Study Finds Few Aging Cells Can Cause Early Battery Pack Retirement After Analyzing Over 3 Years of Vehicle Data

Professors Chen Zhongwei from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and Zou Changfu from Chalmers University of Technology in Sweden led a team that analyzed vehicle operational data spanning over three years. The study covered passenger cars equipped with nickel-manganese-cobalt (NMC) lithium-ion batteries and buses using lithium iron phosphate (LFP) batteries, with some vehicles accumulating up to 300,000 kilometers of mileage. The team developed a fleet-scale analytical framework, leveraging vehicle voltage, current, temperature, and state-of-charge data combined with battery model identification and neural networks to estimate changes in cell capacity and internal resistance under standardized conditions. The data show that at lower mileage levels, differences in state-of-health among cells in passenger cars are minimal; however, beyond 170,000 kilometers, some cells begin aging more rapidly. Under a unified retirement criterion, cell inconsistency reduces the usable health status of passenger car battery packs by 6.2% and bus packs by 7.5%. Compared to the average lifespan of cells within the pack, the overall pack lifespan is shortened by 17.7% for passenger cars and 22.8% for buses. Internal resistance inconsistency decreases the output capability of passenger car and bus battery packs by 12.9% and 15.1%, respectively. Under the balancing strategies examined in the study, state-of-charge imbalance typically results in less than a 2% loss in usable charging capacity. Over their full service life, the energy utilization efficiency of passenger car battery packs is 80.7%, while that of bus packs is 72.9%. When the weakest cell triggers pack retirement, approximately 19.3% and 27.1% of potential energy remains unused in passenger car and bus packs, respectively. These findings have been published in *Nature Energy*.