Hidden Battery Imbalances Found in 1.2% of Used EVs, Analysis Shows

Analysis of approximately 10,000 used electric vehicles has identified a hidden battery issue, termed 'battery cell-level imbalances,' in around 1.2% of the cars tested. This imbalance, where the voltage difference between the highest and lowest battery cells exceeds 50 millivolts (mV), can potentially affect an EV's range, charging capabilities, and resale value.

Tests conducted by EV battery diagnostic company Generational through hundreds of automotive dealers in the UK found that while most used EV battery packs function correctly, a small proportion exhibit these cell-level voltage discrepancies. This deviation is more than six times the typical median voltage difference seen in functioning vehicle batteries. Furthermore, about one in 200 tests revealed a 'serious' imbalance exceeding 100mV, a level that could prompt manufacturers to deem the battery pack irreparable, leading to significant replacement costs if outside warranty.

These cell-level issues often go undetected by drivers, as there may be no warning lights, dashboard messages, or noticeable changes in vehicle behavior until the problem becomes more advanced. This lack of visible indication can result in costly complications for dealers and finance companies, including vehicle returns and payment cessation, leading to extended dispute resolutions.

Generational emphasizes that its findings do not suggest one in 85 used EVs requires an immediate battery replacement. Instead, the analysis highlights the need for deeper, cell-level testing beyond the standard State of Health (SoH) metrics, which primarily measure overall battery capacity. A stable SoH reading does not guarantee that all individual cells within the pack are operating within normal parameters.

Oliver Phillpott, CEO of Generational, stated that while SoH reporting is essential, it does not provide a complete picture of battery health. He explained that cell imbalance problems are often imperceptible in their early stages but can later impact range, charging, or value. He advocates for cell-level testing as the practical method for early risk detection.

"Our cell-level testing gives the market a deeper layer of transparency, with a full vehicle audit history to give both dealers and finance companies peace of mind," Phillpott said. "It also helps strong vehicles prove their quality, and it helps dealers identify the small number of vehicles that need attention before they become a customer problem. That is how we build a healthier, more confident used EV market."

Confidence in used EV batteries is crucial, as a battery pack can constitute over 40% of a vehicle's total cost. The importance of individual battery condition testing is growing, particularly as the average age of EVs in the UK increases. Used EV transactions reached record levels in the first quarter of 2026, with battery confidence identified as a key factor influencing consumer decisions.

Groups such as Electrifying.com, New AutoMotive, and BatteryIQ are urging the government to implement mandatory battery State of Health certificates for all used EVs sold by dealers. They argue this would provide a level of transparency similar to MOT certificates, service histories, and mileage records available for used internal combustion engine vehicles, thereby removing a significant barrier to mainstream EV adoption. These organizations contend that the primary issue is not battery degradation itself, but the scarcity of clear, independent information available to consumers at the point of sale.

In a typical EV battery pack, hundreds of individual cells work in unison. For reliable performance, these cells must remain balanced. If one cell's voltage drifts significantly, the battery management system may restrict charging or discharging to protect it. This can lead to reduced range, inaccurate State of Charge estimates, altered charging behavior, and, in severe cases, necessitate costly module or battery replacements.

Generational recommends regular testing, including annual checks, to identify cells that are beginning to drift. Early detection allows for potential remedial work while the vehicle's battery warranty is still valid. Such proactive measures are seen as vital for building consumer trust and ensuring a robust used EV market.