One in 85 used electric vehicles in the UK has a hidden battery fault that could degrade driving range, reduce charging capabilities, and lower resale value, according to new data from EV diagnostic company Generational.
Analysis of approximately 10,000 battery-powered cars conducted through hundreds of automotive dealerships across the country found battery cell-level imbalances in roughly 1.2 per cent of second-hand electric vehicles tested.
While the vast majority of used electric car battery packs operate correctly, a small proportion recorded cell-level voltage imbalances exceeding 50 millivolts (mV). That is more than six times the median voltage difference found between the highest and lowest cells in a typical electric vehicle battery.
Generational also warned that about one in 200 diagnostic tests revealed a serious imbalance above 100mV. An imbalance of that magnitude can be severe enough to cause vehicle manufacturers to write off the entire battery pack, requiring a complete replacement under warranty or imposing substantial repair costs on the owner if the warranty period has expired.
The findings arrive as electric vehicle campaign groups joined forces to urge the UK Government to introduce mandatory battery State of Health (SoH) certificates for all dealer-sold used electric cars. Advocates argue that mandatory testing would remove one of the largest remaining barriers to mainstream adoption.
Hidden risks in the second-hand market
Generational explained that cell-level imbalances remain completely invisible to drivers. In many cases, no warning light appears on the dashboard, no warning message displays, and no noticeble change in vehicle behaviour alerts a driver to imbalances until they become far more advanced.
These hidden issues create costly complications for motor retailers and vehicle finance businesses. Buyers returning vehicles to dealerships and stopping monthly finance payments result in lengthy dispute cases that require resolution.
Generational clarified that its analysis does not suggest one in 85 second-hand electric cars requires a replacement battery, but rather that testing needs to be undertaken at a deeper level.
Cell-level diagnostics provide retailers with a more in-depth view of battery condition beyond overall capacity. By interrogating individual cells within the battery pack, the tests help dealers identify whether a battery is operating within normal parameters or showing signs of divergence.
Generational pointed out that current battery State of Health data shows overall capacity, but does not necessarily reveal whether every individual cell within the pack is operating correctly. As such, a vehicle can display a stable State of Health reading while still developing faults at the cell level.
Expert calls for cell-level testing
Oliver Phillpott, chief executive officer of Generational, emphasized that capacity checks alone do not provide a complete picture of battery condition. "Reporting on State of Health is essential for used EVs, but it does not tell the full story of battery condition. It measures capacity, not necessarily whether the cells inside the pack are functioning correctly," Phillpott said.
"Cell imbalance problems are often imperceptible while the battery is developing a fault that could affect range, charging or value later on. The only practical way to see that risk early is to test for it," he added.
Phillpott noted that the electric vehicle sector has demonstrated the durability of modern battery systems. "The industry has made significant progress in showing that EV batteries are generally far more robust than many people assume," he said.
"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. 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," Phillpott said.
Mandatory certification and industry context
Full confidence in used EV batteries is vital for buyers, as a battery pack can often represent more than 40 per cent of a vehicle's total cost. A high-voltage traction battery is the most expensive single component in an electric vehicle, making independent condition testing essential as the UK electric vehicle fleet ages.
Data from the Society of Motor Manufacturers and Traders (SMMT), the trade body representing the UK automotive industry, shows used EV transactions reached record levels in the first quarter of 2026. At the same time, industry research continues to identify battery confidence as one of the key factors shaping consumer consideration of electric vehicles.
Campaign groups Electrifying.com, New AutoMotive, and BatteryIQ say it is vital for used EVs to have certification confirming they are still in good working order. They argue this would deliver the same level of transparency drivers receive from an MOT certificate, service history, and mileage records when buying a pre-owned petrol or diesel car.
The organizations argue that the real problem facing buyers is not battery health itself, but the lack of clear, independent information available when a vehicle is sold.
How cell imbalances degrade battery function
In a modern electric vehicle battery pack, hundreds of individual cells operate together as a single integrated system. For the pack to deliver reliable driving performance and range, those cells need to remain closely balanced with one another.
If one cell begins to drift significantly from the others, the vehicle's automated battery management system may limit charging or discharging to protect the weakest cell. This internal protection mechanism can potentially affect driving range, State of Charge estimates, and charging behaviour, and in more serious cases require expensive module or battery replacements.
Generational is advocating regular testing, recommending annual checks that identify cells beginning to drift while owners or dealers still have time to investigate. Catching issues early could allow remedial work to be completed while the vehicle's battery warranty remains valid.
