The Used EV Battery Question: What 80,000 Tests and 30,000 Drivers Actually Show

Kerry Dunstan and his partner went shopping for a used electric car this summer. The first question they asked the dealer was not about mileage, service history or tyres. It was about the battery.

They found a 2021 Nissan Leaf with 29,000 miles on it. The dealer told them its state of health, the industry's standard measure of how much capacity a pack has lost, was still around 93%. They paid £12,500 and drove it home.

Three months later, Mr Dunstan, a cabinetmaker who also runs an electric Volvo SUV, is not in love with the car. "I like sporty, jazzy cars, and it's just a bit 'meh'," he told the BBC in October. But the Leaf has performed exactly as expected, he said. That is the quiet finding emerging from a growing pile of battery data: the second-hand electric car is usually a better bet than its reputation suggests.

What the auction data shows

Cox Automotive runs used car auctions around the country. It has now evaluated batteries on nearly 80,000 used EVs before they go back on sale. The average state of health across that pool is 92%, according to NPR, which reported the figures in March. Adam George, a vehicle services director at Cox, told NPR the company had expected something worse.

"We were expecting battery health to be experiencing mass degradation over the first one to three years of owning a vehicle," George said. "What we have seen, though, is that these 2, 3, 4-year-old off-lease cars that are coming back have battery health scores well upwards of 95%."

Recurrent, a research firm that gathers data from more than 30,000 EV drivers, sees a similar pattern. Its data shows cars from most major brands retain 95% or more of their expected range after three years. Liz Najman, Recurrent's director of market insights, attributes part of that to software and battery management systems designed to compensate for early degradation and keep the range number steady on the dashboard. For buyers, the practical takeaway is that the steep early drop many feared is closer to 5%, give or take.

The shape of decline

Battery ageing is not a straight line. Lithium-ion packs degrade in two ways. Calendar ageing happens slowly with time, even when the car sits unused. Cyclical ageing comes from repeated charging and discharging. Recurrent describes the combined effect as an "S curve": a quick early dip, a long plateau, then a sharper fall at the end of life.

Najman compared it to breaking in a pair of shoes. "It's very much like breaking in a pair of shoes," she told NPR. The shoes feel stiff at first, then soften slightly, and then last a long time before the final decline. Applied to a car, that means the middle years of an EV's life are often the most predictable ones.

The evidence from older cars backs this up. Among EVs that are ten years old or older in Recurrent's community data, only 8.5% have ever had a battery replacement. More than 90% are still running on their original pack. For cars of any age with more than 150,000 miles and no replacement, the same dataset shows they are still getting at least 83% of their original range. Recurrent's Najman is careful about the limits of this. The dataset is small, because few EVs were built more than a decade ago, and some of the oldest models cannot connect to Recurrent's opt-in network at all. Still, her conclusion is blunt: "EV batteries are holding up phenomenally well."

Why SOH has become a market

The gap between real-world battery health and what a dealer or dashboard claims has created a small industry. Austria-based Aviloo sells two tests to buyers and auction houses. The premium version uses a data logger roughly the size of a glasses case, plugged into the car for several days while the driver runs the battery from full down to about 10%.

The second product, a flash test, reads data from the car's own battery management software at a standstill. "We can do this at a standstill in under two minutes," Patrick Schabus, Aviloo's chief product officer, told the BBC. Aviloo provides battery health certificates for the major UK auction outlet British Car Auctions.

Aviloo's chief executive, Marcus Berger, says his company's results sometimes differ substantially from the SOH percentages produced by cars' own analytics systems. He also pushes back on the idea that 80% SOH is a hard cutoff for a usable car. "An EV with a state of health below 80% can still be a great car," he said. "It just needs to be priced appropriately."

Dealers who use independent reports say they help close sales. David Smith is sales director at Cleevely Electric Vehicles in Cheltenham, which uses reports from the battery analytics firm ClearWatt. He told the BBC that customers ask for the data and that the dealership cannot alter the reports. "Once customers have seen the report, that aids the sale nine times out of 10," he said. Matt Cleevely, the company's managing director, adds that swapping individual cells or modules inside a pack is often possible, and far cheaper than fitting a whole new battery.

The replacement bill, and the warranty

When a high-voltage pack does fail, the cost is not small. A replacement can run in the ballpark of $5,000 to $20,000, according to NPR. That figure is the reason battery anxiety exists, and it is also why warranties matter. EV battery warranties typically cover at least eight years and 100,000 miles, and automakers will replace a pack that fails within that window.

George, of Cox Automotive, frames a premature failure as the EV equivalent of a blown engine or transmission: rare, expensive, and exactly what the warranty is for. There have been several large-scale battery recalls tied to defects, and any individual pack can carry a manufacturing flaw. Those replacements are not a financial hit for the owner while the car is covered.

For older cars beyond warranty, the second-life market offers an alternative to scrapping. Paul Chaundy, of Second Life EV Batteries in Dorset, told the BBC that packs no longer good enough for driving still work for stationary storage. Some of his customers run businesses with, say, six electric forklifts and a grid connection sized for only two or three charging ports. A retired EV battery fills the gap.

What buyers should actually do

Three things follow from the data. First, ask for an independent SOH report rather than relying on the dashboard figure. Manufacturers calculate those numbers with different methodologies, and they do not always agree with third-party tests. Chaundy's view, as he put it to the BBC, is that "we need more standards around that."

Second, treat 80% as a negotiating position, not a cliff edge. Range is what matters for the buyer's actual journeys. Lucy Hawcroft, who bought a used Nissan Leaf in New Zealand with her husband, saw the SOH drop fairly quickly after purchase. The car still does around 160km (100 miles) on a full charge, which covers the couple's typical trips of up to 10km.

Third, charging habits are not settled science. Simona Onori at Stanford University told the BBC there is "likely a sweet spot between frequent fast charging and avoiding it altogether," but added that to her knowledge this has not been thoroughly studied. Mr Dunstan, for his part, charges both his EVs to 100% and does not worry about it.

The broader trend is on the buyer's side. Max Reid, head of battery costs at the market research firm CRU, told the BBC that older batteries might last 500 to 1,000 charging cycles, while some new EV cells are reaching 10,000 cycles. The used Leaf on a forecourt today is not the used Leaf that will be on one in five years.