## 1 Introduction  
Batteries are the most common electrical energy storage device in vehicles as a replacement of traditional fuel (petrol, gasoline) [1; 2; 3; 4]. The capacity of a battery is gradually reduced due to erosion, passivation, outgassing, temperature, decomposition of materials, and changes on the electrode surface during its operation [5; 6]. Also, the battery weakening process can be increased when the battery is subjected to operate beyond its specified safe operating conditions. The battery state is also used to estimate the expected lifetime of the battery and can simply be described by two parameters: state of charge (SOC) and state of health (SOH). The SOC mostly depends on current in and out and its initial charge condition of a battery at a given temperature [7]. The SOH is a measurement that reflects the general condition of a battery and its ability to deliver the specified performance compared to a fresh battery. In smaller vehicles, monitoring the voltage and current of each cell reduces the reliability because of the increased amount of circuitry. Also, these circuits require some power to operate, therefore, they induce a self-discharge. The battery monitoring may have inputs as the total voltage, current, and temperature, but requires a reference point to know that all cells are full. For the battery to have a long life, it requires an appropriate battery management system (BMS) to equalize the cells in a battery string. Several cell-balancing strategies have been presented in the literature [8; 9; 10; 11; 12; 13; 14; 15]. Slight differences between the series-connected cells in a lithium-ion (Li-ion) battery pack can produce imbalances in cell voltages, and this effect causes great reductions in the charge capacity [16; 17]. There are three main types of battery management systems that are commonly found in commercial and industrial lithium battery-based systems, such as the shunt resistor equalizing method, capacitor equalizing method, and inductor/transformer equalizing method. The aim is to retain a low bill of material (BOM) and still achieving a fast equalization. In lithium types, the voltage is mainly dependent on the charging state when the battery is either full or empty. The easiest way is to equalize when at the almost-full state. There are multiple balancing systems, which can roughly be subdivided into the following two categories: The active cell balancing method and passive cell balancing method. The shunting resistor cell balancing method is a passive method that shunts the charging current around each cell as they become fully charged. It removes excess energy from the higher voltage cells by bypassing the current of the cell, and waits until it is the same level as low voltage cells. This can be categorized into two subcategories: fixed or switched resistor balancing methods [18; 19; 20; 21; 22]. The first method uses continuous bypassing of the current for all cells. It can be only used for lead-acid and nickel batteries because they have the capability to be in overcharge conditions without damage. The second method is a controlled shunting resistor. This method removes energy from high cells in a controlled manner by using switches. When the unbalancing happens, it will be detected by a sensor that is measuring the voltage across cells that decides what resistor should be shunted. The disadvantages of this method are the requirement for a large-power dissipating resistor, high-current switches, and thermal management requirements. A switched resistor cell balancing is shown in Figure 1a. The capacitive shunting balancing method is an active cell balancing method. An active cell balancing employs an active charge shunting element to remove energy from one cell to another cell. The capacitor shunting balancing method can be implemented in the following three categories: switched capacitor, single switched capacitor, and double-tiered switched capacitor [10; 23; 24; 25]. The switched capacitor has two conditions, charging and discharging.