Performance and comparison of equalization methods for lithium ion batteries in series

Deviations between batteries in series appear gradually and increase with the number of cycles. This inconsistency reduces the lifetime of battery packs, increases the cost of using them, and may lead to security issues. Equalization is an important means of reducing battery differences. The relevant research has focused on the design of equalization circuits and the improvement of equalizer efficiency while neglecting a comparative analysis of methods of equalization on the performance of battery packs, which hinders technicians from making the correct choice during application. A quantitative analysis is provided in this paper to compare the effects of methods of equalization on reducing the differences among batteries. Multiple parameters are introduced to describe differences among equalization methods, and the results indicated that equalization worked only at appropriate current rates and failed at large ones. With the same current rate in charging and discharging, the relative equalization time of active equalization was shorter than that of hybrid equalization. Active equalization was better than passive equalization in reducing battery capacity differences. The maximum difference in state of charges among batteries with active equalization at a current rate of 0.25 C decreased from 10% to 9.207% in discharging, while that with passive equalization dropped from 10% to 9.492%. Therefore, active equalization was suitable for short period charge and discharge. Moreover, the values of the equalization current of the equalization methods were similar, but the effect of hybrid equalization on reducing the differences among batteries was more significant. The average hybrid equalization current was 0.073 A larger than the average active equalization current. However, the capacity of the battery pack with hybrid equalization was 0.369 Ah higher than that with active equalization. Hybrid equalization was more conducive to the regular maintenance of the battery pack. In the life cycle of the battery pack, an equalization management mode of "single-cycle active equalization + hybrid equalization regular maintenance" could be introduced. On this basis, fast equalization within a single cycle could be achieved and consistency among cells during long-term cycling could be guaranteed. This study provides a scientific basis for engineering practice and helps choose an appropriate method of equalization.