## 3 Results and Discussions
### Temperature Change during LIB Cycle
It may cause great damage to the internal structure of the LIB and seriously affect the cycle service life of LIB. The PCM can have a good cooling effect on the temperature and make the average of the maximum temperatures for the six cycles of LIB keep at about 54.4 \({}^{\circ}\)C. The average of the maximum temperatures for the six cycles of LIB under the experimental condition of the combination of PCM and heat dissipating fins is about 54.3 \({}^{\circ}\)C, which is not significantly different compared to PCM cooling condition. The result shows that the effect of heat dissipation fins is not significant and the pure PCM cooling is the best choice due to the cost and complexity of the device structure. On the other hand, the LIB temperature rose at a slower rate and the temperature curve shifted slightly to the right as the cycle progressed. The heat generated inside LIB consisted of five parts: electrochemical reaction heat, ohmic internal resistance heat, polarization heat, electrolytic decomposition heat, and SEI decomposition heat. The electrolytic and SEI decomposition heat were very small and could be ignored when LIB was in the normal operating temperature range. The LIB would produce heat during the charging and discharging cycles. For discharging condition, the total chemical reaction was an exothermic reaction and the temperature rose theoretically, and for charging condition, the total chemical reaction was an endothermic reaction and the temperature drops theoretically. However, in the case of natural convection in Figure 3b, the temperature remains unchanged or even rises within a short period of time (region A) during the battery charging process. In the LIB charge process, although it is an endothermic process, the LIB generated heat such as ohmic internal resistance heat, polarization internal resistance heat, and electrochemical reaction heat more than itself heat dissipation, so that the temperature remains stable or even slightly rises. Figure 4 shows the temperature changes of LiFePO4 battery in charging and discharging cycles under four different working conditions. It can be found that the highest temperature of the LIB under extreme conditions is about 40.6 \({}^{\circ}\)C and the lowest temperature is about 35.1 \({}^{\circ}\)C when the LIB is shelved after charging. The temperature of LiFePO4 LIB will not be too high compared with Sanyo LIB when it works for a short time under extreme conditions due to its smaller LIB capacity, and lower internal polarization and ohmic resistance heat. The maximum temperature of Sony LIB is about 35.1 \({}^{\circ}\)C under PCM cooling, 37.5 \({}^{\circ}\)C under natural convection, 40.6 \({}^{\circ}\)C under extreme condition. It is found that  
Figure 3: The temperature changing of LIBs during charging and discharging. the LIB temperature under PCM cooling decreases by 2.4 \({}^{\circ}\)C compared with the natural convection. In the case of the combination of PCM and heat dissipation fins, the temperature decreases 0.4 \({}^{\circ}\)C than PCM cooling due to the temperature of battery itself is low. In addition, the Sony LIB temperature in extreme conditions cannot be lowered to the level of other conditions during charging and shelving due to the heat is not dispersed in time. It can accelerate the degradation of the LIB and influence the LIB performance in extreme condition for a long time, which further prove the necessity of BTMS.