Used EV Battery Health: What to Check Before You Buy
The good news is that EV batteries degrade far more slowly than early fears suggested, and most used EVs on the UK market have plenty of healthy life left. The bad news is that a neglected battery can be a five-figure problem. This guide explains how batteries age, what to check before you buy, and how to spot the red flags that should send you walking away.
How EV batteries degrade
All lithium-ion batteries lose capacity over time, and two forces drive it. Calendar ageing is the slow chemical decline that happens whether you drive the car or not — think of it like a slow leak. Cycle ageing comes from charging and discharging. In practice, most modern EVs lose roughly 1 to 2 percent of capacity per year in normal UK use, with the fastest loss in the first year or two before the curve flattens.
Heat is the enemy. Batteries kept cool by liquid thermal management — as in Teslas, the MG4, the Kia e-Niro and most modern EVs — age far better than those cooled only by air. The early Nissan Leaf, which relied on passive air cooling, is the famous cautionary tale: high-mileage early Leafs can show serious degradation, especially cars that lived in hot climates or were rapid-charged constantly. Fast charging itself is not the villain it is sometimes made out to be, but habitual charging to 100 percent and leaving the car sitting full, or regularly running it to empty, does accelerate wear.
Mileage matters less than you might think. A 60,000-mile EV that was charged gently on a home charger to 80 percent will often have a healthier battery than a 30,000-mile car that was rapid-charged twice daily as a taxi. Charging habits and thermal management tell you more than the odometer.
What degradation looks like in practice
A battery at 90 percent state of health (SoH) simply means it holds 90 percent of its original usable capacity. A car rated at 250 miles new will do roughly 225. For most drivers that is barely noticeable. Problems start when degradation is severe — say 75 percent SoH on a car that was marginal on range to begin with — because every journey needs more planning and the car's resale value drops.
Real-world data is reassuring. Fleet telematics studies of hundreds of thousands of EVs show average degradation of around 1.8 percent per year, and many cars with 100,000 miles still retain over 85 percent of original capacity. Batteries rarely fail outright; they fade gradually, and the car's battery management system protects against the worst abuse.
Get a battery health report
The single most useful document when buying a used EV is a battery health report showing the state of health percentage. Several routes exist. Many dealers now provide reports from diagnostic tools as part of approved used preparation. Independent specialists across the UK offer battery health checks for around £50 to £100, measuring SoH, cell balance and sometimes fast-charge history. Some manufacturers expose battery data through their apps — Tesla's service mode and third-party apps can read detailed figures, and brands like BMW and Volkswagen show battery health in dealer diagnostics.
Ask the seller for a recent report before you travel to see the car. A seller who cannot or will not provide one is not necessarily hiding something, but you should factor a £50 to £100 independent check into your plans. If the reported SoH seems too good for the age and mileage — a six-year-old car claiming 98 percent — treat it with scepticism and verify independently.
What the numbers mean
Above 90 percent SoH: excellent, buy with confidence.
80 to 90 percent: normal for a car of four to eight years old, perfectly usable.
75 to 80 percent: acceptable only if the price reflects it and the range still covers your needs.
Below 75 percent: walk away unless the car is priced as a bargain and you understand the range limitation.
Check the battery warranty
Almost every EV sold in the UK carries a separate battery warranty, typically eight years or 100,000 miles, guaranteeing the battery against excessive degradation — usually defined as dropping below 70 percent capacity. This is your safety net, and on a used car you should check how much of it remains.
Confirm the warranty transfers to new owners — it almost always does — and check the exact terms. Some warranties cover only outright failure, not degradation, so read the wording. Also confirm the car has been serviced to schedule where required; a few manufacturers tie battery warranty validity to service history. A car with three years of battery warranty remaining is a fundamentally safer buy than one with three months.
Look at the charging history
If you can, find out how the car was charged. A car with a home charger that was charged overnight to 80 percent is the ideal history. Heavy rapid-charging leaves traces: some diagnostics report the number of DC fast-charge sessions, and taxi or fleet history is a warning sign, though not a dealbreaker if the SoH checks out.
Ask the seller direct questions: did you charge to 100 percent regularly, was it rapid-charged often, was it left plugged in at full charge for long periods? Honest answers are a good sign in themselves. Vague or evasive answers suggest you should lean harder on the diagnostic report.
Do a real-world range test
On the test drive, go beyond a loop around the block. Note the battery percentage and the predicted range at the start, drive a mixed route of at least 20 miles, and compare the percentage used against the miles covered. A car showing 100 percent and 200 miles that drops to 70 percent after 40 gentle miles has a problem — or at best a very pessimistic gauge worth investigating.
Watch for uneven behaviour: range estimates that jump around wildly, sudden drops in percentage, or the car entering reduced-power mode unexpectedly can indicate cell imbalance or a weak module. Some imbalance is normal and can improve with a full slow charge to rebalance the cells, but severe symptoms need professional diagnosis.
Check for recalls and software updates
Battery-related recalls have affected several models — LG-supplied batteries in early Hyundai Kona and Chevrolet Bolt cars, for example, and software updates that changed charging behaviour on others. Check the DVSA recall database for the specific car, and ask the dealer to confirm all battery software updates have been applied. An unapplied battery update can affect both safety and warranty validity.
Inspect the car like any used car
Do not let the battery distract you from the basics. Check tyres, brakes, suspension and bodywork as normal — EVs are heavy and can wear tyres and suspension faster. Check that both charging cables are present, that the car charges on AC and, if applicable, DC, and that the charge port flap and locking pin work. A pre-purchase inspection by an EV-savvy mechanic, costing £150 to £250, is money well spent on any used EV above £10,000.
Know the replacement cost reality
Out-of-warranty battery replacement is expensive — £5,000 to £15,000 depending on the car — which is why the health check matters so much. But outright replacement is rare. Module-level repair, where a specialist replaces only the failed cells, is increasingly available in the UK for £1,000 to £3,000 and can restore a degraded pack. Factor this into your thinking: a slightly degraded battery is a negotiating point, not necessarily a dealbreaker, because repair options exist beyond the main dealer.
Red flags that should stop the deal
No battery health data and a seller unwilling to allow a check.
SoH below 80 percent on a car under six years old.
Warning lights or error messages relating to the battery or charging system.
A car imported from a very hot climate with an air-cooled battery.
Evidence of accident damage near the battery pack or undertray.
Battery warranty already expired with significant degradation present.
Bottom line: a used EV is one of the best-value buys in the UK market right now, but only if the battery is healthy. Get a state-of-health report showing 85 percent or better, confirm how much of the eight-year battery warranty remains, check the charging history and recalls, and do a real-world range test before you commit. A clean bill of health on the battery turns a used EV from a gamble into one of the cheapest cars to run that you can buy — prices mentioned here are guidance only, so check current listings before you decide.