Improvement of Usable Capacity in Second Life LFP Cells by Internal Resistance Guided Placement Strategy

Abstract

This study evaluates the effect of internal-resistance-guided cell placement on capacity and voltage balance in an 8S battery module built from 280 Ah second-life LiFePO4 cells showing swelling and degradation. Internal resistance, open-circuit voltage, and capacity loss were first characterized using a DCIR-based procedure with 0.5C charge–discharge steps. Then restorative actions, including mechanical compression (strapping), terminal surface cleaning, full charge–discharge cycling, and resting, were applied, and the evolution of internal resistance was tracked. Based on these data, a placement strategy was adopted in which lower-resistance cells were located at the module center and higher-resistance cells at the ends. The resulting series module was tested to assess voltage distribution, usable capacity, and balancing demand. Results show that, under operating conditions, a placement rule can improve module efficiency, reduce inter-cell voltage deviation, and mitigate balancing requirements without additional hardware or complex modeling, offering a practical approach for second-life LFP modules.