## 5 Experimental Verification
Figure 9 shows the hardware waveforms of the developed prototype operating in buck mode with V\({}_{\text{pack}}\) as 15 V and balancing current I\({}_{1}\) as 1 A and is aimed to verify the veracity of the system, where v\({}_{\text{gs1}}\) is the gate signal voltage of MOSFET Q\({}_{1}\), v\({}_{\text{gs2}}\) is the gate signal voltage of MOSFET Q\({}_{2}\), v\({}_{\text{ds1}}\) is the voltage across the MOSFET Q\({}_{1}\) from drain to source, v\({}_{\text{ds2}}\) is the voltage across the MOSFET Q\({}_{2}\) from drain to source, i\({}_{\text{L}}\) is the current passing through the L\({}_{\text{m}}\), and i\({}_{\text{D}}\) is the current flowing through the MOSFET Q2, and i\({}_{1}\) is the current at low side output and regards as the balancing current. Then, the prediction and actual measurement results are processed by the mean absolute error (MAE). The formula is shown in Equation (18), where \(n\) is the number of comparisons, \(\eta_{\text{rea\_i}}\) is the measured efficiency, and \(\eta_{\text{test\_i}}\) is the estimated efficiency. In Figure 10, it can be observed that the maximum error is 5.06%, the minimum error is 0.02%, and the average absolute error is 1.48%. \[MAE=\frac{1}{n}\sum_{i=1}^{n}\left|\left(\eta_{\text{est\_i}}-\eta_{\text{rea \_i}}\right)\right| \tag{18}\]  
Figure 8: Photos of the proposed cell balancing system based on a bidirectional flyback converter (a) laboratory setup (b) battery management system. Figure 9: Hardware waveforms of the developed prototype (a) high side (b) low side. The efficiency distribution of the proposed can be obtained in Figure 9. In the literature, there is hardly any discussion about the relationship between balancing current and operation efficiency. Therefore, this research still collects the efficiency of the system mentioned in Section 1. The comparison is shown in Table 3, although they are all operated in different situations. The capacitor shuttle system like [PERSON]'s design will have the highest efficiency, the system proposed by [PERSON] has a lower efficiency in his test condition, and the individual charging cell balancing systems in all the research mentioned own the system efficiency from 70% to 90%. Therefore, the purpose of the power loss analysis in this research is highly necessary to achieve higher efficiency in the system. Another important purpose of this research is to improve the performance of the proposed cell balancing system by applying the control strategy. To verify the effect of the control strategy, the experiments will be conducted under the conditions shown in Table 4. During the testing, \(V_{cr}\) will be set as 50 mV, \(I_{1}\) as 1 A, and \(T_{1}\) as 30 s. To compare among the pack-to-cell, the cell-to-pack, and the bidirectional architecture, the proposed cell balancing system will operate in three diverse control strategies. First of all, the system will complete the balancing process the same as the pack-to-cell architecture by only carrying out the pack-to-cell procedure. Secondly, the proposed system will merely operate with the cell-to-pack program to reach battery module equilibrium in a similar way as the cell-to-pack architecture. The last one, implemented in this research, is that the system combines the cell-to-pack and the pack-to-cell functions, including both of the advantages, to get the battery module balanced. The experimental results will be shown in Figures 11-13, where Figures 11a, 12a and 13a are the actual hardware measurements of each cell's voltage by the differential voltage circuit shown \begin{table}
\begin{tabular}{c c} \hline \hline
**System Designer** & **Efficiency (\%)** \\ \hline [PERSON] [6] & 60–98\% \\ [PERSON] [7] & 71.5–74.5\% \\ [PERSON] [8] & 70–90\% \\ [PERSON] [9] & 83–90\% \\ [PERSON] [10] & 95–97\% \\ [PERSON] [11] & 90\% \\ [PERSON] [12] & 80\% \\ This work & 54.5–83.1\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Cell voltage of the unbalanced 6 SIP battery module for the testing condition.