Uneven Battery Cell Aging Cuts EV Range and Lifespan

New research indicates that electric vehicle batteries degrade faster than expected because individual cells within a pack do not age at the same rate. This uneven wear, known as cell-to-cell inconsistency, reduces the total energy a battery can hold and shortens its useful life. The findings come from a long-term study of real-world vehicle data rather than controlled laboratory tests.

The study, reported by Tech Xplore, analyzed operational records from 133 electric vehicles, including passenger cars and city buses. These vehicles traveled up to 300,000 kilometers over more than three years. The data revealed that the overall performance of a battery pack is ultimately limited by its weakest component, much like a chain is held back by its weakest link.

Cell variance drives performance loss

Battery packs consist of many individual cells connected in series to deliver power. While manufacturers aim for uniformity, small differences in capacity and internal resistance exist between cells from the start. As vehicles are charged and discharged repeatedly, these differences widen. The result is that the pack cannot deliver its full potential capacity because the system is constrained by the cells that have degraded the most.

Researchers from Chalmers University of Technology and other institutions found that this effect significantly impacts both range and power delivery. In a series-connected system, the total available energy is determined by the cell with the lowest remaining capacity. This means that even if most cells are still healthy, the pack's overall energy storage is reduced, directly limiting how far the vehicle can drive on a single charge.

Real-world data differs from lab tests

Previous studies on battery degradation have largely relied on laboratory simulations, which often use idealized charging patterns and controlled temperatures. This new work distinguishes itself by using data from actual driving conditions, which include varying temperatures, different driving speeds, and diverse charging habits. This approach provides a more accurate picture of how batteries behave over years of daily use.

The study covered two main types of battery chemistry: NMC, which uses nickel, manganese, and cobalt, and LFP, which uses lithium iron phosphate. Both types showed signs of uneven cell aging. By isolating actual aging from temporary changes caused by temperature or state of charge, the researchers were able to quantify the long-term loss in battery health and energy utilization.

Implications for vehicle longevity

The findings suggest that current strategies to balance cell performance may need refinement. While engineers have developed methods to equalize charge levels between cells, the physical degradation of the cells themselves remains a challenge. The uneven aging process means that simply balancing the electrical load does not reverse the underlying loss of capacity in the weaker cells.

For consumers and fleet operators, this highlights a trade-off: while electric vehicles offer lower emissions, their long-term reliability depends on how well battery management systems can mitigate the effects of cell inconsistency. Understanding these dynamics is crucial for predicting battery lifespan and planning for potential replacements or upgrades in the future.