Used EV battery health: state of health explained

A used electric car's battery health is usually given as its state of health: the battery's usable capacity today as a percentage of its capacity when new. It is the most useful single number on a used electric car, because it sets how far the car goes on a full charge and how much battery warranty cover is likely to matter. The MOT does not measure it. So ask for a dated report before you buy, and treat a seller who cannot produce one with care.

What state of health means

A battery loses some of its capacity as it ages and as it is used. State of health puts a number on that. A battery with a state of health of, say, nine tenths can store about nine tenths of the energy it held when new, so a full charge gives you about nine tenths of the range it gave when new, in the same conditions.

Two things it is not:

• It is not the state of charge. State of charge is how full the battery is right now. A car showing 100% charge can still have a worn battery.
• It is not the range on the dash. The range estimate depends on recent driving, temperature and heating, so it moves around even when the battery is healthy.

Different tools can also report state of health in slightly different ways, for example against the gross or the usable capacity. Compare figures from the same kind of test where you can.

How state of health is measured

There are three common routes. None is perfect, so more than one is better.

A dealer diagnostic report

A franchised dealer for the make can usually read the battery data from the car's control units and print a report. It uses the manufacturer's own tools, so it is usually the figure to give most weight. Ask for the date and the mileage on the report.

An independent battery test

Independent specialists offer battery health tests, usually by plugging a reader into the car's diagnostic port, sometimes with a short drive or a controlled charge. Ask what the test measures and whether it reports a percentage, a capacity in kWh or both.

The car's own display

Some cars show battery health or capacity in a menu. Others show nothing directly. The car's own figure is a starting point, not proof, and a range estimate on the dash is not a health measurement.

What wears a battery faster

Battery ageing depends on the chemistry and on the cooling system, as well as on how the car was used. Material published by the US Department of Energy notes that climate affects how long electric car batteries last, with harsher climates shortening life. Beyond that, these are the factors most often linked to faster capacity loss:

• Heat. High temperatures, especially when the battery is kept at a high state of charge, are hard on cells.
• Frequent rapid charging. Rapid charging is part of normal use, but a car that lived on rapid chargers has had a harder life than one charged slowly at home.
• Long periods at full or empty. Many handbooks advise against leaving the battery at 100% or near zero for long spells.
• Age and mileage. Both add up over time. A low-mileage car is not automatically healthy if it sat for years at full charge.

We do not publish an average yearly loss figure, because results vary by model, chemistry and use, and any average hides that spread. Judge the car in front of you on its report.

Why the MOT does not cover it

The MOT checks safety and emissions items listed in the DVSA inspection manual. Brakes, tyres, steering, lights and the body are tested on an electric car as on any car. The traction battery's capacity is not. Where the battery can be seen, the tester only checks it visually, for example for leaks. A car can pass its MOT with a battery that holds noticeably less charge than it did when new. Our guide to the MOT for electric cars covers what is and is not tested.

What to ask the seller

1. Do you have a battery health report? Ask for a copy, with the date, mileage and method.
2. How was the car usually charged? Mostly at home, or mostly on rapid chargers?
3. How much battery warranty is left? Read the terms yourself. Our guide to battery warranties explains what to look for.
4. Has any battery work been done? Ask for the invoice for any module or pack repair.
5. Have the software updates been applied? Battery management software can change charging behaviour and the displayed figures.
6. Can I see it charge? Plug it in at the viewing and check that charging starts.

If the seller will not allow a test before you commit, factor that into the price or walk away. A dealer selling to you has to supply a car of satisfactory quality under the Consumer Rights Act 2015, and durability is part of that. A private sale gives you fewer rights, so the report matters more.

How battery health fits the rest of the checks

State of health is one check, not the whole car. You still need to read the MOT history, look at tyres and brakes, check the paperwork and run a vehicle history check. Our guide to buying a used electric car puts the battery in the order we would do things. If anything looks wrong, see the car and ask a mechanic who works on electric cars.

Frequently asked questions

How do I check the battery health of a used electric car?

Ask the seller for a dated battery health report from a dealer or an independent test. If there is none, ask to arrange a test before you commit. The range on the dash and the car's own menus are a starting point, not proof.

What is a good state of health for a used EV battery?

It depends on the car's age, mileage and model, and on how much range you need. Work out whether the range from a full charge, at that state of health and in winter, covers your regular journeys with room to spare.

Does the MOT test an electric car's battery?

No. The MOT does not measure the traction battery's capacity or state of health. At most the battery gets a visual check where it can be seen, so a pass tells you nothing about how much charge it holds.

Does rapid charging damage the battery?

Rapid charging is normal use, but a car that relied on it heavily has had a harder life than one charged mostly at home. Ask how the car was charged and judge it on its battery report.