Researchers analyzing years of EV driving data discovered how differences between individual battery cells can intensify over time.
An electric vehicle (EV) battery can reach the end of its useful life even when many of its individual cells still have substantial capacity remaining. New research shows that a small number of rapidly aging cells can shorten the lifespan of an entire battery pack, leaving a significant share of its potential energy resources unused.
Published in Nature Energy, the study analyzed batteries operating in real vehicles over more than three years. Researchers found that differences between individual cells can reduce battery lifespan, restrict power output, and become more pronounced as vehicles accumulate mileage.
Why a Few Weak Cells Can Limit an Entire Battery
EV battery packs contain numerous cells that must work together to store and deliver energy. In a series-connected battery pack, however, the weakest cells can determine how much of the pack’s total capacity is actually available. During charging, a cell with reduced capacity may reach its maximum allowable voltage before the others. During discharge or high-power operation, a cell with greater internal resistance can reach its operating limits first.
These differences develop for several reasons, including variations in raw materials, manufacturing, cell grouping, temperature, and driving conditions. As cells age at different rates, the pack can eventually reach its retirement threshold even though many of its cells remain functional.
What Years of Real-World Driving Revealed
Led by Zhongwei Chen of the Dalian Institute of Chemical Physics (DICP) at the Chinese Academy of Sciences (CAS) and Changfu Zou of Chalmers University of Technology, the research also involved scientists from Beijing Institute of Technology and Zeekr Technology Europe AB.
The team examined electric passenger cars using nickel-manganese-cobalt (NMC) batteries and electric buses equipped with lithium-iron-phosphate (LFP) batteries. Some vehicles had traveled as far as 300,000 kilometers (about 186,400 miles). Using routinely recorded voltage, current, temperature, and state-of-charge data, the researchers combined battery modeling with neural networks to estimate changes in individual cell capacity and internal resistance. Their approach accounted for differences in operating conditions and seasonal temperatures, allowing more consistent comparisons of battery aging.
In passenger cars, differences in cell health generally remained small at lower mileage. Once vehicles exceeded approximately 170,000 kilometers (about 105,600 miles), however, some cells in many battery packs began aging more rapidly, increasing the gap between stronger and weaker cells. The timing and severity of this deterioration varied among vehicles.
How Much Battery Life Is Being Lost?
Using a common retirement threshold, the researchers calculated that uneven cell aging reduced usable pack-level state of health by 6.2% in passenger cars and 7.5% in buses. Compared with the average potential lifespan of their individual cells, battery packs had lifetimes that were 17.7% shorter in passenger cars and 22.8% shorter in buses.
Differences in internal resistance also reduced available power capability by 12.9% and 15.1%, respectively. By comparison, differences in state of charge typically reduced usable charged capacity by less than 2% under the charging and balancing strategies used in the studied vehicles.
Across their calculated lifetimes, passenger-car battery packs utilized just 80.7% of their potential energy resources, while bus batteries utilized 72.9%. Under the study’s retirement criteria, this meant that approximately 19.3% and 27.1% of potential energy resources, respectively, remained unused when the weakest cells reached their limits.
Extending Battery Life by Addressing Cell Differences
The findings suggest that battery lifespan cannot be accurately assessed using only the average condition of its cells. Comparisons between the vehicle fleets also indicate that battery balancing strategies may influence how differences between cells develop over time.
The researchers identified several potential improvements, including more consistent cell manufacturing, better cell grouping, advanced balancing controls, and improved thermal management. Reconfigurable battery systems, which could adjust how cells are connected and used, may offer another way to reduce the constraints imposed by weaker cells.
“Our findings show that the performance and lifetime of an EV battery pack are not determined solely by the average aging level of their cells, but can be strongly constrained by a small number of faster-aging cells,” said Prof. Chen. “By quantifying this weakest-cell effect under real-world EV operation, our study provides a basis for improving battery utilization, lifetime management, and system-level optimization.”
Reference: “Quantifying the impact of cell-to-cell inconsistency on electric vehicle battery degradation and utilization” by Litao Zhou, Xiaolei Bian, Yizhou Zhang, Zhenpo Wang, Zhongwei Chen, Zhiyu Mao and Changfu Zou, 25 August 2026, Nature Energy.
DOI: 10.1038/s41560-026-02131-5
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