Interaction of Temperature and Current Differences among parallel-connected Lithium-Ion Cells in Dependency of the thermal Battery Design

Automotive batteries contain a high number of lithium-ion cells connected in parallel and serial to fulfill the power and energy demands of electrified vehicles. Cell to Cell Variances (CtCV), regarding the cell resistance and capacity, as well as influences of the cell stacking cause temperature and current imbalances among parallel-connected cells. Due to the temperature dependence of the cell resistance and the quadratic correlation of the arising irreversible heat with the cell current, a high interaction exists between current and temperature differences. For this reason, it is of utmost relevance to investigate the influence of thermal battery design on thermal and electrical inhomogeneities among cells to ensure battery safety and longevity. In this paper, the influence of CtCV, cell topology and thermal boundary conditions on current and temperature gradients between parallel cells is experimentally investigated. Two cell groups consisting of three and four parallel cells are loaded with continuously alternating discharge/charge pulses under various thermal conditions. The measurements prove a significant correlation between temperature gradients and current differences. Heterogeneous cooling and thermal insulation of the cells leads to a considerable increase of temperature and current imbalances among the cells.