How to check battery State-of-Health (SoH) before you buy
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The most important check on any used electric car is its battery state of health (SoH) - the percentage of the original usable capacity that remains. It drives range and resale value more than mileage does, and unlike engine wear, you can measure it. This guide shows you how, from the two-second dashboard glance to a precise diagnostic read.
Why this is the check that matters
On a petrol car, mileage tells you most of what you need. On an EV, two cars with the same distance on the clock can have very different batteries, depending on climate, charging habits and how often they were rapid-charged. SoH is the number that captures the difference, so it should drive your offer.
The quick check: in-car clues
Before any equipment, look at what the car tells you. Some models, like the Nissan Leaf, show a battery capacity gauge on the dashboard - a column of bars that drop off as the battery ages. A new Leaf shows twelve capacity bars; a car that has shed several early in life has usually had a hard time. It is a rough guide rather than a precise figure, but it takes seconds and needs no kit.
On most other EVs there is no simple bar gauge, so you move to a diagnostic read for anything precise.
The exact check: a diagnostic read
For an exact SoH figure, the EV community uses a small Bluetooth or Wi-Fi adapter that plugs into the car's diagnostic (OBD-II) port, paired with an app on your phone. The app reads the battery management system directly and reports SoH, often to one decimal place, along with extra detail like cell balance.
What you need
- An inexpensive ELM327-style OBD-II adapter (Bluetooth for Android, Wi-Fi for iPhone).
- A battery-health app suited to the car you are viewing.
- A couple of minutes with the car, ignition on - you do not need to drive it.
Step by step
- Find the OBD-II port - on most cars it sits under the dashboard, near the steering column.
- Plug in the adapter and switch the car on.
- Pair your phone to the adapter over Bluetooth or Wi-Fi.
- Open the app and let it connect, then read the state-of-health figure.
- Note the SoH and cross-check it against any in-car clues - a healthy car has them lining up.
Checking specific models
The exact app and menu differ by make, but the workflow above is the same. Our model-specific notes live alongside each landing - for example used Nissan Leaf, Tesla Model 3 and Hyundai Kona Electric listings each show their SoH up front, so you can shortlist before you ever plug in an adapter.
What to do with the number
Once you have a figure, compare it with other cars of the same model and age, and treat anything well below its peers as a bargaining point or a reason to move on. For how to read whether a figure is good, see what SoH percentage is good on a used EV. For the bigger picture on health and range, start with the battery health and real range pillar.
Frequently asked questions
Can I check an EV's battery health myself?
Yes. Many electric cars expose a state-of-health figure through a cheap OBD-II adapter and a diagnostic app on your phone. Some models also show capacity clues on the dashboard. The exact tool depends on the make, but the principle is the same - read the measured SoH rather than guessing from mileage.
What is a Verified SoH badge?
It means the seller uploaded a battery diagnostic screenshot and we checked it against the listing, so the state-of-health figure is backed by a real read rather than a claim. Verified cars take the guesswork out of the most important number on the car.