## 5 Experimental Verification  
As shown in Figure 8, the photo of the implemented cell balancing system is used to verify the loss model proposed in this study through the test results. This research measures the efficiency of the bidirectional flyback converter by adjusting the operating current. The type of cell used in this study is the lithium ternary cell UR18650 NSX produced by Panasonic, with a rated capacity of 2500 mAh and a voltage range of 2.5 V-4.2 V. Through the test of the 6S1P battery module, which is composed of the series connection of these kinds of cells, the voltage V\({}_{\mathrm{pack}}\) range at high side is 15 V-25.2 V, and the voltage V\({}_{\mathrm{Bn}}\) range at low side is 2.5 V-4.2 V. In the experiment, the efficiency test is carried out under the condition that the voltage V\({}_{\mathrm{pack}}\) is 20.9V and the voltage V\({}_{\mathrm{Bn}}\) is 4.2 V at the low side. The balance current I\({}_{1}\) changes from 0.5 A to 2.0 A to observe the variety of efficiency. 0 shows the parameters and specifications of the model, where the model of MOSFET Q\({}_{1}\) and Q\({}_{2}\) is CSD18510 KCS manufactured by TI company. It is worth mentioning that the parameter R\({}_{\mathrm{ds(on)}}\) of MOSFET will change in different situations. In this research, the temperature of MOSFETs does not change much because the converter's power is just about 8.4. W and the losses caused by MOSFETs is only about 0.05 W which will not increase the temperature of MOSFETs during operation. According to the datasheet of CSD18510 KCS, this research estimates R\({}_{\mathrm{ds(on)}}\) as 1.4 m\(\Omega\) based on MOSFETs' temperature as 25 \({}^{\circ}\)C and v\({}_{\mathrm{gs}}\) as 15 V. The balancing system is implemented by utilizing TLP250H as gate drivers, LM358 as operational amplifier to achieve the voltage sensing, and LT318 as photoMOS relays. The loss analysis formula is shown in Equations (7)-(13). The results are shown in Figure 9 and compared with the hardware measurement which using the DC Power (62100H-600S) and DC Load (63204A-600-280) manufactured by Chroma Company to acquire the system efficiency in the different operating currents. It can be obtained that the balance current of the designed bidirectional flyback converter is proportional to the operation efficiency under the test conditions shown in Table 2. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** \\ \hline MOSFET \(Q_{1}\) parasitic capacitance & \(Coss\) & 2 nF \\ MOSFET \(Q_{1}\) internal resistance & \(R_{\mathrm{ds1(}m)}\) & 1.4 m\(\Omega\) \\ MOSFET \(Q_{1}\) rise time & \(t_{\mathrm{on}}\) & 1 ns \\ MOSFET \(Q_{1}\) fall time & \(t_{\mathrm{off}}\) & 29 ns \\ MOSFET \(Q_{2}\) internal resistance & \(R_{\mathrm{ds2(}2m)}\) & 1.4 m\(\Omega\) \\ Wire resistance & \(R_{s}\) & 135 m\(\Omega\) \\ Operation period & \(T_{\mathrm{sw}}\) & 10 us \\ Operation frequency & \(f_{\mathrm{sw}}\) & 100 kHz \\ Transformer’s turn ratio & N & 0.3333 \\ Primary field inductor & \(L_{m}\) & 117 uH \\ Transformer’s DC internal resistance & \(R_{L(d)}\) & 0.1 m\(\Omega\) \\ Transformer AC impedance factor & \(\lambda\) & 1.4 \\ Transformer’s volume & \(V_{e}\) & 15.305 cm\({}^{3}\) \\ Magnetic induction intensity variation & \(\triangle B\) & 0.225 T \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Electrical parameters in the balancing system.