fuel Cell	Photovoltaic and wind	•A new MPPT step size algorithm. •Storage life is everlasting.	•Costly hydrogen storing.
[130]	Fly wheel	Photovoltaic and wind	•Storage lifespan: 20 years. •Cost-effective single storage.	•The longer charging time. •Storage has lower energy density.

6.2.2.2. Topology of Hybrid system energy storage

Fig. 47 shows the hybrid-stage ESS-based CS. The advantage of hybrid storage system is that it can complement other local ESSs by using plug-in electric vehicles as a storage medium to store surplus power during off-peak hours and return it to the charging point during peak hours. A hybrid super capacitor- and battery-based storage system for standalone micro grids is depicted in Ref. [129]. Because super capacitors are high-power storage devices, but batteries have a large energy storage capacity, a hybrid system like this one is necessary. A two-layer hybrid ESS uses Li-ion and lead acid batteries for typical loads and super capacitor-based storage for unexpected power fluctuations [131]. A hybrid EV charging point could charge about 50 vehicles daily using a mix of Li-ion battery, hydrogen, and ammonia-based storage. The comparison of hybrid-system energy storage topologies is shown in Table 17.

Fig. 47.

Topology of hybrid-system energy storage.

Table 17.

Comparison of different hybrid-system energy storage topology.

Ref.	The kind of storage	RES	Benefits	Drawbacks
[131]	Super capacitor and Li-ion Battery	Photovoltaic	•Design of the economic system. •Storage of energy for a long time •As comparison to lead-acid batteries, Li-ion storage offers a higher energy storage density.	•SC merely regulates electricity. •Storage expands.
[132]	Ammonia, hydrogen, and Li-ion battery storage	Photovoltaic and wind	•50-vehicle remote station operation.	•Limited Li-ion battery charging cycles. •Storage's low energy density. •FC vehicle charging is ignored.
[133]	both hydrogen and fuel cells	Photovoltaic	•Hazard model requires solar, Fuel Cell, and hydrogen unpredictability.	•High-priced storage facility. •Fuel-poor nations can't afford diesel generator backups.
[134]	Fuel cell and batteries	Photovoltaic and wind	•Charging EVs and Fuel Cell Hybrid Buses independently.	•Hydrogen storage costs. •Vehicle State of Charge ignored.
[129]	both a fuel cell and a super capacitor	Photovoltaic and wind	•Fuel Cell-Super Capacitor economical combo.	•Super Capacitor discharge rate is high. •No design for automobile charging.

6.2.2.3. Topology of swappable system energy storage

In swappable energy storage system design, the readily available, fully charged battery from the CS replaces the depleted battery of the EV. The structure of a battery-swapping type of CS is mainly a charging point area that the EV can freely enter and leave. The system consists of a battery stack unit for receiving batteries, a replacement battery robot installed in the charging point area to perform battery substitution activities, and a data acknowledgement unit for collecting information about each EV that enters the charging point, such as the vehicle's make and model, battery size, and state of charge, as well as the date and time of delivery and charging.

A battery exchange van is suggested in Ref. [135]. The van's battery charge monitoring system is coupled with a location and communication system. According to the established priority, it can transport batteries from a renewable energy-powered rural CS to EVs everywhere. The topology for a swappable ESS has been shown in Fig. 48.

Fig. 48.

Topology of swappable system energy storage.

6.3. Architecture of energy-storage-based fast-charging stations

Each EV must be galvanically insulated from the DC bus because ground current leakage between numerous EVs connected to an identical DCFCS does not pose any safety problems. Hence, all configurations have a high-power isolated DC-DC converter between the EV and the DC bus.

6.3.1. Battery-ESS (BESS)-Based architectures

A typical DC-bus BESS DCFC architecture has to provide AC–DC conversion, PFC, voltage regulation, and battery-grid isolation [121]. In the event of damaged battery housing, the battery-grid isolation prevents ground problems. The necessity for isolation between the grid and the ESS is an inherent limitation not present in DCFC stations without storing energy, where isolation is only essential between the grid and the EV. Due to active management of the reactive power flow via pulse width modulation control, the dynamic front end can also provide PFC. A DC-DC converter that is isolated and has a fixed DC bus voltage is used in a DCFC design, as shown in Fig. 49.

