Generational Finds Hidden Battery-Cell Imbalances in One of 85 Used EVs

About one in 85 used electric vehicles in a UK dealer dataset had battery-cell voltage divergence above Generational’s 50-millivolt threshold, even though such an imbalance may not trigger a dashboard warning or undermine an overall capacity score. The battery-diagnostics company reported the result on August 26 after analyzing tests from approximately 10,000 battery-electric vehicles processed through hundreds of UK dealers.

The finding matters because a traction battery operates as a pack, but its limits can be set by an individual weak cell. A used EV can retain apparently stable overall capacity while one cell begins behaving differently from the rest. That divergence may eventually influence usable range, charging behavior, state-of-charge estimates and repair decisions.

What the one-in-85 figure measures

Generational found that around 1.2% of the tested vehicles had a difference exceeding 50 millivolts between the pack’s highest- and lowest-voltage cells. That threshold was more than six times the median difference in a typical vehicle within its dataset. Around one in 200 tests exceeded 100 millivolts, a level the company characterized as serious and potentially sufficient for a manufacturer to write off a pack.

Those thresholds are Generational’s diagnostic categories, not universal regulatory limits. The analysis also does not mean that one in 85 used EVs needs a replacement battery. A flagged voltage spread identifies a condition requiring further investigation; it does not, by itself, establish the cause, repair path, warranty outcome or remaining service life.

Important details needed to judge how broadly the result applies were not disclosed. The makes, models, ages, mileages and regional distribution of the vehicles were not identified, nor was enough diagnostic methodology provided to compare the results across battery chemistries and pack architectures. The dataset therefore describes vehicles moving through participating UK dealers, not the entire UK fleet and not the U.S. used-EV market.

Capacity and cell balance answer different questions

A conventional battery State of Health figure generally describes how much capacity remains compared with the battery when new. That is useful for estimating degradation, but it does not necessarily show whether all cells are operating consistently. Generational CEO Oliver Phillpott said State of Health measures capacity, not necessarily whether the cells inside the pack are functioning correctly.

Cell-level testing asks a different question: how far apart are the highest- and lowest-voltage cells under the test conditions? When one cell reaches a protective upper or lower limit before the others, the battery-management system may restrict charging or discharging to protect that weakest cell. The driver experiences the pack-level restriction even if most cells could accept or deliver more energy.

This weakest-cell effect is consistent with published battery-pack simulation research showing that cell-capacity variation can reduce usable EV range nonlinearly because the lowest-capacity cell constrains a series-connected pack. That research does not validate Generational’s prevalence estimate or thresholds, but it helps explain why aggregate capacity and cell-to-cell consistency are separate diagnostic dimensions.

What a useful used-EV certificate would need

The UK result supports closer scrutiny of what a dealer-provided battery certificate should contain. A single percentage is easy for buyers to understand and compare, but it can compress several distinct conditions into one number. Capacity retention, cell-voltage spread and the circumstances of the test are not interchangeable measurements.

Electrifying.com, New AutoMotive and BatteryIQ have called for mandatory State of Health certificates for dealer-sold used EVs in the UK. That proposal has not been adopted as government policy. Generational separately advocates regular cell-level checks, particularly annual testing, so developing issues can be investigated while any applicable battery warranty remains valid.

For U.S. buyers and policymakers, the immediate gap is evidence rather than a ready-made rule. Comparable American data would need to identify vehicle age, mileage, model, battery chemistry, test conditions and subsequent repair outcomes. It would also need to establish whether adding cell-voltage information improves consumer decisions without generating unnecessary pack replacements from isolated readings.

The UK analysis makes a narrower but consequential point: stable overall battery capacity does not prove that every cell is tracking normally. Whether that distinction becomes part of standardized U.S. used-EV disclosures will depend on transparent methods and fleet-specific validation that the current dataset does not yet provide.

By Thomas Caldwell — AMI’s senior editor for mechanical and mobility engineering, covering vehicle electronics, systems integration, electrification, chassis systems, propulsion, and safety policy.