###### Abstract  
This research proposes a power loss analysis and a control strategy of an active cell balancing system based on a bidirectional flyback converter. The system aims to achieve an energy storage application with cells connected in 6 series and 1 parrarel (6S1P) design. To reduce the structural complexity, Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) array commonly used in balancing system is replaced with the photovoltaic Metal-Oxide-Semiconductor (photoMOS) array. Power loss analysis is utilized for the system operating in the proper current to reach higher efficiency. The proposed loss models are divided into conduction loss, switching loss, and copper and core loss of the transformer. Besides, the models are used to estimate the loss of converter operating in different balance conditions to evaluate the system efficiency and verified by the implemented balancing circuit. By way of the loss models, the balancing current can be determined to reach higher efficiency of the proposed system. For further improvement of the balancing process, the system has also applied a control strategy to enhance the balancing performance that reduces 50% maximum voltage difference than traditional cell-to-pack architecture, and 47% balancing duration than traditional pack-to-cell architecture. power loss analysis; control strategy; active cell balancing; bidirectional flyback converter 202022 June 2020202 June 2020202 June 2020202 June 2020202 June 2020202 June 202020202 June 202020202 June 2  
cell in the battery module to ensure that the battery module works in a stable condition; the protection function provides over-voltage, under-voltage, over-current, over-temperature, and under-temperature prevention. Thus, appropriate protection ensures that the battery module operates safely and normally; the balancing function is mainly to improve the imbalance problem in the battery module due to the different characteristics of each cell. The balance circuit rearranges the energy among cells until the power of each cell reaches equilibrium to maximize the energy capacity and prolong the lifetime of the battery module. So far, many researchers have proposed various kinds of balance circuit architectures [4; 5; 6; 7; 8; 9; 10; 11; 12]. [PERSON] employed a passive balancing BMS that combines the power resistor and Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) internal resistance as a balancing resistor to save the space [4]. [PERSON] proposed a BMS architecture based on cell redundancy [5]. These two studies belong to passive balancing methods. Passive balancing methods have the least system cost but must take into account the heat issue. The balancing current is usually limited to less than 50 mA. If the current is set too large, the heat generated by the power consumed by the resistor may damage the battery module. On the other hand, if the current is low, the balancing time will significantly increase. [PERSON] introduced an inductance-capacitance (LC) series resonant circuit as an energy carrier, which transfers the balancing energy directly from the highest charged cell to the lowest charge cell [6]. However, the LC series resonant method needs to control the switches rapidly to achieve the balancing process. [PERSON] utilized megahertz multiple-receiver wireless power transfer to achieve battery cell equalization [7]. However, wireless power transfer has low efficiency and it is unable to control the balancing current for each cell. [PERSON] described the design and safety verification of a novel charge equalizer with a fully distributed architecture, which not only perfectly fits the typical partitioning of a battery into modules, but also achieves the system with a simple circuit structure [8]. Whereas this cell equalizer cannot reach the adjustable balancing current. [PERSON] also designed a high-efficiency digitally controlled charge equalizer for series-connected cells based on switching converter and super-capacitor [9].