The short answer
On a Model 3, a Model Y, or a Model S or X built from 2021, ask the car. Open Controls, then Service, then Battery Health. It needs software 2025.8 or later, and Tesla's manual warns that some cars may not have the test; software 2025.26 then removed it from many newer cars and left some older ones showing only their last result. A green box saying the battery is healthy means Tesla's battery computer thinks the pack is where it should be for its age and mileage. Older Model S and X don't have the screen, so on those, and on any car where it is gone, you do it with arithmetic: the range at a full charge against the range the car had when it was new. Either takes a couple of minutes on a test drive.
What the green box means
The box shows up whether or not anyone has run Tesla's full test. Without a test it's the battery computer's own estimate, which Tesla's service documentation calls reliable in most cases. A measured figure only exists once the test has run, and the screen dates it. I saw this on a real car: a 2026 Model Y, just under a year old, read with its owner's permission. Its battery health status said OK and its internal health figure was 93.6%, though no test had ever been run on it.
A check that works on any Tesla
Switch the range display from percent to miles. Divide the miles by the battery percentage and multiply by 100, and you have the range at a full charge. Divide that by the EPA range the car was sold with, which depends on the trim and the wheels.
On the Model Y above, 201 miles at 60% one day and 177 miles at 53% five days later both work out at about 334 miles. The EPA rates that car at 357 miles on its 19-inch wheels. 334 divided by 357 is 93.6%, the same figure the car keeps for itself. You don't need to charge to 100% first. A fuller battery only gives a sharper answer, because the screen rounds.
Getting the division right
Use the EPA figure for the wheels the car was sold on, because the same car is rated lower on bigger wheels, and a car whose wheel setting is wrong shows the wrong range. Don't do it with a cold battery, which shows less than it holds. On an older Model S or X, make sure the display is set to rated range rather than ideal.
The estimate behind the range can also drift: a car that only ever tops up a little at a time can show less than its pack holds. Tesla's advice is to use the whole battery for a while, charging above 80% and running below 30%, and to let the car rest at both ends. It can take about eight weeks to settle. The full test sidesteps that, because it measures instead of estimating.
Running Tesla's full test
Where the car still offers it, the test lives on the same screen. It needs an AC charger that delivers at least 5 kW, which means a wall charger rather than a Supercharger, and up to a day. The car runs its battery down, rests, charges to 100% and rests again, and the climate control is off while it runs. Afterwards the range estimate can jump up or down, because a measurement has replaced an estimate. The battery itself hasn't changed.
A seller can run it the night before you come. Check the date under the result, though: Tesla's service staff are told to rerun anything more than a month old.
If the test won't start
Before it starts, the screen lists what it needs, and one line is that no battery or thermal alerts are present. Tesla blocks the test on purpose when there are. The alerts it waits on include a pack out of balance, a charge limit the battery has put on itself, low coolant, and a coolant pump or valve that isn't working. On a car you're about to buy, a test that won't start is an answer in its own right. Ask which alert it is.
What normal looks like
A new pack loses a few percent in its first months while a protective layer forms on the cells, and then the loss slows right down. Tesla's service documentation treats a car under 18 months old and under 20,000 km, about 12,400 miles, as ordinary early loss as long as its health is above 83%. Past that stage Tesla's yardstick is the range at full charge: a fall of less than 3% over six months is within expected bounds. The Model Y above, at 93.6% after 11 months and about 10,600 miles, sits inside the early allowance, and its screen said OK.
What the number can't see
Battery health is stored energy and nothing else. Tesla's documentation spells out what it ignores: corrosion inside the pack, mechanical damage, wear in the internal connections, moisture, and damage to the outside. A car with a crash or a flood in its past can show a green box. Look underneath, and read the title.
When the box isn't green
If the test finishes and there's no green box, Tesla's own screen is saying the pack has lost more than Tesla expects for its age and mileage.
On a car inside its battery warranty, that's where a claim for a replacement pack starts. The warranty promises at least 70% of the original capacity for eight years, with a mileage cap between 100,000 and 150,000 miles depending on the model. A service centre will run the test for you, but it charges for it even under warranty. Run it yourself. It's free on the car.
Model S and X built before 2021
Tesla hasn't given these cars the battery health screen, and its service documentation ties that to the warranty: most of them were sold with no capacity promise at all. A Model S or X sold before 29 January 2020 has an eight-year warranty against the battery failing, with no minimum capacity. On those cars a shorter range isn't a warranty claim, however far it has fallen. The division above is the check.
Cars built between that date and the 2021 redesign did get the 70% promise, and for those Tesla's staff can share an estimated figure, with a warning that it's only an estimate. For anything older, they're told no figure applies.
What the factory record adds
No VIN lookup can tell you today's battery health, ours included. What Tesla's record does hold is the pack it lists for the car, usually the one it was built with, and the day it was built: the capacity it started with, and a good guide to which warranty it was sold under. The free check on this site shows both.