Statistical Relationships between the Numerous Retired Lithium-Ion Cells and Packs with Random Sampling for Echelon Utilization

Retired batteries are repurposed widely in energy storage packs as an economical and eco-friendly method to achieve echelon utilization. However, pack performance is strongly affected by the variations of retired cells and the configuration of pack. Quantifying this effect with various pack configurations and cell-to-cell variations is crucial for predicting the performance of numerous packs. Therefore, under a random sampling scenario, we developed statistical models of relationships between retired cells and packs in capacity and resistance based on the probability and statistics, which provides great theoretical foundation for designing and optimizing the pack structure. It is proved that parallel configuration improves the utilization efficiency and variation of pack-level capacities. Meanwhile, both parallel and series configurations reduce pack-level resistance variation. Moreover, the capacity statistical performance of pack with parallel connections in series is superior than that of the pack with series connections in parallel, though their resistance statistical characteristics are the same. Furthermore, based on the models, a capacity screening criterion are proposed to reduce the capacity variation of the pack while making full use of the retired cells. Results verify that retired cells with capacities in the range of μC±2σC should be accepted to randomly compose the energy storage packs