Internal resistance variances in lithium-ion batteries and implications in manufacturing

Abstract

This thesis addresses issues in manufacturing that lead to cell DC internal resistance (DCIR) variance, provides an overview of generally accepted cell degradation mechanisms and modeling techniques associated with IR as a function of cycling, models the cycle life of a simple battery pack of two parallel-connected cells that start off as unbalanced, and validates it with experimental data from cycle life testing of parallel-connected LiFePO₄ cell groups. Experimental results from samples of commercially available cells show that variance in the thickness of the electrode layer is correlated to differences in capacity and DCIR. In cycle-life testing, parallel-connected cell groups with larger differences in DCIR between the cells experienced faster cycle life degradation. The proper matching of DCIR values within a battery pack, relative to the designed C-rate capability of the pack, is important to ensuring maximum useful life of the battery pack. This is especially important for parallel-connected cell groups, where the current distribution to each cell is typically not monitored in order to reduce battery management system complexity.