**3. Charging Systems of the Batteries of Electrical Vehicles**

It is very important to store as much of the energy produced as possible. V2G technology works with specially designed bidirectional charging stations that allow the electrical vehicle owners to charge their vehicles while discharging the vehicle battery. The electricity in the batteries of electrical vehicles is transferred to the grid to compensate for the supply–demand balance in the electricity grid [40].

The ability of the batteries to charge and discharge depends on many factors, such as the design of the batteries, their charge status, temperature, former cycle history, and use. Depending on the charging strategies and charger size of the electric vehicle batteries, the peak power demand of the grid can vary [41]. This multiple dependency makes the determination of the charge status of the battery and the charging methods complicated. The battery charging methods used in the literature are constant current charging, constant voltage charging, and constant current–constant voltage charging. The charging current in the constant current scheme is equal for all battery groups that are connected in a series. As the charge status increases in batteries, the voltage must be increased in a constant manner to continue charging at constant current as the internal resistance also increases [42]. However, the charging current to be selected is very important in this method. This is because a high value of charging current allows the battery to be charged in a short time; however, it also causes damage to the battery because of overcharging and overheating. Charging the batteries at low current increases the charging times. At constant voltage, the battery charge draws a high current at the initial stage from the source because of the low battery internal resistance. This high current is limited to avoid damage to the elements. In constant voltage charging, the charging is started at full current of the charger by applying voltages that cannot cause damage to battery elements. After reaching the voltage level, called the float voltage, the current gradually begins to decrease. The charging current decreases in time because of the increasing battery internal resistance that stems from the increase in the charge [43]. This allows the charge to be completed with the leakage current, and in this way, the possibility of overcharging the battery is reduced. Because of the reduction in the charging current, the charging time of the battery becomes longer. Constant current–constant voltage charging is applied in two steps. For the purpose of eliminating the negative conditions, such as the overcharging of the batteries and pulling the overvoltage from the batteries, the battery is charged with a constant current until it reaches the preset voltage level; then, charging is continued with the constant voltage level [44].

The discharge levels, temperatures, and charge method parameters of the batteries of electrical vehicles affect the battery life cycle. To protect the battery life cycle, it is necessary to have a charging topology with a high efficiency or to select proper charging topology befitting the characteristics of the battery. The most important characteristics of these charge topologies are to provide the proper voltage level according to the energy flow direction and bidirectional energy flow [45]. Also, the methods are standard for battery charging of electrical vehicles. There are three main charging methods, named Type-1, Type-2 and Type-3. These are classified on the basis of their usage and applications. Type 1 charging method is used for vehicles, which are usually parked in residences and workplaces for a long time because of its single-phase system. The battery charging time being long and slow does not cause overload to the existing grid. For this reason, overnight charging is carried out to benefit from cheap electricity. When the battery is full, it provides power up to 3.7 kW and a maximum current of 16 A in Type 1 charging mode [46]. Type 2 charging method is used in places where there is heavy density, such as hotels, markets, hospitals, universities, airports, and shopping centers. It provides medium-speed charging within 1–4 hours of periods. It has a three-phase AC grid, and provides power between 11 and 22 kW, and a maximum current of 32 A [46]. Type 3 charging method is also defined as the method of fast charging. The battery charging time varies between 15 and 30 minutes, and these stations offer the possibility of charging batteries within short times in areas such as short breaks where there is an urgent need for energy. Although it has both the AC and DC model, it causes too much load for the network because of its high current value. The technical specifications of the charging stations used to charge batteries of EVs are categorized according to the type of the charging stations. They provide powers up to 43 kW for AC, and the maximum values for DC are 500 V and 125 A. In Type 3 charging method, there are safety measures present, such as the verification of the cable connection, not giving voltage when the cable is not connected, checking the ground connection, and reporting the maximum current capacity of the charger [47,48].

