Study: 1 in 85 Used EVs Has Hidden Battery Cell Imbalance
CEO Oliver Phillpott says testing beyond State of Health can catch imperceptible faults before they worsen.
Generational, a UK-based electric vehicle battery diagnostics company, has published new analysis of individual battery cells from approximately 10,000 used-EV battery tests conducted through hundreds of automotive dealers across the UK.
The data shows that while the overwhelming majority of used EV batteries remain well balanced, a small but important minority exhibit cell-level imbalances that are not visible through headline State of Health data.
Across Generational's test dataset, the median used EV showed a voltage difference of just 8 millivolts (mV) between the highest and lowest battery cells – indicating that the typical battery is operating within a tightly balanced range.
However, around 1 in 85 tests showed a significant imbalance above 50 mV, a level requiring investigation. Around 1 in 200 tests showed a serious imbalance above 100 mV, indicating a potential underlying fault requiring further assessment.
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.
'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.'
Cell-level testing – the benefit of better transparency
In a modern EV battery pack, hundreds of individual cells operate together as one system. For the pack to deliver reliable performance, those cells need to remain closely balanced. If one cell begins to drift materially away from the others, the battery management system may limit charging or discharging to protect the weakest cell – potentially affecting range, state-of-charge estimates, charging behaviour and, in more serious cases, requiring expensive module or battery replacements.
Generational's cell-level diagnostics is designed to give dealers a deeper view of battery condition beyond overall capacity. 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.
While State of Health remains an important measure of battery capacity, it does not necessarily reveal whether every individual cell within the pack is operating correctly. A vehicle can display a stable State of Health reading while still developing faults at cell level.
The cost of voltage imbalance and lack of insight for the industry and consumers
The company's analysis shows that while such issues remain uncommon, they are often invisible to the driver. In many cases, there may be no warning light, dashboard message or noticeable change in vehicle behaviour until the imbalance becomes more advanced.
These issues can prove to be costly for dealers and finance businesses in both time and repairs, with consumers returning vehicles to dealerships and ceasing finance payments – resulting in protracted cases to be resolved.
Given that EV batteries can often represent over 40% of the 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. SMMT data shows used BEV transactions reached record levels in the first quarter of 2026, while industry research continues to identify battery confidence as one of the key barriers shaping consumer consideration of electric vehicles.
The company stresses that the battery is the single most valuable component in an EV, and that understanding of how batteries work has not yet caught up with how many are now passing through the used market.
An EV battery is built from several hundred cells, connected in groups wired in series, and they must work in unison. The pack stops charging when the first cell fills and stops discharging when the first cell empties, so one weak cell limits the whole battery – which a pack-level average will not show.
Cells also drift apart and back together as the battery charges and discharges, so the state of charge and recent usage plays a critical role. For example, a borderline imbalance measured at a low state of charge does not always indicate a persistent fault, while a large imbalance that remains after an attempted rebalance is materially more significant.
When an issue is identified, Generational works with customers to support next steps – including helping them attempt a guided cell rebalance, testing the vehicle at rest and confirming whether the issue persists before deciding whether further investigation, warranty support or repair action is required.
Generational's insights into the current state of play
The analysis builds on Generational's wider battery diagnostics platform, which is designed to help automotive retailers, leasing companies, remarketers and workshops assess used EV batteries quickly and consistently.
Generational's recent Battery Performance Index, based on more than 8,000 EV battery assessments across the UK, found that average battery State of Health stood at 95.15%, with many older and higher-mileage vehicles performing strongly. However, the Index also found that variance between vehicles increases with age, reinforcing the importance of transparent testing to identify the minority of underperforming batteries within a broader population of healthy vehicles.
The company champions battery transparency as becoming essential infrastructure for the automotive industry, as the used EV market continues its upward trajectory. Verified condition data can help retailers price vehicles more confidently, protect residual values, reduce acquisition risk and give consumers clearer information about the most valuable component in an electric vehicle.
About the 2025 Battery Performance Index
The Index is based on more than 8,000 battery health assessments conducted in 2025 across the UK used EV market, covering:
0-12-year-old vehicles (2013–2025 model years)
0-160,000+ miles
36 manufacturers
Passenger BEVs, PHEVs and LCVs
The report will be published annually, establishing a recurring national benchmark for battery condition in the UK.
About Generational
Generational is a UK-based innovator in electric-vehicle battery-condition testing solutions. The business provides simple, intuitive electric-vehicle battery-condition checks and state-of-health (SoH) certification services to car dealers and remarketers.
Generational was founded by a team of automotive software engineers and data scientists with deep expertise in EV technology, from Red Bull Racing, Imperial College London, and the University of Cambridge.
Its ultimate mission is to facilitate a thriving market for used EVs – helping businesses sell vehicles faster and more profitably, maximise residual value, and build a robust business-case for investing in the pre-owned EV market.