Fig. 49.

DCFC design with Battery-ESS that provides isolation.

6.3.2. Flywheel-ESS (FESS)-Based architectures

An active front-end AC-DC converter allows bi-directional power transfer between the flywheel and the AC grid in an AC-input flywheel system. Where the FESS should be placed in the architecture will depend on the style of input connection employed by a commercial flywheel system. Table 18 compares the various topologies for swappable systems for energy storage.

Table 18.

Comparison of different swappable -system energy storage topologies.

Ref.	The kind of storage	RES	Benefits	Drawbacks
[136]	Li-ion battery	Photovoltaic	•Size of the generation and ESS was optimized. •The issue with the lengthy charge times is resolved.	•No particular method of energy management is used. •Only PV generation increases in price.
[137]	Lead-Acid battery	wind	•Inexpensive system. •Wind energy interruptions are resolved.	•Pollution from lead-acid batteries. •Short life in storage.
[138]	Li-ion battery and Pumped heat energy storage	Photovoltaic, wind and geo-thermal	•Micro grid battery swap station performance.	•Location Restriction.
[137]	Li-ion battery	Photovoltaic and wind	•Bi-level micro grid-battery swap station scheduling. •Maximizing revenue.	•Micro grid reactive power is ignored.
[137]	Li-ion battery	Photovoltaic and wind	•Micro grid energy management strategy. •Energy management includes micro-grid reactive power.	•BSS design for vehicles. •Only for the Tesla EVs.

6.3.3. Hydrogen-ESS (HESS)-Based architectures

The electrolyzer in a DCFC system with hydrogen storage is not required to be galvanically separated from the grid, but the fuel cell, as a generation source, must be isolated [139]. As fuel cells operate like current sources with a low voltage and a high current, they need a DC-DC converter at their output. Hence, architectures use fuel cell output DC-DC converters [18]. The FESS-based DCFCs with AC input and DC input are shown in Fig. 50 (a) and (b), respectively [140].

Fig. 50.

Fly wheel-ESS based DCFC with (a) AC-input (b) DC-input.

6.3.4. Flywheel-Battery Hybrid-ESS (FBHESS) based architectures

The HESS-equipped DCFC schematic is shown in Fig. 51 [18].Hybrid ESSs with flywheels and batteries must keep the grid and energy storage parts separate, just like DCFC designs with BESS and FESS. An independent DC-DC converter is used to offer isolation on the grid side. Also, a battery-connected DC-DC converter maintains a constant DC bus voltage while transferring energy to and from the battery. The fixed bus voltage also reduces the complexity of the DC-DC converter designs [140].

Fig. 51.

DCFC design with Hybrid-ESS.

6.4. SST based ultra fast charging stations (UFCS)

A UFCS infrastructure that can quickly recharge EV batteries is crucial for widespread adoption and achieving the Net Zero Emission (NZE) target. The design and deployment of a high-power, high-voltage UF charger is costly and challenging, especially for MV connections. The cost of building a UF charger depends on factors such as location, electrical equipment repair, foundation, and conduits from the source to the service transformer and UF charger [5].The details of advanced DC fast chargers are shown in Table 19 [97].

Table 19.

Information about advanced DC fast chargers.

Production company	Design	Supply voltage(V) (AC)	electrical output(V) (DC)	Produced current(A)	Power(kW)	Have to add 200 miles (Minutes)	Efficiency (%)
ABB	Terra 53	480	200–500	120	50	72	94
			50–500
Tritium	Veefil-RT	380–480	200–500	125	50	72	>92
		600–900	50–500
PHIHONG	Integrated	380 ± 15 %	200–750	240	120	30	93.5
		480 ± 15 %
Tesla	Super charger	380–480	50–410	330	135	27	91
EVTEC ABB	Espresso Terra HP	400 ± 10 %	170 + 500	300	150	24	93
			150–920	375	350	10	95

With an estimated use of 0.3 kWh/mile, a 7.2 kW level-2 on-BC takes just over 500 min to add 200 miles to an EV's range. A 50 kW DC quick charger and a 135 kW T charger can complete the same activity in 75 and 27 min, respectively. In addition, the recently developed 350 kW Ultra-Fast charger cuts the filling time to just 10 min, making it much more competitive with the refueling period of fossil fuel vehicles [5].

