Used EV battery health: what the numbers actually show

Recurrent pulls data from more than 30,000 EV drivers. The research firm describes battery ageing as an "S curve": a rapid decline at the start, a long level stretch, then a sharper fall at the end. Liz Najman, the firm's director of market insights, told NPR the process is "very much like breaking in a pair of shoes."

For second-hand buyers, the useful part is where most cars sit on that curve. Cox Automotive has tested nearly 80,000 EVs and found an average battery health of 92%, according to NPR. Adam George, a vehicle services director at Cox, said the company had expected "mass degradation over the first one to three years of owning a vehicle." Instead, two, three and four-year-old off-lease cars came back with battery health scores "well upwards of 95%." That gap between expectation and result is the core of the used EV story so far.

What the older cars show

The oldest EVs are the hardest to judge, because few were built. NPR reported that only a million EVs were sold worldwide between 2010 and 2018, against more than a million a year now.

Recurrent's data, based on its opt-in community, found that among EVs ten years old or older, only 8.5% had ever had a battery replacement. More than 90% were still on their original pack. For cars that have covered distance rather than time, Recurrent looked at EVs of any age with more than 150,000 miles. Najman said those without a replacement battery were getting at least 83% of their original range. NPR noted one common reason for very early replacement: a manufacturing defect. Because new EVs carry warranties, typically at least eight years and 100,000 miles, that kind of failure is not a financial hit for the owner.

"An EV with a state of health below 80% can still be a great car… It just needs to be priced [appropriately]."

That last point matters more than it sounds. Marcus Berger, chief executive of the Austria-based battery analytics firm Aviloo, told the BBC he challenges the conventional wisdom that a battery below 80% state of health is finished.

Aviloo sells two tests. The premium version plugs a data logging box the size of a glasses case into the car for a few days and runs the battery from 100% down to 10%. The flash test reads the battery management software at a standstill. Patrick Schabus, the company's chief product officer, said the flash test takes "under two minutes."

The measurement problem

Independent reports are already part of the sales process. David Smith, sales director at Cleevely Electric Vehicles in Cheltenham, told the BBC that most customers ask for battery data and that his company uses 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."

The catch is that carmakers generate their own state of health figures using different methods. Berger said Aviloo's results sometimes diverge "substantially" from the percentages produced by cars' built-in systems. Paul Chaundy of Second Life EV Batteries in Dorset put it plainly to the BBC: "We need more standards around that, I think."

Charging habits add another variable. 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. Battery hardware has moved on regardless. Max Reid, head of battery costs at the 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."

For buyers, the practical picture is less dramatic than the early predictions. In 2010 the New York Times wrote that estimates of EV battery pack lifespan ranged "upward from seven years," as NPR recalled. Fifteen years on, the data pooled from tens of thousands of vehicles points the other way, even if the label on the report still depends on who produced it.