Progression of cell-to-cell variation within battery modules under different cooling structures

Cell-to-cell variation generally exists within battery packs, due to factors attributed to manufacturing and operating. Non-uniform temperature distribution, caused by the uneven cooling condition, contributes significantly to cell-to-cell variation over time, particularly to capacity variation, as temperature significantly influences the battery degradation rate. Especially for parallel-connected cells, the lack of individual current sensing and actuation makes it challenging to detect and control the capacity variation. In order to understand how cell-to-cell variation evolves, we investigate the effect of cooling structures on the progression of variation within parallel-connected battery cells using an electro-thermal-aging model for battery cells and a thermal model of the cooling system. The simulation result shows that the cell-to-cell variations increase initially because of uneven cooling conditions, but then the variation decreases over time, thanks to the self-balancing mechanism among parallel-connected cells. Moreover, when comparing the sequential and round cooling structures, it is found that the round cooling structure, which provides a more uniform cooling condition to all cells, has significant advantage in terms of suppressing the cell-to-cell variation, especially for large battery strings. The even cooling structure is preferred in practical applications; however, the trade-off between cooling system complexity and performance needs to be carefully considered.