Quick answer: State of health (SOH) is the battery's usable capacity today compared with when it was new, shown as a percentage. A battery at 90% SOH holds about 90% of the energy it held new, so it gives roughly 90% of the original range.
How a battery loses capacity
Every lithium-ion battery slowly loses capacity, in your phone and in your car. EV batteries are much bigger and better managed, but the same chemistry is at work. Two things drive it:
- Calendar ageing: time itself, made faster by heat and by sitting at a very high or very low charge for long periods.
- Cycle ageing: the wear from charging and discharging, which adds up over many thousands of kilometres.
The battery management system (BMS) in your car watches the pack constantly. It balances the cells, controls temperature and keeps an estimate of how much capacity the battery has left. That estimate is where SOH comes from.
What's a normal SOH?
It's common for a battery to lose a few percent in its first couple of years, then settle into a slower, steadier decline. How fast depends on the car, the battery chemistry, the climate and how the car is charged. That's why one measured result for your car tells you more than any average.
Most new EVs sold in Australia come with a battery warranty of around 8 years, usually with a kilometre limit and a minimum capacity, often about 70%. Check your car's warranty booklet for the exact terms, because they differ between brands and models.
Why the range on the dashboard isn't SOH
The range estimate on your screen is a prediction, not a measurement. It's based on your recent driving, the outside temperature, air-conditioning use and the charge level right now. A cold morning or a run of freeway driving can move it a lot, even though the battery hasn't changed.
SOH looks past all of that. It compares the battery's actual capacity with its capacity when new.
What SOH means in kilometres
A simple way to picture it: multiply the car's original range by the SOH.
| State of health | Car with 500 km range new | Car with 400 km range new |
|---|---|---|
| 100% | 500 km | 400 km |
| 95% | 475 km | 380 km |
| 90% | 450 km | 360 km |
| 85% | 425 km | 340 km |
| 80% | 400 km | 320 km |
Real range also depends on weather, speed and driving style, so treat these as rough figures. You can try your own numbers with the calculator on our battery health check page.
How SOH is measured
A diagnostic tool connects to the car's diagnostic port and reads data from the BMS. Depending on the car, that includes:
- the state of health figure, or the data needed to work it out
- the voltage of every cell group, which shows whether any are lagging
- module temperatures
- any battery fault codes, current or stored
Some cars report SOH directly. For others it has to be worked out from the capacity data, and a few models share less information. When you book a test with us, tell us your make and model and we'll confirm what we can read before we come out.
When it's worth checking
- Before you buy a used EV, so you know what you're paying for.
- Before you sell, to give buyers a recent, independent figure.
- As the battery warranty gets close to ending, while there's still time to act on a problem.
- If your range has dropped noticeably and the weather doesn't explain it.