## 3 Experiment on Low-Temperature Battery Characteristics
### Charging Strategy  
Based on the experimental results in Sections 3.1 and 3.2, the magnitude of the charging current would affect the actual capacity of the battery at different temperatures. The relationship between battery capacity and charging currents reversed between \(-10\)\({}^{\circ}\)C and \(-20\)\({}^{\circ}\)C. The critical ambient \begin{table}
\begin{tabular}{c c c} \hline \hline
**Temperature (\({}^{\circ}\)C)** & **Charging Current (A)** & **Battery Capacity (Ah)** \\ \hline \(-10\)\({}^{\circ}\)C & 1 A & 40.0 \\ \(-10\)\({}^{\circ}\)C & 2 A & 45.4 \\ \(-10\)\({}^{\circ}\)C & 3 A & 47.1 \\ \(-20\)\({}^{\circ}\)C & 1 A & 30.0 \\ \(-20\)\({}^{\circ}\)C & 2 A & 27.4 \\ \(-20\)\({}^{\circ}\)C & 3 A & 25.0 \\ \(-30\)\({}^{\circ}\)C & 1 A & 16.7 \\ \(-30\)\({}^{\circ}\)C & 2 A & 16.5 \\ \(-30\)\({}^{\circ}\)C & 3 A & 16.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Battery-capacity calculation results of the charging experiment. Figure 8: (**a–c**) Charging curve for the lead–acid battery at temperatures of \(-10\), \(-20\), and \(-30\)\({}^{\circ}\)C under the charging currents of 1, 2, and 3 A, respectively. temperature of the relationship between battery capacity and charging currents was therefore tested experimentally. The temperature range for the experiment was from \(-10\)\({}^{\circ}\)C to \(20\)\({}^{\circ}\)C. The interval of the experimental temperature was set as \(1\)\({}^{\circ}\)C. The experimental method and other experimental conditions were the same as those in Section 3.2. The experimental results shown in Figure 9 demonstrate that when the ambient temperature was less than a critical value, the battery capacity increased with the increase of the charging current and decreased when the temperature was greater. The temperature of \(-16\)\({}^{\circ}\)C was observed as the critical level. Thus, a low-temperature charging strategy for lead-acid batteries is proposed. When the temperature is above \(-16\)\({}^{\circ}\)C, the battery has a strong activity of active substances. At this time, a large current can be selected for charging. When the temperature is lower than \(-16\)\({}^{\circ}\)C, a small current can be selected for charging. \(I_{s}\) is the solar charging current; \(I_{w}\) is the wind turbine charging current; \(I_{B}\) is the charging current of battery, and \(I_{B}\) is equal to the sum of \(I_{s}\) and \(I_{w}\); \(T_{a}\) is the ambient temperature; \(T_{c}\) is the critical temperature mentioned above (\(-16\)\({}^{\circ}\)C). The charging strategy of the hybrid wind-solar system is shown in Tables 5 and 6. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**Environmental Conditions**} & **Current 1** & **PV Output Status** & **Wind Turbine Output Status 1** \\ \hline Polar day & Wind energy available & \(I_{s}+I_{w}>I_{max}\) & MPPT & Current-limiting \\ Polar day & Wind energy available & \(I_{s}+I_{w}<I_{max}\) & MPPT & MPPT \\ Polar day & Wind energy available & \(I_{s}>I_{max}\) & Current-limiting & Unloading \\ Polar day & Wind energy unavailable & \(I_{s}>I_{max}\) & Current-limiting & Not working \\ Polar day & Wind energy unavailable & \(I_{s}<I_{max}\) & MPPT & Not working \\ Polar night & Wind energy available & \(I_{w}>I_{max}\) & Not working & Current-limiting \\ Polar night & Wind energy available & \(I_{w}\leq I_{max}\) & Not working & MPPT \\ Polar night & Wind energy unavailable & \(I_{s}+I_{w}=0\) & Not working & Not working \\ \hline \hline \end{tabular}  
* \(I_{max}\) is the lower limit of small charging current.  
\end{table} 
Table 6: The charging strategy of the hybrid wind-solar system (\(T_{a}<T_{c}\)). Figure 9: Discharge curves of the lead-acid battery at low temperature. If \(T_{a}>T_{c}\), there are three operating conditions for hybrid wind-solar system. Operating condition 1 (the first and second rows of Table 5): In the polar day, the PV always works in MPPT mode. And if the value of wind speed is between \(V_{c}\) and \(V_{F}\), which means wind energy is available, the wind turbine also works in MPPT mode.