## 4 Discussion on Storage and Maintenance Methods for Ni-MH Battery Modules
### Advantages of the Proposed Storage and Maintenance Method  
Assuming that a batch of Ni-MH battery modules needs to be stored for 90 to 360 days from production to application, for a single Ni-MH battery module, the total maintenance time of the proposed storage and maintenance method was compared with the six benchmark methods 4-9. The methods 1, 2, and 3 only used the _DDC_ mode, which makes the Ni-MH battery module inactive. Thus, these three methods were not considered suitable any more. Method 10 represents the proposed storage maintenance method. In method 10, the _FDC_ mode and _DDC_ mode are utilized alternatively; thus, there are two single maintenance times, 41 min and 130 min. The single maintenance time of the _DDC_ mode is 41 min and that of the _FDC_ mode is 130 min. The comparison results between the seven methods are given in Table 12 and Figure 24.  
Method 4 and method 10 are taken here as examples to illustrate the results. Method 4 is maintained for 192 min once, and when the number of storage days is less than 180 days, it only needs to be maintained once because the second maintenance period is not reached; thus, the total maintenance time is 192 min. When it reaches 180 days but less than 270 days, it needs to be maintained twice, and the total maintenance time is 384 min. For method 10, when the storage time is less than 240 days, it needs to be maintained once by _FDC_ mode, which takes 130 min. When it reaches 240 days but  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \begin{tabular}{c} **Maintenance** \\ **Period** \\ \end{tabular} & \begin{tabular}{c} **Storage** \\ **SOC** \\ \end{tabular} & \begin{tabular}{c} **Charge–Discharge** \\ **Rate** \\ \end{tabular} & \begin{tabular}{c} **Rest** \\ **Time** \\ \end{tabular} & \
\begin{tabular}{c} **Charge–Discharge** \\ **Mode** \\ \end{tabular} &
\begin{tabular}{c} **Experiment Process** \\ \end{tabular} \\ \hline \multirow{4}{*}{120 days} & \multirow{4}{*}{30\% SOC} & & & & 1. & Discharge to cut-off voltage; \\  & & & & 2. & Rest for 5 min; \\  & & & & 3. & Charge (1 C) for 1 h; \\  & & & & 4. & Rest for 5 min; \\  & & & & 5. & Discharge for 42 min. \\ \cline{1-1}  & & & & & 1. & Discharge to cut-off voltage; \\  & & & & 2. & Rest for 5 min; \\  & & & & 3. & Charge (1 C) for 18 min. \\ \hline \hline \end{tabular}
\end{table}
Table 11: New storage and maintenance method.  
less than 360 days, the second maintenance is performed by \(DDC\) mode, and the single maintenance time is only 41 min and the total maintenance time reached 171 min.  
It can be seen from Table 12 and Figure 24 that the total maintenance time increases smoothly with the increase of storage time. When the storage time is less than 180 days, the maintenance time of method 9 is 100 min, less than that of method 10. This is because both method 9 and method 10 are maintained in a \(FDC\) mode, but the charge rate of method 9 is 2 C, which is higher than the 1-C charge rate of method 10 thus, method 9 has a shorter single maintenance time. When the maintenance time does not reach 180 days, both methods are maintained only once, and the total time is equal to the single maintenance time; thus, method 10 takes longer to maintain. However, the analysis of the experimental results shows that the 1-C charge rate is more conducive to the maintenance of the charge-discharge power and efficiency of the Ni-MH battery module.  
However, if the storage time exceeds 180 days, the advantage of saving the maintenance time endowed by the proposed method becomes more and more significant. The reason is that method 10 uses the \(DDC\) maintenance mode during even maintenance. The single maintenance time is only 41 min, which makes the total maintenance time increase less. Compared with the six benchmark methods, the proposed method provides superior storage and maintenance outcomes and significantly saves maintenance time.