6.4.1. Architecture of UFCS

A solid-state transformer or a medium voltage (MV)/low voltage (LV) low-frequency transformer (LFT) can step down the voltage, coming to the level that a UF charger can use when connecting a modern UFCS to a 33 MV grid. Fig. 52 depicts a DC-input flywheel hybrid architecture in which the battery voltage can influence the DC bus voltage.

Fig. 52.

DCFC design with Flywheel-Battery Hybrid-ESS.

6.4.1.1. Standard AC Bus design in UFCS

The conventional AC bus in a UFCS is connected to the MV grid via a step-down transformer in AC bus architecture. In order to ensure that an EV's battery is charged with a consistent DC voltage, each UF charger is connected to an AC bus with its own dedicated AC-DC and DC-DC converters. Also, DC-DC and DC-AC converters connect the RES and ESS on-site to the AC bus [141]. The AC bus design offers sophisticated AC-DC and DC-AC conversion technologies, well-established AC distribution system standards and processes, standardized AC switchgear, and protective relays [142]. Fig. 53 shows the LFT-based UFCS with standard AC and DC bus designs.

Fig. 53.

LFT based UFCS (a) with a conventional AC bus design (b) with a conventional DC bus design.

6.4.1.2. Standard DC Bus design in UFCS

It combines several UF chargers, RES, and ESS to a DC bus via DC-DC converters using a pivotal DC-DC converter after LFT. Unlike AC bus design, this method eliminates the AC-DC converters needed for each load and source, improving system efficiency and cost. The DC bus voltage should be less than 1 kV to reduce insulator expense and support modern battery voltage ranges. DC bus design simplifies grid-connected and islanded operation modes with a single coupling to the MV grid. Additional benefits include being simple to regulate and having no reactive power [142].

6.4.1.2.1. Design of a DC bus based on medium/high-frequency transformer

In order to communicate with the MV power network and provide IEEE 1547-compliant isolation, medium/HFT inside an isolated DC-DC following front AC-DC converters may substitute the heavy LFT. Fig. 54 includes a DC-DC isolated converter and an active front-end (AFE) that uses pulse width modulation to operate the active switches to provide PFC with AC-DC conversion [143].

Fig. 54.

Design of a DC bus based on Medium/HFT with bidirectional AFE and isolating stages.

6.5. RES based charging stations

An EV battery-swapping station employing wind energy and eight lead-acid batteries is created. These batteries work as backup during wind oscillations and can be swapped for approaching cars [144].

The significance of the RES based CSs is given in the following.

• •Connecting RESs to EVs can reduce emissions and boost resource efficiency through energy storage.
• •The development of a DC micro-grid driven by non-polluting energy sources that is capable of efficiently and effectively balancing power to satisfy load demand and charging EVs.
• •Utilizing fuzzy logic control and sparrow search algorithms (SSA) to optimize micro-grid regulation settings for different kinds of environments.
• •Using a hybrid SSA and fuzzy controller, power management on the DC bus may be accomplished despite variations in irradiance and load uncertainty.

6.6. Role of different CSs in existing power distribution network

From the above CS topologies, the role of different CSs in existing power distribution system network is discussed as.

• •ESS based CS:

A DCFCS should use an ESS to minimize or eliminate grid power. When power rates are low, and demand is high, the ESS will be charged. At a predetermined flat rate, the ESS can charge more electricity for EVs without overtaxing the system. Establishing an ESS-based CS might be more accessible and less expensive than upgrading the grid infrastructure in rural areas with limited grid capacity [121].

• •SST based UFCS:

By emulating the refueling network of an EV powered by conventional-based combustion, a UFC infrastructure can shorten the charge time to just 5–10 min [145]. SST, a novel technology, can impact smart grids, UFCSs, RES power systems, and more. SST improves power quality, frequency and voltage support, fault current regulating and isolating, energy management, and total harmonic distortion faster than LFT tap-changing [5].

