## 2 Experimentation
### Test Items
#### 2.4.2 The Charge-Discharge Power Test  
We used the HPPC (hybrid pulse power characteristic) test to calculate the charge and discharge powers of Ni-MH battery modules with different SOC values--80%, 50%, and 20%. The detailed charge-discharge power test flow is shown in Figure 3.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Method** & **Maintenance** & **Storage** & **Charge** & **Rest** & **Charge–Discharge** & **Experiment Process** & **Number of** **Samples** \\
**Number** & **Period** & **SOC** & **Rate** & **Time** & **Mode** & & **Sampling** \\ \hline \hline \multirow{4}{*}{5} & \multirow{4}{*}{120 days} & \multirow{4}{*}{50\% SOC} & \multirow{4}{*}{1 C} & \multirow{4}{*}{5 min} & \multirow{4}{*}{FDC + 10\%} & 1. & Discharge to cut-off voltage; & \multirow{4}{*}{4} \\  & & & & & & 2. & Rest for 5 min; & \\  & & & & & & 3. & Charge (1 C) for 1 h and then & & \\  & & & & & & & charge (0.5 C) for 12 min; & & \\  & & & & & & 4. & Rest for 5 min; & \\  & & & & & & & 5. & Discharge for 30 min. & \\ \hline \hline \multirow{4}{*}{7} & \multirow{4}{*}{90 days} & \multirow{4}{*}{50\% SOC} & \multirow{4}{*}{0.5 C} & \multirow{4}{*}{5 min} & \multirow{4}{*}{FDC 3} & 1. & Discharge to cut-off voltage; & \multirow{4}{*}{4} \\  & & & & & & 2. & Rest for 5 min; & \\  & & & & & & 3. & Charge (0.5 C) for 2 h; & \\  & & & & & & 4. & Rest for 5 min; & \\  & & & & & & & 5. & Discharge for 30 min. & \\ \hline \hline \multirow{4}{*}{8} & \multirow{4}{*}{90 days} & \multirow{4}{*}{50\% SOC} & \multirow{4}{*}{1 C} & \multirow{4}{*}{5 min} & \multirow{4}{*}{FDC 3} & 1. & Discharge to cut-off voltage; & \multirow{4}{*}{4} \\  & & & & & 2. & Rest for 5 min; & \\  & & & & & & 3. & Charge (1 C) for 1 h; & \\  & & & & & & 4. & Rest for 5 min; & \\  & & & & & & 5. & Discharge for 30 min. & \\ \hline \hline \end{tabular}  
* \({}^{1}\) Direct discharge and charge; \({}^{2}\) full discharge and charge plus 10% overcharge; \({}^{3}\) full discharge and charge. Note: All discharge rates used in experiments 1–9 were 1 C.  
\end{table}
Table 1: _Cont._Figure 3: Charge–discharge power test flow of Ni-MH battery modules.  
Figure 2: Capacity test flow of Ni-MH battery modules.  
The charge-discharge resistance can be calculated using the following equations:  
\[R_{d}=(U_{occ1}-U_{t1})/I_{maxd}, \tag{1}\]  
\[R_{c}=(U_{t2}-U_{acv2})/I_{maxc}, \tag{2}\]  
where \(I_{maxc}=0.75\)\(I_{maxd}\), \(I_{maxd}=90\) A is the discharge current, \(I_{maxc}=67.5\) A is the charge current, \(R_{d}\) is discharge resistance, \(R_{c}\) is charge resistance, \(U_{occ1}\) is the initial OCV before discharging at each SOC value, \(U_{occ2}\) is the initial OCV before charging at each SOC value, \(U_{t1}\) is the OCV after discharging for 10 s with 90-A current at each SOC, and \(U_{t2}\) is the OCV after charging for 10 s with 67.5-A current at each SOC.  
The peak discharge power can be calculated using the following equations:  
\[P_{1d}=U_{d\text{-}cutoff}\times\left(U_{occ1}-U_{d\text{-}cutoff}\right)/R_ {d}, \tag{3}\]  
\[P_{2d}=I_{maxd}\times(U_{occ1}-R_{d}\times I_{maxd}), \tag{4}\]  
\[P_{d}=min\{P_{1d},P_{2d}\}, \tag{5}\]  
where \(P_{1d}\) is the peak discharge power limited by the discharge cut-off voltage, \(P_{2d}\) is the measured peak discharge power, \(U_{d\text{-}cutoff}\) is the discharge cut-off voltage, and \(P_{d}\) is the final value of the discharge power.  
The peak charge power is computed using the following equations:  
\[P_{1c}=U_{c\text{-}cutoff}\times\left(U_{c\text{-}cutoff}-U_{occ2}\right)/R_ {c}, \tag{6}\]  
\[P_{2c}=I_{maxc}\times(U_{occ2}+R_{c}\times I_{maxc}), \tag{7}\]  
\[P_{c}=min\{P_{1c},P_{2c}\}, \tag{8}\]  
where \(P_{1c}\) is the peak charge power limited by the charge cut-off voltage, \(P_{2c}\) is the measured peak charge power, \(U_{c\text{-}cutoff}\) is the charge cut-off voltage, and \(P_{c}\) is the final value of the charge power.