One in 85 used electric vehicles has a hidden issue with its battery that could affect the car's range, charging capabilities and resale value.New analysis has found what it calls 'battery cell-level imbalances' in around 1.2 per cent of second-hand EVs analysed.Tests of approximately 10,000 battery-powered cars conducted through hundreds of automotive dealers across the UK show that, while most used EV battery packs are functioning correctly, a small proportion have a cell-level voltage imbalance above 50 millivolts (mV).This is more than six times the typical vehicle's median voltage difference in mV between the highest and lowest battery cells.The extensive examination of cell-level data by EV battery diagnostic company Generational also flagged that around one in 200 tests showed a 'serious' imbalance above 100mV.This could be severe enough for manufacturers to write off the battery pack, requiring a replacement under warranty or, if outside the warranty period, at significant cost to the owner.It comes as electric car campaign groups joined forces this week to call on the Government to introduce mandatory battery State of Health (SoH) certificates for all dealer-sold used EVs, arguing the move would remove one of the biggest remaining barriers to mainstream adoption. Generational analysis finds one in 85 used EVs has a hidden battery cell imbalance that could affect performance Generational's analysis raises questions about EV battery health and the longevity of used EV batteries, as the issues flagged are invisible to drivers.In many cases, it said there was no warning light, dashboard message or noticeable change in vehicle behaviour to alert a driver to imbalances until they became more advanced.These issues can prove costly for dealers and finance businesses in both time and repairs, with consumers returning vehicles to dealerships and ceasing finance payments, resulting in lengthy cases that need to be resolved.However, Generational is not suggesting that one in 85 second-hand EVs needs a replacement battery, simply that testing needs to be undertaken at a deeper level.Cell-level diagnostics are designed to give dealers a more in-depth view of battery condition beyond overall capacity, as the test interrogates individual cells within the battery pack, helping retailers identify whether a battery is operating within normal parameters or showing signs of divergence.Generational points out that battery State of Health data currently 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. Generational points out that battery State of Health data currently shows capacity but does not necessarily reveal whether every individual cell within the pack is operating correctly Oliver Phillpott, CEO of Generational, said: '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.'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: 'The industry has made significant progress in showing that EV batteries are generally far more robust than many people assume.'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.'Full confidence in used EV batteries is needed, as a battery can often represent more than 40 per cent of a vehicle's cost.The importance of testing battery condition on a case-by-case basis is paramount, particularly as the UK's EV parc ages.Society of Motor Manufacturers and Traders data shows used EV transactions reached record levels in the first quarter of 2026, while industry research continues to identify battery confidence as one of the key factors shaping consumer consideration of electric vehicles.Electrifying.com, New AutoMotive and BatteryIQ say it is vital for used EVs to have certification confirming they are still in good working order, delivering the same level of transparency drivers receive from an MOT certificate, service history and mileage records when buying a used petrol or diesel car.The organisations argue that the real problem is not battery health itself, but the lack of clear, independent information available when a vehicle is sold. Generational is advocating regular testing, particularly annual checks that identify cells beginning to drift while owners or dealers still have time to investigate How cell-level problems can stop a battery working – and why catching issues early is vital In a modern EV battery pack, hundreds of individual cells operate together as a single system.For the pack to deliver reliable performance, those cells need to remain closely balanced. If one cell begins to drift significantly from the others, the battery management system may limit charging or discharging to protect the weakest cell.As mentioned, this can potentially affect range, State of Charge estimates and charging behaviour and, in more serious cases, require expensive module or battery replacements.Therefore, Generational is advocating regular testing, particularly 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.