How to read an EV battery health score, and why 80% is not the scrap line

When Kerry Dunstan and his partner bought a 2021 Nissan Leaf this summer, the dealer told them the battery's state of health (SOH) was around 93%. They paid £12,500 for a car with 29,000 miles on the clock. Three months in, the Leaf has "performed exactly as expected", Dunstan told the BBC, even if he finds it a bit "meh" next to his electric Volvo SUV.

That SOH figure is the one buyers pounce on. It is also the one nobody produces the same way twice.

What SOH actually measures

Batteries degrade in two ways. Calendar aging happens with time, even if the car sits unused. Cyclical aging happens with use, as the pack is charged and discharged. "That means there's no way to dodge degradation," NPR reported on 2 March 2026. Drive a lot or drive a little, and the pack will eventually hold less energy.

The decline is not a straight line. Recurrent, a research firm that pulls data from over 30,000 EV drivers, describes it as an "S curve": a fast initial drop, a long plateau, then a sharper fall at the end. Liz Najman, Recurrent's director of market insights, compared it to breaking in a pair of shoes: stiff at first, then comfortable, then suddenly worn out.

The first slope is gentler than feared. Adam George, vehicle services director at Cox Automotive, told NPR his firm had expected "mass degradation over the first one to three years". Instead, two to four year old off-lease cars are coming back with scores "well upwards of 95%". Cox has tested nearly 80,000 EVs and found an average battery health of 92%. Recurrent's data shows most major brands retaining 95% or more of expected range after three years, helped by software that compensates for early losses.

Ten year old cars are the harder test. Among EVs that age or older in Recurrent's community, only 8.5% have ever had a battery replacement, and more than 90% still run on the original pack. For cars with over 150,000 miles, including those that never had a swap, the figure is at least 83% of original range.

Where the 80% rule comes from

Conventional wisdom treats a pack below 80% SOH as finished. Aviloo, an Austria-based battery analytics firm, disputes that. "An EV with a state of health below 80% can still be a great car. It just needs to be priced appropriately," chief executive Marcus Berger told the BBC in an article published on 16 October 2025.

Aviloo sells two tests. The premium one uses a data logging box about the size of a glasses case, plugged into the car, which monitors a discharge from 100% down to 10% over a few days. The flash test reads data from the car's battery management software and runs it through a computer model. "We can do this at a standstill in under two minutes," chief product officer Patrick Schabus said.

Berger also said Aviloo's results sometimes diverge "substantially" from the SOH percentages cars report through their own built-in systems. That is the nub of the problem. There is no single method.

Paul Chaundy, of Dorset-based Second Life EV Batteries, put it plainly to the BBC: "We need more standards around that, I think."

For dealers, independent reports have become a sales tool. David Smith, sales director at Cleevely Electric Vehicles in Cheltenham, uses SOH reports from ClearWatt. "They're completely independent. We can't interfere with the reports," he said. "Once customers have seen the report, that aids the sale nine times out of 10." Matt Cleevely, the firm's managing director, noted that replacing individual cells or modules is often possible and far cheaper than a whole new pack.

What a bad score does not tell you

Model matters as much as the number. The Nissan Leaf that Dunstan bought was built without the liquid-based battery cooling common to many other EVs, and earlier Leafs show considerable year-on-year range loss, according to data analysed by US insurance and research firm NimbleFins. Nissan has changed the design in the latest generation.

Charging habits matter too, but the evidence is thinner than the internet suggests. Simona Onori at Stanford University told the BBC there is "likely a sweet spot between frequent fast charging and avoiding it altogether", adding that to her knowledge this has not been thoroughly studied. Dunstan charges both his EVs to 100% and does not worry about it.

Cell chemistry has moved on regardless. Max Reid, head of battery costs at market research firm CRU, said older batteries might last 500 to 1,000 charging cycles, while "now, it's 10,000 cycles in some of these new EV cells coming out".

Warranties still backstop the worst cases. They typically cover at least 8 years and 100,000 miles, and a defect replacement is on the manufacturer, not the owner. NPR put the cost of an out-of-warranty pack at roughly $5,000 to $20,000. In 2010 the New York Times wrote that estimates of pack lifespan ranged "upward from seven years", against an average car age of more than 12 years.

Retired packs do not necessarily go to waste either. Some of Chaundy's customers use former EV batteries to store electricity at their premises, for example where a grid connection is only big enough to charge two or three of six electric forklifts.

For a used buyer, the practical takeaways are narrower than the marketing. Ask who produced the SOH figure and how, because a car's own dashboard estimate and an independent test can disagree. Treat 80% as a negotiating position rather than a cliff edge. And check the specific model's cooling design and warranty terms before the number.