A study of cell-to-cell variation of capacity in parallel-connected lithium-ion battery cells

Capacity variation among battery cells can occur due to inconsistent manufacturing processes and operating conditions, such as uneven temperature distribution. For a battery string made of parallel-connected cells with only one voltage and one current sensor, the lack of independent current sensors makes it difficult to detect or control the degradation variation. In order to investigate the progression mechanism of cell-to-cell capacity variation, this paper adopts an electric aging model and analytically determines the relationship between variation progression and cell degradation characteristics. Assuming all cells have similar temperatures, the capacity variation will decrease over time for cells with a convex or linear degradation curve (i.e., the most common case), providing a self-balancing mechanism for parallel-connected cells. Compared to battery strings with uniform cell capacities, battery strings with an initial cell-to-cell variation will degrade slightly faster. State-of-charge imbalance and uneven heat generation are analyzed using a thermal model. Assuming the same coefficient of heat transfer (i.e., same cooling condition), simulation results further verify the self-balancing mechanism for a parallel battery string consisting of 5 LiFePO4 battery cells.