• •RES based CS:

Since EV demand continues to rise, more CSs are necessary to meet energy needs, making RES essential for sustainable goals. Smart charging using RES is required to utilize EVs as green transportation. Several advancements have enabled sustainable green EV charging technologies, reducing operational costs through daytime solar irradiation or night time wind energy. EV charging technology now only uses sun, wind, and hydropower, despite the availability of many RESs. Future studies should explore using variable RESs such as geothermal, marine, and tidal energy. Integrating geo-location-specific RES, such as tidal and wind energy in coastal areas, could assist EV charging [146].

7. TENDENCES for the future EV charging stations

The charging infrastructure for EVs is progressing past range anxiety to address other issues and improve the EV driving experience. Increasingly, people who purchase EVs demand various charging infrastructures to be compatible with one another. To better protect against power outages in the case of an emergency, communities should investigate V2G and vehicle-to-home (V2H) applications in anticipation of the increasing number of EVs on the road.

Future developments in EV charging include.

• •Establishing off-BC that can operate at high voltages (up to 1000 V DC) to shorten charging times, lower line currents, and, ultimately, lessen thermal stress on distribution system components.
• •Coordinated EV charging and V2G/V2V adoption can reduce grid congestion and EV owners' charging bills.
• •The company is currently working on designing wireless power transfer (WPT) chargers that prioritize efficiency as a critical feature.
• •Wide-band gap semiconductors offer a higher power density, improved efficiency, and reduced thermal stress than traditional semiconductors.
• •Hydrogen is gaining popularity as a means of storing renewable energy and is increasingly used in transportation.

7.1. Ultra-fast charging stations with Solid State Transformers (SST) base

Like an LFT, an SST conducts the MV/LV transformation via AC-DC, DC-DC, and DC-AC conversion stages, but it is more flexible and simpler to operate. SSTs are called single-stage, two-stage, or three-stage topologies based on how many stages of conversion they have. Three-stage SSTs specifically provide a DC bus to connect EVs, ESSs, and RESs, unlike single- and double-stage SSTs. It is one of the best types of topologies for UFCS applications. Its two DC links allow it to support multiple topologies. However, modularity and distributed control requirements increase control complexity and require whole-system synchronization. A 3-Ø SST takes up 80 % less area than an LFT.

As a new technology, SST can affect smart grids, UFCSs, power systems with RES, and more. SST provides frequency and voltage support services faster than LFT tap-changing, power quality enhancement, fault current limiting and isolating, power flow control, and superior total harmonic distortion performance [5]. [139] describes a front-end (FE) AC-DC stage and a regulation stage DHB DC-DC stage. An ESS is incorporated into the UFCS using a non-isolated boost-type converter between the rectifying and regulation stages. The above mentioned approach has accomplished bi-directional power flow in contrast to previously described topologies. However, using more active switches leads to the UFCS underutilization and reduced efficiency. The control circuitry is more complicated with bi-directional power flow. In Ref. [120], a UF charger with a 6-Ø interleaved boost converter is linked to an 8 kV MV line, and eight similar sub modules (SM) were serially connected to split the input voltage uniformly. Each SM's rectification stage consists of an unregulated diode rectifier, and two paralleled unidirectional 3-level boost converters as the PFC stage. The DC-DC regulation stage comprises two HB LLC conversions with parallel and series input and output connections. In order to achieve maximum efficiency, the LLC converters have been designed to operate in an open-loop configuration with a duty cycle of 100 %. The SST topologies with single, two and three stages consisting of LVDC and HVDC links are shown in Fig. 55 [5]. The SST topology comparison for different functions is described in Table 20 [5].

Fig. 55.

Topologies of Solid State Transformer (a) 1-Stage topology (b) 2-Stage topology with LVDC DC-link (c) 2-Stage topology with MVDC DC-link (d) 3-Stage topology with LVDC and MVDC link.

Table 20.

SST topology functional comparison.

