In a recent study published in Nature Energy, researchers have quantified how cell-to-cell inconsistency limits the performance, lifetime, and resource utilisation of electric vehicle (EV) battery packs under real-world operating conditions. The study revealed that a small number of faster-ageing cells can constrain the performance and lifetime of an entire battery pack.
The study was led by Prof. CHEN Zhongwei from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS), Prof. ZOU Changfu from Chalmers University of Technology, and researchers from Beijing Institute of Technology and Zeekr Technology Europe AB.
Quantitative evaluation of the impacts of cell-to-cell inconsistency on EV battery pack state of health, lifetime, state-of-charge utilisation, power capability, and lifetime energy-resource utilisation based on real-world operation data. (Image by ZHOU Litao)
EV battery packs typically consist of many individual cells. Differences in raw materials, manufacturing processes, cell grouping, local temperature distribution, and real-world operating conditions can cause cells within the same pack to age at different rates.
As a result, a series-connected battery pack exhibits a classic "weakest-cell" effect: during charging, the lower-capacity cell hits the upper voltage limit first; during discharge and high-power operation, the higher-resistance cell reaches safety constraints earlier; and once the fastest-ageing cell crosses the retirement threshold, the whole pack may have to be taken out of service prematurely.
While previous studies have mostly relied on laboratory test data, the long-term, system-level consequences of cell-to-cell inconsistency under real-world driving and operating conditions have not been systematically quantified until now.
To address this issue, the researchers developed a fleet-scale framework for cell-level ageing diagnosis and system-level performance evaluation. The dataset covers electric passenger cars equipped with nickel-manganese-cobalt (NMC) batteries and electric buses equipped with lithium-iron-phosphate (LFP) batteries, with vehicle operation spanning more than three years and individual vehicle mileage reaching up to 300,000 kilometers.
Using voltage, current, temperature, and state-of-charge signals routinely recorded during vehicle operation, the researchers combined battery model identification with neural networks to estimate the ageing trajectories of individual cell capacity and internal resistance under unified reference conditions.
According to the researchers, this approach helped reduce the influence of seasonal variation and differences in state of charge and operating conditions, enabling more consistent comparisons of cell ageing.
Based on these estimates, the researchers then established six metrics—covering cell state of health dispersion, pack health utilisation, lifetime utilisation, state-of-charge utilisation, power capability utilisation, and energy-resource utilisation—to systematically quantify the effects of cell-to-cell inconsistency.
The researchers found that, in the passenger-car fleet, cell-to-cell variation in state of health remained generally small at lower mileage. However, after accumulated mileage exceeded roughly 170,000 km, individual cells in many vehicles began to enter accelerated ageing regimes, and differences among cells increased markedly—although the onset and severity vary across vehicles.
Using a unified retirement threshold, cell-to-cell inconsistency reduced the usable pack-level state of health by 6.2% for electric passenger cars and 7.5% for buses. Relative to the average lifetime of the constituent cells, pack lifetime was shortened by 17.7% and 22.8%, respectively. Power capability losses caused by internal-resistance heterogeneity reached 12.9% and 15.1%, respectively. By contrast, under the charging and balancing strategies represented in the studied vehicles, the impact of state-of-charge imbalance on usable charged capacity was typically below 2%.
Taking into account premature retirement and state-of-charge imbalance, the lifetime energy-resource utilisation of the electric passenger-car and bus battery packs was only 80.7% and 72.9%, respectively. In other words, when the weakest cell causes the entire battery pack to reach its retirement criterion, approximately 19.3% and 27.1% of potential energy resources remain under-utilised. These results demonstrate that battery pack degradation cannot be evaluated solely by the average ageing level of its cells, as a small number of faster-aging cells can significantly constrain pack lifetime, power capability and resource utilisation.
The comparison between the two vehicle fleets further indicates that balancing strategies can influence the long-term evolution of cell-to-cell inconsistency. Improving EV battery utilisation therefore requires coordinated control of cell differences from battery manufacturing, cell grouping, and operational management. Reducing initial cell-to-cell variation and optimising cell grouping can help suppress the amplification of inconsistency during service, while advanced balancing control, thermal management, and reconfigurable battery systems may help mitigate weakest-cell limitations.
"Our findings show that the performance and lifetime of an EV battery pack are not determined solely by the average ageing level of their cells, but can be strongly constrained by a small number of faster-ageing cells," said Prof. CHEN. "By quantifying this weakest-cell effect under real-world EV operation, our study provides a basis for improving battery utilisation, lifetime management, and system-level optimization."