In order to make the most of depleted resources, the battery is stored as power energy and used again when needed. However, battery fires make it difficult for batteries to enter the market. Battery fire accidents still occur. A lot of research is in progress, but the focus is on coping after a fire rather than finding the cause of a fire. This study aimed to find the root cause. Implement high-voltage systems using series-connected batteries. The permissible capacity fluctuations of series-connected batteries greatly affect the price of battery packs. Using Matlab R2019a simulink, we simulated the effect of change in allowable capacity deviation of series-connected batteries on battery aging. The lithium battery and charger library used the basic model provided by Matlab. Comparisons of ±1% and ±2% for acceptable dose deviation. The aging parameters of batteries are all the same. The voltage of each battery was charged with 4.1V applied by EV. The aging simulation time proceeded for 10e+06 sec. Due to the low cell balance current, the cell balance function does not operate normally. Therefore, the simulation was conducted except for the cell balance function in the aging simulation. The simulation analyzed the voltage and SOH results. It was confirmed that capacity deviation in serial-connected batteries occurs when charging with a single power source and voltage distribution is not uniform. It was also confirmed that the aging progression rate is different due to uneven charging voltage and C-rate. Furthermore, this phenomenon became more serious as the allowed capacity deviation of the serial-connected batteries increased. As the aging progression rate of each battery is different, the difference in the capacity ratio of each battery increases and the charging voltage of the relatively smaller battery gradually increases over time. In the end, it was confirmed that the peak voltage during the charging of the relatively smaller battery non-linearly increases. This study compared the results of simulations. When the battery's SOC exceeds 100%, there is no DCIR information on the battery, causing errors in the simulation results. Therefore, it was not possible to confirm the results by applying the charging voltage of 4.2V used in ESS, which occurs when the battery's SOC exceeds 100%. We think that practical tests are necessary for fires occurring in large-capacity ESSs. There have been many papers that deal with safety in battery fire cause studies. However, they mainly focus on the problems of individual battery cells. This paper is characterized by analyzing the structure of the battery system and researching how the charging of relatively small batteries can become a fire cause due to progressive damage caused by serial connections when the maximum voltage continues to rise during charging. It can be said that this research clarifies the fundamental cause of battery fires, which is different from other papers. The authors of this research have a background in developing BMS for electric vehicles and large-capacity ESS since 2009, and this research is approached based on their knowledge. The BMS has a protective circuit and functions to interrupt the overvoltage of the battery. Typically, it interrupts when the voltage of the battery exceeds 4.3V. Therefore, the highest voltage that can be reached due to progressive damage is 4.3V. Afterward, I plan to study the voltage state higher than OVP due to temperature change in the OVP voltage of the battery.

Prof. Dr. Seok-Cheol Kee, Professor, Chungbuk National University