S.No.	Function	1-Stage	2-Stage	3-Stage
1.	Power flow in both directions	Yes	Yes	Yes
2.	Input Current Limit	No	Yes	Yes
3.	Output Current Limit	No	Yes	Yes
4.	Reactive power grid support	No	Yes	Yes
5.	Power Factor Independent	No	Yes	Yes
6.	Frequency Independent	No	Yes	Yes
9.	Modularity	Easy	Difficulty	Easy

7.2. EV battery with high voltage

An EV equipped with a high-voltage DC bus typically has powertrain wiring that is comparatively smaller, resulting in a reduction in weight. The primary advantage of a high-voltage battery pack is the ability to provide high charging energy without exceeding the current limits of the connector. However, high-voltage EV batteries do come with their own set of challenges, which are discussed below.

• 1)The additional cost associated with the use of power electronic switches with a higher voltage rating.
• 2)Increased requirements for voltage insulation throughout the powertrain.
• 3)More battery cell balancing is needed to ensure that all cells connected in series have the same amount of charge.
• 4)On-board voltage conversions increase losses.

7.3. DCFC's use beyond automotive

Due to efficiency and underground air quality advantages over diesel mining trucks, electrification is accelerating. The haul trucks and other heavy-duty mining trucks use high-voltage batteries to get the power they need to haul ore uphill. These trucks are essential to mine productivity, so they must be charged quickly [111,147]. NASA is in the process of developing an electric aircraft propulsion system. Large onboard batteries would necessitate a high charging current for these aircraft. Charge-Point and Uber Elevate propose a 2-MW electric aircraft charger [148].

7.4. Rechargeable storage batteries

Rechargeable EV batteries reduce the initial investment. Such second-life battery packs with 60%–80 % storage capacity seem best suited for stationary BESSs because their performance requirements are lower than those of EV batteries [118]. The modular multiport converter modes for grid applications with rechargeable batteries from various manufacturers are discussed [47]. Second-life battery replacements must be considered when calculating the system's cost.

8. Conclusion and future SCOPE

The article provides a thorough analysis of battery charger technologies include their operations, advantages and disadvantages. Various types of EV battery chargers are described in the literature with their advantages, disadvantages and configurations. Different charging power levels, connectors, and standards are detailed. The review also covers numerous charging techniques and CS topologies based on configuration, ESSs, REs and UFCSs.

The tendencies for the future EVCS are explained.

• •Implementing V2G and V2V technologies is crucial for properly utilizing decentralized ESS.
• •Wide band gap devices improve on-BC and off-BC size, weight, and volume. Furthermore, they boost energy levels, which reduce charging time.
• •The main advantage of using an off-BC is its ability to charge the battery rapidly. However, it requires a robust infrastructure to function correctly. It's crucial to have a quick and up-to-date EV charging infrastructure that relies on sustainable energy and reduces the burden on the grid. Off-BCs are high-power chargers that can recharge an EV's battery to 80 % capacity in just a few minutes. However, overusing off-BC technology can cause damage to the utility system and result in PQ disturbances. One method of resolving the above problem involves integrating energy storage and distributed energy resources at the level of the CSs.
• •Inductive charging is a technology that can lead to a reduction in battery size and an improvement in the efficiency of the vehicle.
• •By incorporating innovative techniques such as V2G, V2V, and UFCS for EV charging, the power system can be stabilized, and RESs can be optimally utilized. Additionally, incorporating energy sources like V2G, V2H, and solar PV-based charging can potentially decrease future storage requirements and costs for the power grid.
• •In order to make sure that semiconductor soft-switching is efficient, it is essential to implement novel modulation techniques that involve the integration of machine learning.
• •Innovations in power conversion topologies, standards, control schemes, and advancements in wide band gap (WBG) power devices, digital controllers, and magnetic materials allow fast charging stations to resemble traditional gas stations in refueling speed and convenience.
• •Advanced energy management technologies can enable the automation of the electrical grid, driven by the Energy Internet-based grid technology of the future.

Materials and data availability

The data provided in this study can be obtained from the corresponding author upon request.

Ethical approval

Not applicable.

CRediT authorship contribution statement

V.N. Saraswathi: Conceptualization, Methodology, Writing – original draft, Investigation. Vijaya Priya Ramachandran: Software, Validation, Formal analysis, Data curation, Writing – review & editing, Visualization.

Declaration of competing interest

The authors declare no conflict of interest.