High-speed Target SOC Alignment Algorithm for Second-life Battery Pack Maintenance

Since the retired battery pack from EV piles up, reusing the battery pack for the battery energy storage system (SL-BESS) is a promising solution. Due to the different aging characteristics, the performance of the cells inside the battery pack becomes mismatched, which raises a safety issue for the cells. For the maintenance, the battery pack requires a full diagnosis to scan the characteristics and to equalize the state of charge level. Although the cell equalizer exists in the battery pack, it requires a long time of operation to align the SOC of cells and does not provide a target SOC balancing. This paper proposes a high-speed SOC alignment algorithm for such a seconds-life battery application. The performance of the alignment algorithm is verified by the real-time simulations for the 20-series battery string. The SOC levels of the cells are adjusted to the target level and the SOC deviation is within 3%. The proposed method only requires 50% operation time of the conventional method.