How to check a used electric car's battery before you buy

In a used electric car, the high voltage battery is the most expensive part and the hardest to judge on a viewing. Nothing you can see or hear tells you how much of its original capacity it still holds. This guide covers what to look at, what to ask for and how to read the answer.

What State of Health means

State of Health (SoH) is the usable energy or capacity of the battery today, as a percentage of what the same battery variant offered when new. At 100% the battery is as good as new. At 90% it holds about 90% of the energy it held when new, so a full charge takes the car less far than it once did.

Every battery loses capacity with time and use, and heat, long spells at a very high or very low charge and frequent fast charging can speed this up. That is why two cars of the same age and mileage can be in quite different condition. For the longer story, read Battery health is the new mileage.

Why the range on the dashboard is not enough

The range on the dashboard is an estimate of how far the car can go on its current charge. Many cars base it on recent driving, so it rises after a gentle drive and falls after a fast motorway run or a cold morning with the heating on.

The charge gauge does not help either. It reads 100% when the battery is full, whatever the battery can hold, so a worn battery and a healthy one both show 100% after a full charge. Some cars show a battery health figure in a menu or an app. Note it, but the screen does not say how it was worked out or how precise it is.

A full battery always reads 100%. The question is how much a full battery holds.

What to ask the seller

First, ask for a recent battery certificate. A certificate records the battery on the day of the test, and a BattGrade certificate is valid for 6 months from the day it is issued. If there is none, ask the seller to have the battery tested: it takes a few minutes with the car switched on and writes nothing to the car. In the EU, the Battery Regulation gives anyone who has bought a battery, or a third party acting for them, read-only access to the battery management system data used to estimate its state of health (Regulation (EU) 2023/1542, Article 14). Our page for car buyers has a message you can send.

Second, ask for the charging and service history, if the seller has it. Some cars and charging apps record charging sessions, and service records can show battery-related work, recalls and software updates. Ask directly whether the battery, or any module in it, has been repaired or replaced. A repair is not bad news in itself, but you want to know. Check, too, how much of the manufacturer's battery warranty is left.

How to read a BattGrade certificate

A BattGrade certificate is a PDF with a QR code. An inspector produces it with the BattGrade app and an OBD2 adapter plugged into the car, and BattGrade's server calculates and signs the result from the raw data read from the car, so neither the inspector nor the seller can type it in. BattGrade is a private company, not a public authority and not an accredited test lab. Read the certificate in this order.

The SoH. The headline number, rounded to 0.1 and never shown above 100%.

The method. Energy-based and capacity-based values are worked out from the car's own energy or capacity data. A BMS-reported value is the figure the car's battery management system (BMS) gives, used as it is.

The uncertainty band. The SoH comes with a band in percentage points either side. For example, 86.0% with a band of ±2.0 points covers 84.0% to 88.0%, which straddles the line between two grades. Two cars whose bands overlap may be closer than their headline numbers suggest.

The grade. A letter that comes from the SoH alone, on the same bands for every make: A from 85%, B from 70%, C from 55%, D from 40% and F below 40%. Nobody assigns it by hand.

The checks. Cell voltage spread, temperature spread, insulation and signs of a BMS reset, wherever the car provides the data. They do not change the grade by themselves, so read them even when the grade looks good.

The car and the test. The make, model and VIN, the mileage entered by the inspector, the test date, the adapter and who ran the test. How it works explains every field on an example certificate, and the full rules are in the methodology.

Red flags worth a second look

None of these means you should walk away automatically, but each one deserves a question.

Signs of a BMS reset. Flagged when the SoH reported by the battery management system is more than 5 points above the capacity-based value, or when a near-new SoH sits on a car whose lifetime discharged energy is too low for its mileage. A BMS reset or a battery replacement can cause it. Ask for the service history and the invoice for any battery work.

Uneven cells. A gap of more than 30 mV between the highest and the lowest cell voltage is marked for attention, and more than 50 mV as poor. It can point to an unbalanced or weak module.

An insulation warning. Insulation resistance below 500 Ω/V is a warning, and below 100 Ω/V a failure, following UN Regulation No. 100. Have the high voltage system inspected by a qualified technician before you buy.

A certificate that does not match. The make, the model and the visible part of the VIN should match the car in front of you, and the mileage at the test should make sense against the odometer today. If the verification page says the certificate could not be verified, its data no longer matches its signature. Do not rely on it.

An expired or revoked certificate. After 6 months a certificate shows as expired, and the battery may have changed since. A revoked certificate was withdrawn by the inspector, for example after a mistake. Either way, ask for a new test.

How to verify a certificate

Do not rely on the PDF alone, and never on a photo of it. Scan the QR code, or type the certificate ID at battgrade.com/verify. The page checks the certificate's digital signature every time it opens.

A green banner means the data on file matches the signature, so nothing has changed since the certificate was issued, and it is within its 6 months. Amber means it has expired. Red means it could not be verified or has been revoked. If the ID does not exist at all, a printed certificate carrying it may be counterfeit.

When to get an independent test

Ask for a new test when there is no certificate, when it has expired, when it carries a BMS reset or insulation flag, when its details do not match the car, or when the uncertainty band crosses a grade boundary that matters to the price.

A test run by the seller's own team is not a problem in itself: the result is calculated by BattGrade's server and the certificate names who ran the test. Still, you may prefer a test you arrange yourself. There is no public list of inspectors yet, but on the page for car buyers you can tell us which car it is and where, and we reply by email.

A battery certificate is not a full inspection of the car or a warranty, so have the rest of the car checked as you would any used car.

The short version

Ask for SoH, not range. Ask for a recent certificate and verify it yourself at battgrade.com/verify. Read the method, the band and the checks, not only the grade, and treat any flag or mismatch as a question to answer before you agree on a price.