Battery health at 90%: does that mean the battery is fine?

You're at a dealer, looking at a used electric car. The screen shows battery health at 90%, and the salesperson adds: "The battery's in great shape, practically new." That takes care of half your worries.

In short: not necessarily. Battery health tells you how much energy the whole pack can still hold, not whether one cell inside is falling behind. Most used EVs have well-balanced cells; a few deserve a closer look^[1].

Battery health and cell balance aren't the same thing

Battery health (state of health, or SoH) is how much energy the whole pack can hold now, compared with when it was new. Generational, a UK company that tests EV batteries, puts it plainly: health measures capacity, not necessarily whether every cell inside is working properly^[1].

A pack is really several hundred cells, joined in series groups^[1]. Cell balance is about whether their voltages line up. It's usually measured as the gap between the highest and the lowest cell, in millivolts (mV), thousandths of a volt^[1].

Cells in series are like a group of hikers roped together: the group moves at the pace of its slowest member. When you charge, the weakest cell fills first and the whole pack stops charging. When you drive, it runs out first and the whole pack stops too^[1].

Why cells drift apart

A 2026 study in the journal Nature Energy, by researchers in China and Sweden, sets out the causes^[2]. Cells leave the factory very slightly different, from their materials, their manufacturing and the way they are grouped. Inside the pack, position and cooling layout mean some run warmer than others, so they age at different rates. A few cells carry hidden defects and can fail early^[2].

Even in identical conditions, cells don't age together. The Technical University of Munich stored 24 cells of the same type at 25°C for 10 months. Each one aged at its own rate, and the spread in their capacity almost doubled^[3]. The same study found that small differences in self-discharge between cells have almost no effect on balance^[3].

That's why the battery management system balances the cells while charging, slowly bringing their charge levels into line. In the Nature Energy study, the cars balanced continuously while charging, and their cell voltage gap rarely went above 100 mV. The buses, which didn't balance, reached gaps of up to 300 mV^[2].

In other words, balancing is like asking the hikers at the front to wait. It doesn't make the slowest one any stronger: balancing fixes cells that are out of line, not a cell that has weakened.

10,000 UK tests: most cars are well balanced

Generational analysed around 10,000 used-EV battery tests from hundreds of dealers across the UK^[1]. For the typical car (the median), the highest and lowest cells were just 8 mV apart^[1].

A few cars were different. About 1 in 85 had a gap above 50 mV, which Generational says needs investigating. About 1 in 200 went above 100 mV, which it says points to a potential underlying fault^[1]. Gaps like these don't show up in the health figure, and usually no warning light comes on^[1].

This is UK data, and 50 and 100 mV are Generational's own thresholds; other testers may use different ones. Taiwan is hotter than the UK, and there are no public figures yet on whether the share is the same here.

What happens when the gap grows?

Range and charging: when the gap grows, the battery management system limits charging or discharging to protect the weakest cell. That can mean less range, a less accurate charge reading, or charging that behaves differently^[1]. If it comes to a repair, that can mean replacing a module, or even the whole battery^[1]. In Taiwan, a taxi driver recently said Tesla linked his car's battery fault to cell voltage differences that hadn't been balanced: see "Out of warranty by 3 days, NT$650,000 out of pocket".

Battery life: the Nature Energy study followed 116 electric cars and 17 electric buses in China, up to 300,000 km each^[2]. By the end, the weakest cells left 6.2% of the cars' battery health unusable on average, and about 17% in the worst car^[2]. Because of the weakest cell, the packs also retired 17.7% earlier than their average cell would have lasted^[2]. Cells that were simply out of line cost far less, usually under 2%, and balancing largely deals with that^[2].

For a car with 500 km of range on a full charge, 6.2% is about 31 km. But that's the average at the end of the study, not what every car will see. In this fleet, the cell gap mostly stayed small up to about 170,000 km. After that, some cars of the same model pulled apart sharply, while others stayed tight all the way to 300,000 km^[2].

What to ask when buying used

1. Start with battery health: the car's own figure is a good starting point, telling you how much the whole pack still holds^[1].
2. Then ask for the cell voltage gap: how many mV between the highest and the lowest cell, and whether it was measured with the car at rest^[1].
3. Read it against that report's thresholds: they differ between testers. Generational's: above 50 mV, look further, perhaps with rebalancing or a repair; above 100 mV, a possible fault^[1].
4. If the gap is clear, ask an expert: let the maker or a workshop that knows EVs decide whether it needs work^[1]. If the car is still under warranty, ask the maker first.
5. Measure again later: the cell gap changes with mileage, and readings compared over time tell you more than a single one^[2].

Most used EVs have well-balanced cells: in the UK, the typical car's were just 8 mV apart^[1]. Battery health tells you how much the battery holds; cell balance tells you whether its cells are keeping together. Look at both. CELLPROOF's test reads the cell voltage gap with the car at rest and lists it beside battery health.

Battery health covers the whole pack; cell balance watches the weakest cell.