Overcharging of the batteries causes disruptions in the chemical structure and shortens their usage of life cycle. Charging systems have to work together with the battery management system to avoid overcharging. In addition, energy management systems are needed to ensure that batteries can be used safely under normal operating conditions and even in the event of accidents. The basic functions of battery management systems are to provide protection for the cells, heat management, charge/discharge control, data collection, communication with modules, data storage, and cell balancing [49]. In the case of the battery of the vehicle wearing out, the battery becomes eligible for a second-time use. The batteries are known as 'second life batteries', of which their major source is EVs. Such batteries can be repurposed for use in residences, telecommunication towers, building loads, and in power and transmission support [50]. They are very much cost-effective for their use in residential areas for following loads and backing-up the systems, the same as in other commercial and industrial areas [51].

The general energy flow diagram of charge/discharge processes of V2G and V2H structures is given in Figure 6. Different AC/DC and DC/DC topologies are used to increase the efficiency and performance of this system. There is a need for the control system to manage the energy flow, and, for this reason, all these topologies are employed in this respect. The charging time of electrical vehicles is longer when compared with fuel filling time. Fast charging methods are employed to shorten this time. In this method, the energy flow-control is applied up to 80% of the battery, and voltage control is applied over 80% [52].

The Li-ion battery pack is the battery of the vehicle, which is connected to a charger module. The grid supplies power to other loads and, by means of a suitable transformer to alter the magnitude, to the charger module. The charger module has two converters, firstly an AC/DC converter to convert the AC grid power into DC required by the battery, and secondly a DC/DC converter to change the magnitude of the DC power as necessary. Both these converters are controlled by the mechanism of pulse width modulation (PWM). This system is bidirectional and can work in either V2G or G2V technologies [52]. Li-ion batteries used in electrical vehicles have approximately 5000 life cycles. One charging and one discharging of the battery constitutes a cycle [53]. In Figure 7, when the number of cycles increases, the energy holding capacity of the battery decreases. Reduction of capacity also means shortening of battery life. A battery management system is needed to increase the operating time [54].

**Figure 6.** Generalized energy flow diagram for V2G and V2H system [52]. PWM: pulse width modulation.

**Figure 7.** Li-ion battery usage curve: the energy storing capacity of the battery is found to drop with increasing number of charging/discharging cycles [55].

Determining the charging locations has fomented significant research interest among scientists and engineers. Optimum charging points are determined using various heuristic algorithms such as the genetic algorithm [56], non-linear auto regressive [57], flow refueling location model [58], maximum covering location problems [59], and agent-refueling multiple-size location problem [60]. This is usually important for V2B applications, as the buildings are densely located and the intermittency of loads are high. Queuing algorithms are efficient in such cases, wherein the charging stations need to balance the loads to minimize the charging time [61,62]. In such stations, the chargers are specified in three levels on the basis of their charging power and charging circuit [63]. Queuing models according to these levels helps to design the stochastic resource-sharing network to accommodate the convoluted distribution [64] and traffic [65,66] networks to make V2X more accessible.

Battery management systems (BMS) are important components in the provision of conditions such as safe operation, long-term reliability, and low cost of a battery. BMS increase battery life cycle and prevent damage to the battery, ensuring correct and reliable operation of the system [67,68].

The curve showing the relationship between temperature and battery output voltage is depicted in Figure 8. Accordingly, the battery temperature increases when the battery draws excess current. Therefore, the increased temperature also adversely affects the output voltage [69]. For this reason, the current is required to be limited to a certain value when the battery is in constant current mode. The limitation process is performed through BMS.

**Figure 8.** Relationship between temperature and battery output voltage: the output voltage reduces linearly with increasing battery temperature [69].

The BMS provide the balance of the voltage values of each cell that makes up the battery pack in order to maximize the capacity of the batteries and to prevent overcharging when charging [70]. In case of over voltage or under voltage in any cell, the system interferes in case of unbalanced voltage balances and the system enters the cutting [71]. When necessary, it provides balance by transferring the energy from the most filled cell to the least charged cell. In this way, BMS intervene in the system and prevent damages in the statement of system failure by interrupting. This is an extremely important system for the protection of high capacity and high cost battery packs [72].

BMS provide the protection of the system by interfering with the system when optimal values are exceeded, done by measuring the values presented to the user. BMS interfere with the high current which is drawn from batteries and interfere with the system during the high charge, the low voltages during discharge, the high temperature, the low temperature, and the leakage current formation. [73].
