Analysis: Every 85th used electric car shows hidden cell deviation
Image: Porsche (illustrative)
The British testing specialist Generational analyzed individual battery cells from around 10,000 tests of used electric vehicles conducted through hundreds of dealerships in the United Kingdom. The vast majority of the tested batteries were well-balanced. The voltage difference between the cell with the highest and lowest voltage was only 8 millivolts (mV) on average, according to the company.
However, roughly every 85th test showed a cell deviation of more than 50 mV, which Generational states should be investigated. In about every 200th test, the difference exceeded 100 mV, indicating a possible underlying fault that required further examination. Such deviations are not necessarily visible in the aggregated State-of-Health data (abbreviated as SoH).
"The State of Health indicator is essential for used electric vehicles, but it does not tell the whole story about the battery’s condition," says Generational CEO Oliver Phillpott. The value reflects capacity but does not necessarily indicate whether the individual cells in the battery pack are functioning properly. As a result, an electric vehicle can display a stable State of Health while still developing faults at the cell level.
An electric vehicle battery consists of several hundred cells that are connected in series in groups and must function together. The battery pack terminates charging as soon as one of the cells is fully charged first, and it stops discharging once one of the cells runs out first. According to Generational, this means a single weak cell can limit the entire battery—something that an average value at the overall battery pack level does not reveal. If a cell deviates significantly from the others, the battery management system may also limit charging or discharging to protect the weakest cell—this can affect range, battery level indicators, and charging behavior.
Cell problems often go unnoticed
According to the company, such problems are rare and often not immediately apparent to drivers. Warning lights, alerts in the cockpit, or noticeable changes in driving behavior may not appear until the issue has progressed further. In more severe cases, costly module or battery replacements may be necessary.
Electric vehicle batteries can often account for more than 40 percent of the vehicle’s cost. For dealers and financing companies, issues with cell defects can lead to time-consuming and costly repairs, as vehicles are returned and financing payments halted. Therefore, Generational emphasizes the importance of checking the battery condition in individual vehicles.
According to the expert, when evaluating cell deviations, the battery charge level and recent vehicle usage play an important role. A marginal deviation at a low charge level does not necessarily indicate a permanent fault, whereas a significant deviation that persists even after attempting to balance the cells is much more concerning.
Generational’s latest “Battery Performance Index” was based on over 8,000 battery tests conducted in the United Kingdom, determining an average State of Health of 95.15 percent. Meanwhile, the variation among vehicles increased as they aged. According to Generational, verified condition data can help dealers price vehicles more accurately, protect residual values, reduce sourcing risks, and provide consumers with clearer information about the most valuable component of an electric vehicle.
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About the author
Thomas Langenbucher is an expert in electromobility with experience in the automotive and financial industries. Since 2011, he has been covering electric vehicles, sustainable technologies, and mobility solutions for ecomento.de. Learn more.
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