## 2 Overvoltage of a Battery
### Chemical Reaction of a Battery  
Batteries are components that charge and discharge electrical energy through chemical reactions. The activity of the reaction changes with the temperature. To determine whether changes in the activity of the chemical reaction affect the voltage of the battery, experiments and simulations were conducted. The configuration of the equipment was as follows [51]. * Battery test system: MACCOR 4000 Series
* Chamber: JEIO Tech TH-G
* Digital recorder: YOKOGAWA MV1000
* Battery: Polymer Li-ion Recharged Battery DTP 6565294 (3.7 V/4000 mAh)  
The following information is based on the specifications listed in the detailed product specification sheet for the battery and was used as the basis for the charging and discharging conditions [51]. * Charge limited voltage: 4.20 V
* Discharge cut-off voltage: 2.75 V
* End of charge current: 0.01 C
* Standard charge: 0.2 C (800 mA)
* Standard discharge: 0.2 C (800 mA)
* Operating temperature range:  
* Charge: 10-45 \({}^{\circ}\)C Discharge: \(-\)20-60 \({}^{\circ}\)C
* Storage temperature range: 10-45 \({}^{\circ}\)C
* Operating and storage humidity range: 65 \(\pm\) 20% RH  
This study was conducted by reanalyzing past experimental data and discovering new facts to write about. In the Case of Having Cell Balancing and in the Case of Not Having It  
Table 6 summarizes the experimental results for the temperature-dependent voltage changes observed after performing a full charge and then varying the temperature. The range of temperature changes was based on the operating temperature range allowed by the BMS for automotive use, and voltage changes were accordingly observed. The largest voltage change occurred when the battery was charged at room temperature (25 \({}^{\circ}\)C) and then subjected to temperatures of \(-\)20 \({}^{\circ}\)C and 55 \({}^{\circ}\)C, resulting in a voltage change of 18 mV. It was noticed that the largest increase in the chemical reaction activity occurred when the battery was charged at room temperature and then, after complete charging, the temperature was increased to 55 \({}^{\circ}\)C, resulting in the influx of thermal energy. In Table 7, experiments were conducted to observe the voltage changes with temperature variations for each SOC. The charging temperature was based on room temperature, and the charging termination condition was divided into each percentage based on the fully charged Ah. In the experimental results, the voltage change was the smallest, 1 mV, with temperature variations from \(-\)20 to 55 \({}^{\circ}\)C at SOC 40%. In addition, the voltage change was the largest, 32 mV, with temperature variations at SOC 0%. Such voltage changes also occurred even in the case of SOC 100%, showing a voltage change of 18 mV. \begin{table}
\begin{tabular}{c c c c c}  & & Charge: 4.2 V/0.5 C, End of charge: 4.2 V/0.05 C \\
**After End of Charging** & **Charging @ \(-\)20 \({}^{\circ}\)C** & **Charging @ 0 \({}^{\circ}\)C** & **Charging @ 25 \({}^{\circ}\)C** \\ \hline Voltage (V) @ 55 \({}^{\circ}\)C & 4.081 & 4.132 & 4.168 & 4.166 \\ Voltage (V) @ \(-\)20 \({}^{\circ}\)C & 4.074 & 4.124 & 4.150 & 4.154 \\ Voltage deviation (mV) & 7 & 8 & 18 & 12 \\ \hline \end{tabular}
\end{table} 
Table 6: After complete charging, voltage variation due to temperature. Figure 7 is a diagram that estimates the cause of the voltage change based on the experimental results of Table 7. Using the SOC-OCV graph as a reference, the solid line represents the activity change in chemical reactions due to the heat energy input into the battery caused by temperature changes. The resulting voltage change is represented by the dotted line. It can be confirmed that open-circuit voltage (OCV) changes are more pronounced near SOC 0% or 100% than the nominal voltage. Based on the previous experiment, a simulation was conducted by observing the voltage change after changing the temperature from room temperature to 60 \({}^{\circ}\)C. Figure 8a shows the simulation model, Figure 8b shows Chart 1's flowchart, and Figure 8c shows Chart 2's flowchart.