Traction battery
pack capacity

Before you start — the discharge test

This gives you the usable capacity of the battery as it stands today — the energy the car will actually deliver between a full charge and empty. It is measured rather than estimated: the energy is metered by the car itself, and because the reading runs between two displayed percentages, the reserve the car holds below zero never enters it.

It works on any electric car, including makes with no fixed rated figure, where the methods above would give a different answer every time. It needs planning: you must set the trip meter yourself before the drive.

1. Start at 70% or above. It does not need to be freshly charged — the test measures a drop, not a full battery, so wherever the car happens to sit is fine as long as there is enough above you. Starting lower leaves too little room to draw the 40 points this needs without running the car down further than is good for it.
2. With the car parked, note the battery percentage and reset a trip meter at the same moment.
3. Drive normally until the battery has dropped at least 40 percentage points — 70% down to 30%, or wider. One continuous drive is best. A few legs on the same day are acceptable. Spread it over several days and the result will read low.
4. On a small battery — roughly 40 kWh or under — take a wider window than the minimum. The precision depends on how many percentage points you cover, not on how big the pack is, so on a small battery those points cost very few miles. Forty points might be 35 miles where a large pack would need 145.
5. Do not precondition or warm the car from the phone app during the test, and keep climate use while parked to a minimum. That energy leaves the battery but never reaches the trip meter, so the test counts the drop without counting what caused it.
6. Do not charge at any point in between, not even a short top-up. One charge and the result is wrong.
7. Park, and read the battery percentage, the trip miles and the trip efficiency together.

If the car can show efficiency either way, use Wh/mi. A figure like 345 Wh/mi is far more precise than the same reading as 2.9 mi/kWh, where the single decimal place alone costs nearly 2%.

Everything the car spends while stationary — preconditioning, climate, standby, always-on security monitoring — comes out of the battery without appearing in the trip miles. The test then attributes the whole drop to driving, and reads low. Preconditioning is the worst of these because it is deliberate and draws hard: a few minutes of cabin heating before each leg can cost more than the rounding error the test is trying to avoid. One drive, no remote warming, and the figure stands up.