The battery is the most expensive component in a used electric car — typically 30–50% of the vehicle's entire value — and the hardest one to judge by eye. Get it right, and a used EV is one of the smartest purchases on the used-car market. Get it wrong, and you've bought someone else's expensive problem.
The good news: checking battery health is a learnable process, not a black art. It combines one diagnostic report, one test drive, one charging test and one look underneath the car. Here's how to run all four before money changes hands.
First, Understand What SoH Actually Means
The headline metric is state of health (SoH): the usable capacity the battery holds today compared with when it was new. An SoH of 90% means the battery can still store roughly 90% of its original energy.
Three caveats before you anchor on that number:
- SoH isn't standardized. Manufacturers and diagnostic providers calculate it differently, and the battery management system can adjust usable buffers with software updates.
- Some degradation is completely normal. Real-world data from over 22,000 EVs analyzed by Geotab shows average battery degradation of just 2.3% per year — meaning an average pack still holds around 82% of its capacity after eight years.
- The number alone doesn't tell you if the car suits you. A battery at 85% SoH in a long-range EV may serve a commuter better than a 100% battery in a short-range city car.
Some aging is normal. What you're hunting for is unusual aging: unusually high capacity loss, big differences between cell modules, or physical damage.
The Six Checks Before You Buy
1. Verify the vehicle details
Confirm model year, mileage, battery version and original usable capacity. Several popular EVs shipped with different battery sizes in the same model year — make sure you're assessing the pack the car actually has.
2. Request a battery health certificate
Ask for a recent report from the manufacturer, a licensed inspection service or a specialist diagnostic provider. It should include, at minimum:
- The SoH estimate
- Stored fault codes
- Any high-voltage system anomalies
A seller who refuses an independent battery test is giving you information too.
3. Document a full charge
Ask to see the car charged to a known level and note the displayed range. The range estimate alone proves little — it leans heavily on the previous owner's driving style and the weather — but it establishes a baseline.
4. Take a proper test drive
Start with a known state of charge and drive a longer, varied route — city, highway, hills. Record distance, energy consumption and how the charge percentage falls. A sudden drop in displayed charge can indicate weak areas within the pack.
5. Run a charging test
Test both AC charging and, if possible, DC fast charging. Watch for interrupted sessions, error messages, or unusually low power despite a warm battery and a low state of charge. Many used EVs are sold without any charging cable at all, so bring your own: a proper portable Type 2 EV charger lets you test AC charging from a standard outlet on the spot — and if you're shopping in Australia, our guide to choosing a portable Type 2 EV charger covers plug types, adjustable current and cable length. Whatever you plug in, make sure it carries the right certification for your country — our certification explainer decodes RCM, CE, ETL, CSA and TÜV marks.
6. Inspect the underbody
Scratches on the protective panels aren't necessarily serious. But dents, cracks, corrosion or signs the battery housing has struck the ground hard should be assessed by a specialist before you commit — mechanical damage can cause problems that no diagnostic report catches early.
|
Check |
What it reveals |
Limitations |
|
SoH report |
Estimated remaining capacity, stored faults |
Calculation varies by provider |
|
Range display |
Estimate based on past driving |
Depends on weather, speed, driving style |
|
Test drive |
Real consumption, charge-level behaviour |
Short trips tell you little |
|
Charging test |
Charging port and battery system health |
Power depends on temperature and SoC |
|
Visual inspection |
Accident or underbody damage |
Internal damage needs diagnostics |
Documents to Collect Before Signing
- Service and repair history, including any high-voltage system work
- A recent diagnostic report or independent battery health certificate
- Battery and high-voltage component warranty terms
- Records of accident damage, recalls completed and software updates
On warranty: most manufacturer battery warranties run for 8 years with a mileage cap and a guaranteed minimum capacity (commonly around 70% SoH). Check whether the warranty transfers to a second owner, whether all servicing conditions were met, and which measurement method applies to a warranty claim — because that determines whether a borderline battery is someone else's problem or yours.
Red Flags — and One Non-Red Flag
Walk away or renegotiate hard if the seller:
- Refuses an independent battery test
- Downplays warning messages or "just resets" them
- Has an incomplete or missing vehicle history
- Shows large state-of-charge fluctuations or repeated charging interruptions
But don't reject a car for having used fast chargers. Frequent DC fast charging is often blamed for battery damage, yet the data is more forgiving: Geotab found that while heavy reliance on charging above 100 kW roughly doubles the degradation rate (about 3.0% per year versus 1.5%), modern packs remain fit for purpose well beyond typical ownership periods. Recurrent's study of 30,000+ EVs reached a similar conclusion — outside two well-known recalls, fewer than 4% of batteries were ever replaced, and for 2022-and-newer vehicles the rate was just 0.3%. Judge the battery you can measure, not the charging history you can't reconstruct.
OBD Adapters: Useful, Not Sufficient
OBD dongles and apps can read cell voltages, temperatures and a calculated SoH — handy for a first impression and for spotting large voltage differences between cell groups. But they don't replace a professional report: not every adapter is compatible with every car, and misread data leads confidently to wrong conclusions. Treat OBD readings as a screening tool; treat anything abnormal as a reason to get a qualified workshop assessment.
The Bottom Line
Don't buy a percentage — buy a car. A lower SoH doesn't make an EV unusable; what matters is whether the remaining range fits your driving and whether the price reflects the battery's true condition. Diagnostics with no faults, plausible consumption on the test drive, stable charging behaviour and an undamaged housing together give you the most reliable overall picture. If doubts remain, make the purchase conditional on an independent battery test — any reasonable seller will agree.
FAQ
What does the SoH of a used EV mean?
State of health describes remaining battery capacity compared with the car's defined condition when new. Because the calculation method varies between manufacturers and diagnostic providers, the figure should never be assessed in isolation — combine it with a test drive, a charging test and a vehicle history check.
How can I check a used EV's battery reliably?
Request a battery health certificate, verify the exact battery version fitted, document a full charge, take an extended test drive with varied conditions, run both AC and DC charging tests, and inspect the battery housing for damage.
What warning signs indicate battery problems?
Large fluctuations in displayed state of charge, repeated charging interruptions, unusually low charging power on a warm battery, stored high-voltage fault codes, physical damage to the battery housing, and a seller who refuses an independent test.
Is an OBD diagnostic enough to assess the battery?
No. OBD adapters are useful for an initial screening of cell voltages and temperatures, but they're not compatible with every vehicle and misinterpreted readings are common. Use them as a first filter, not a verdict.
What should I check in the battery warranty of a used EV?
The age and mileage caps, the guaranteed minimum capacity, whether the warranty transfers to a second owner, whether servicing conditions were met, and which SoH measurement method applies to warranty claims.