When Kerry Dunstan and his partner set out to buy a new electric car this summer, one of the questions they asked was, “How’s the battery?”.

They’d found a 2021 Nissan Leaf with just 29,000 miles on it, and the dealer told them the condition of the battery, or its state of health (SOH), was still around 93%.

The couple were sold. For £12,500, they got an EV with a big boot and plenty of room for passengers.

Though Mr Dunstan, a cabinetmaker who also owns a somewhat snazzier electric Volvo SUV, hasn’t quite fallen in love with the aging Leaf.

“I like sporty, jazzy cars – and it’s just a bit ‘meh’,” he says.

However, he adds the Leaf has performed exactly as expected during the three months they’ve owned it.

It used to be that age and mileage were the two headline details pored over by would-be buyers of second-hand cars. But as more people shift to electric, scrutinising the health of a car’s battery has become arguably even more important.

How has that battery been treated? Did the last owner regularly fast charge it to 100%, for example? That has the potential to shorten an EV battery’s lifespan.

This battery black box problem has put some consumers off buying a second-hand EV. But battery analytics firms say they can reveal the condition of an old EV’s battery with high accuracy. And industry experts say some EVs are lasting longer than many predicted.

Take Mr Dunstan’s Nissan Leaf. This is a model of EV built without the kind of sophisticated, liquid-based battery cooling system common to many other EVs. While Nissan has rectified this in the latest generation of Leafs, earlier models show a considerable shortening of their range year by year, according to data analysed by US insurance and research firm NimbleFins.

Mr Dunstan is unfazed. “I charge both my EVs to 100% and I put them on charge when I need to charge them – I don’t worry about it,” he says.

For people in the market for a second-hand EV who are plagued by battery anxieties, however, Austria-based firm Aviloo says it has a solution. “We really can, completely independently, determine the state of health of a battery,” says chief product officer, Patrick Schabus.

Aviloo is one of several battery analytics businesses in the market. The company, which provides battery health certificates for major UK outlet British Car Auctions, offers two products.

There’s a premium test, where EV owners plug a data logging box roughly the size of a glasses case into their car so that it can monitor battery performance while they use the car over a few days, going from 100% charge down to 10%.

Or, they can opt for a quicker flash test, which uses a different box to suck up data from the car’s battery management software and then analyse it with the help of a computer model. “We can do this at a standstill in under two minutes,” says Mr Schabus.

The premium test observes battery discharge closely, picking up fluctuations in current or voltage, and can reveal extra detailed information about the health of individual cells in the battery, says Aviloo.

Marcus Berger, Aviloo’s chief executive, says his company’s analytics results sometimes diverge “substantially” from the battery SOH percentages produced by some cars’ own built-in analytics systems.

He challenges conventional wisdom that batteries with an SOH below 80% are too far gone: “An EV with a state of health below 80% can still be a great car… It just needs to be priced [appropriately].”

In New Zealand, EV owner Lucy Hawcroft, who works in a sustainability role for an infrastructure firm, bought a Nissan Leaf with her husband roughly three years ago. She recalls getting an SOH result of 95% or so from the dealership. But a year later an independent mechanic checked the SOH again for them.

“It dropped quite a bit,” she recalls. “My husband was a bit surprised, or concerned, about that.”

However, the car still has a range of around 160km (100 miles) when fully charged. The pair mostly use it for short journeys of up to 10km. Mrs Hawcroft says she has friends whose EVs have much bigger ranges, of around 400km: “That would be ideal.”

For David Smith, sales director at Cleevely Electric Vehicles in Cheltenham, being able to analyse used EVs’ batteries in detail is a deal-maker. Most customers ask for this information, he says. His company uses SOH reports from ClearWatt, another battery analytics firm.

“They’re completely independent. We can’t interfere with the reports,” he says. “Once customers have seen the report, that aids the sale nine times out of 10.”

Matt Cleevely, managing director at Cleevely Electric Vehicles, adds that it’s often possible to replace groups of cells or modules within a battery pack – far cheaper than installing a whole new battery.

As for how you should charge your own EV in order to best take care of its battery, Simona Onori at Stanford University says, “There is likely a sweet spot between frequent fast charging and avoiding it altogether.” However, she adds that, to her knowledge, this is not something that has yet been thoroughly studied.

Despite some consumers’ wariness, battery technology has noticeably improved in recent years, says Max Reid, head of battery costs at market research firm CRU. “The older batteries might last maybe 500 to 1,000 [charging] cycles,” he explains. “Now, it’s 10,000 cycles in some of these new EV cells coming out.”

Batteries that are no longer good enough for the EV they were designed for can still be useful, says Paul Chaundy at Second Life EV Batteries, in Dorset. For instance, some of his customers are businesses that use former EV batteries to store electricity at their premises. They might have, say, six electric forklifts but a grid connection only large enough for charging ports to supply two or three of the forklifts.

Regarding the varying methodologies car manufacturers use to generate SOH reports for their own vehicles, Mr Chaundy says, “We need more standards around that, I think.”