## 2 Hardware Description of the Proposed Cell Balancing System  
In this research, the proposed system is developed for a cells connected in 6 series and 1 parrarel (651P) battery pack. The reason why the system is only for a small pack is that it can be series-connected to reach a higher voltages, such as 48 V or 400 V. The specification can be obtained in Section 4. Figure 1 shows the block diagram of the proposed cell balancing system, which consists of a bidirectional flyback converter, a microcontroller unit (MCU:dsPIC33 FJ64 GS606) manufactured by Microchip Technology in Chandler, Arizona, United States, a photoMOS array, an auxiliary power circuit, a differential voltage circuit, and driver circuits. In this research, the bidirectional flyback is the key part of the system, which can realize energy transfer in both directions, to achieve the balancing function with a simpler structure. The microcontroller unit (MCU) shown in Figure 1 not only provides the control signal for the photoMOS and bidirectional flyback converter but also receives all the voltage of cells by the differential voltage circuit, then applies the control strategy via the signal captured to complete the balancing process more efficiently. Its control strategy can be observed in Figure 2, where \(V_{Bn}\) is the voltage of each cell, \(\Delta V_{Bn}\) is the voltage difference between each cell and the average voltage of all the cells, \(V_{\text{er}}\) is the parameter of the minimum voltage error to activate the balancing process, and T\({}_{1}\) is the time duration for a single balancing process. The MCU will gather the voltages of the cells through a differential voltage circuit. By calculating the \(\Delta V_{\text{Bn}}\) of each cell, the system will decide which cell needs to be balanced. If the chosen cell has the highest voltage, the cell-to-pack solution will be carried out, and the balancing system will be operated in boost mode until the timer counts up to T\({}_{1}\). If the chosen cell has the lowest voltage, the pack-to-cell solution will be carried out, and the balancing system will be operated in buck mode until the timer counts up to T\({}_{1}\).  
The common individual charge battery balance architecture can only realize energy transfer in one direction. For example, the pack-to-cell architecture has the ability to effectively charge the cell which capacity is low, but it cannot efficiently transfer the energy of the cell which capacity is high to the pack; the cell-to-pack architecture is able to transfer the energy from the cell which has a higher capacity to the pack effectually, but it is unable to charge the low capacity cell with the pack very efficaciously. The implemented cell balancing system has the most important feature that the energy can be extracted not only from pack to cell but also from cell to pack. To achieve the bidirectional balancing behavior, the implemented system replaces the rectifier diode in the traditional flyback converter with synchronous rectification MOSFET and simultaneously reduces the losses caused by the output rectifier diode. Compared with the system using a multi-winding transformer or multiple flyback converters [26], it simplifies the structure by applying a bidirectional flyback converter with a switch array instead and reduces the cost further by replacing MOSFET with photoMOS in the switch array. Furthermore, the commonly used converter in balancing systems operates with a low constant current, while the proposed solution can implement an adjustable balancing current not only for various control strategies, but also for efficiency measurement of the system operating in different curens.  
Figure 1: Block diagram of active cell balancing system based on bidirectional flyback converter.  
Figure 2: Power loss analysis of the cell balancing system.