Test methodHow we measure, and what

1. Why the dashboard number is unreliable

Range is an estimate, not capacity

The remaining range on the dashboard is worked out from recent energy use. The same battery shows a different range if you drive harder, run the air conditioning, or drive in winter. It answers the question "how far can I go if I keep driving like this". It does not answer "how much capacity does this battery have left".

So if two cars of the same model differ by 30 km of displayed range at full charge, the batteries do not necessarily differ by that much. And a range that looks good does not mean the battery is fine.

Each brand defines SoH differently

The battery management system (BMS) keeps an internal health figure, usually called SoH. Each brand calculates it its own way. Some look only at capacity. Some include internal resistance. Some barely move until the battery nears the warranty threshold. 92% on brand A and 92% on brand B are not the same thing and cannot be compared directly.

Estimates drift, and they can be reset

The BMS learns its capacity estimate over a long time. If a car is charged and discharged only within a narrow range for a long period, the estimate slowly drifts from reality. On some models the estimate can also be reset or recalibrated with a diagnostic tool. For a while after a reset, the displayed figure may be more optimistic than the truth.

This does not mean every seller tampers. The point is that a buyer looking at one screenshot cannot tell how the number was produced, on which day, or whether it belongs to this car.

2. What we measure

Every number on a certificate is either a measurement, or a recorded value read directly from the car. None of them is an estimate or a prediction.

• Measured usable capacity (kWh), shown next to the factory nominal capacity. SoH is one divided by the other. Both are printed on the certificate, so you can do the division yourself.
• Value shown by the car (kWh). The full-pack capacity the car's own BMS reports. We print it as it is, next to our measurement, so you can see the gap between what the car says and what we measured. It plays no part in the SoH calculation.
• Cell voltage spread (mV). A pack is many cells in series, and the weakest group decides how much energy you can use. We record the difference between the highest and lowest cell voltage while the pack is at rest. A larger spread means the cells are less balanced.
• Temperature spread (°C). The difference between the hottest and coldest cell during the measurement. It shows whether temperature is even across the pack.
• DC fast-charge share. The share of the car's lifetime charging energy that came from DC fast charging, read from the car's history. If the car also reports a number of fast charges, we show that too.
• Pack exterior. We inspect the pack casing under the floor for impact marks, scratches, deformation or signs of leaks, and we take photos. The photos go on the certificate page.

3. How we measure

We have two procedures: the charge method and the drive method. The calculation is the same in both. We integrate current over time, then divide by the change in charge percentage. One measures the energy going in, the other the energy coming out. Each certificate states which one was used.

Charge method

During one AC charging session we log the battery's voltage, current and charge percentage the whole way through. We integrate current over time to get the energy that actually went into the battery. We divide that by the change in charge percentage over the same period to get the usable capacity of the whole pack.

AC charging is low power, long and steady, so it suits an on-site visit using the owner's own charger. It takes about 2 to 4 hours, depending on the starting level and the charging power. Where charging is possible, this is the method we use first.

Drive method

On a fixed route we run a controlled discharge from about 80% down to about 30%. We log the battery's voltage, current and charge percentage the whole way through. We integrate the discharge current over time to get the energy the battery actually gave out. We divide that by the change in charge percentage over the same period to get the usable capacity of the whole pack. It takes about 1.5 hours.

Traffic, temperature and driving style affect the drive method more than the charge method. We hold the route and the speed band fixed to control for them, and the certificate states that the drive method was used.

We use the drive method when there is no way to charge on site. Fleet clients can also choose it, for example when a lot has too few chargers, or when many cars have to be tested in one day.

4. Why the VIN, and why only 6 months

A certificate is bound to the VIN. Not to the owner, and not to the licence plate. The VIN stays with the car through a change of owner or plate. On site, a buyer compares the VIN under the windscreen or on the door frame with the last 6 characters on the certificate page. If they match, the certificate belongs to this car.

Validity is 6 months because a battery changes. In 6 months a car can add ten thousand kilometres, dozens of fast charges, or long periods parked full in the heat. A measurement from half a year ago does not describe today. An expired certificate can still be looked up, and it is clearly marked as expired.

5. What we do not do

• We are not a marketplace. We do not run car ads and we do not match buyers with sellers.
• We are not a dealer. We do not buy or sell cars.
• We take no commission. Whether the car sells or not, and at whatever price, we are paid the same test fee.
• We do not guarantee the future. A certificate describes the battery on the test date. It is not a promise about lifespan, range or warranty.
• We do not endorse the manufacturer's warranty, and we do not replace the manufacturer's warranty inspection.

6. Current limits

Today we test only some Tesla models. Other brands are still being prepared. The full list is on Covered vehicles.

We have not published an accuracy figure, because we do not yet have enough comparison data. Whenever the owner agrees, we also run the car's built-in battery test on the same car and record both results. Once we have enough cars, we will publish the comparison on the home page. Until then we will not claim any margin of error.

Results are affected by air temperature, the starting charge level and the charging equipment. We try to test under similar conditions each time, and we suggest another day when conditions are clearly poor.