###### Abstract
This equalizer he proposed is similar to capacitive shuttle cell balancing, which has energy recycling and the system cost is still low but it still needs an amount of time to complete the balancing process. [PERSON] presented an energy sharing state-of-charge (SOC) balancing control scheme based on a distributed BMS architecture where the dc bus voltage regulation system and the cell balancing system are combined into a single system [10]. Cells in the pack are separated by the converter, and the converters' output is connected in series so that the system should apply current-sharing control to maintain the stability of the direct current (DC) bus. [PERSON] achieved cell-balancing by a bidirectional flyback dc-dc converter [11]. The bidirectional flyback dc-dc converter proposed by the researcher is composed of two traditional flyback dc-dc converters that each one takes charge of one power direction. It will cost more than using a bidirectional flyback dc-dc converter instead which is one of the most important elements in the solution adopted in this work. [PERSON] raised a modularized design of active cell-to-cell/module-to-module charge equalizer [12]. Compared with the equalizer implemented by [PERSON] [6], the design has removed the capacitor of the LC circuit to further reduce the system cost. Nevertheless, the problem has to be solved that the design must deal with the inrush current when the two balancing batteries have a large voltage difference. There are more active cell balancing systems which have not been mentioned above. According to the balancing patterns, active cell balancing systems can be divided into three categories: capacitive shuttle cell balancing [13; 14; 15; 16], inductive shuttle cell balancing [17; 18; 19; 20; 21], and individual charging cell balancing [6; 9; 22; 23; 24; 25; 26]. The capacitor shuttle system has less loss, but it takes a longer time to complete the balancing process. The inductance shuttle system also has a low loss. Nevertheless, the cost is more than the capacitor shuttle method because of its complicated switch structure. The individual charging system works the most efficiently but at a greater expense. However, most of the capacitive shuttle system and the inductive shuttle system can only transfer energy to adjacent cells, which will increase the time required to reach equilibrium in the battery module and reduce the efficiency of energy transfer. In contrast, the individual charging system performs more effectively than the other. It can reduce the balance time and increase the efficiency of energy transfer. The individual charging system has the following forms: buck-boost converter, quasi-resonant buck-boost converter, and flyback converter. Each type of individual method benefits from its structure, such as balancing duration, balancing times, the complexity of architecture, or cost. Taking all the factors into consideration, this research implements an active cell balancing circuit based on a bidirectional flyback converter. The proposed system further improves the system by applying a control strategy which is able to reduce the balancing duration. Moreover, this research also proposes loss models to estimate the bidirectional flyback converter to discuss how to determine the proper balancing current of the proposed circuit to achieve the highest efficiency of the bidirectional flyback converter and reduce the losses in the balancing process.