Words: Richard Edwards
Published 25 August 2026
One in 85 used EVs carries a battery-cell imbalance invisible to the state of health figure buyers rely on, according to analysis of around 10,000 battery tests conducted through hundreds of UK dealers.
The data, from British diagnostics firm Generational, mostly tells a reassuring story. The median used EV in the sample showed just 8 millivolts of difference between its strongest and weakest cells, and the company’s wider index of more than 8,000 assessments puts average state of health at 95.15%, with many older, higher-mileage cars performing strongly.
The exception is the reason for the research. Around one test in 85 found a cell-voltage spread above 50mV, more than six times the typical car and enough to warrant investigation. One in 200 exceeded 100mV, a level at which some manufacturers’ own inspection procedures treat a pack as failed.
Why the headline number misses it
State of health measures how much capacity a battery retains. It does not reveal whether every cell inside is pulling its weight, and a pack is only as good as its weakest cell: charging stops when the first cell fills, discharging stops when the first cell empties. A car can display a healthy state of health while one drifting cell quietly limits range, charging speed and value, usually with no warning light or change in behaviour until the problem is advanced.
“Reporting on state of health is essential for used EVs, but it does not tell the full story of battery condition,” says Generational chief executive Oliver Phillpott. “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.”
Where a fault is flagged, Generational guides dealers through an attempted cell rebalance. Around half of the issues cleared; the other half persisted, with the severity of the imbalance predicting whether it could be fixed without repairs.
Why it matters here
The findings are British, but the buying culture they describe is thoroughly familiar. New Zealand’s used EV market was raised on the Nissan Leaf’s battery bars, and state of health has become the number that sells or sinks a used electric car, quoted in listings and checked with apps before test drives. This data suggests that figure, while necessary, is not sufficient.
It also lands on a nerve: Trade Me research published this month found battery condition has overtaken range as the biggest barrier holding Kiwi buyers back from EVs. Cell-level transparency is precisely the kind of answer that concern is waiting for, and with batteries representing 40% or more of an EV’s value, the case for testing beyond the headline number gets stronger as the fleet ages. For the small minority of packs that do fail, a local repair and upgrade economy is already emerging.
