How Do You Test a Car Battery?
The short answer
A proper battery test is two tests, not one. A voltmeter across the terminals with the car off tells you the state of charge: roughly 12.6 volts is fully charged, about 12.4 is three quarters, and anything near 12.0 is nearly flat. That only tells you what's in it. The second test puts the battery under load and watches whether the voltage holds up, and that's the one that finds a battery about to fail.
This trips people up constantly. A battery can read a perfectly healthy voltage sitting on the drive and still not start the car on a cold morning, because voltage measures how full it is and not how much it can deliver. If you've had a battery checked, been told it's fine, and then been let down by it anyway, this is almost always why.
Common causes and what they mean
A few things could be behind it. Here's what each usually points to, and how serious it is.
Resting voltage, with everything off
Leave the car untouched for a few hours first, ideally overnight. A reading straight after driving is inflated by surface charge and flatters the battery. About 12.6 volts is full, 12.4 is around three quarters, 12.2 is about half, and 12.0 or below means it's nearly empty.
Cranking voltage, while it starts
This is the useful one. Watch the voltage as the engine turns over. It'll dip, which is normal, but on a healthy battery in mild weather it should hold up somewhere around 9.6 volts or better. A battery that collapses well below that under load is on its way out, whatever it read at rest.
Charging voltage, with the engine running
With the engine going you should see roughly 13.7 to 14.7 volts at the battery. Much less and the alternator isn't keeping up, so the battery is being slowly drained every time you drive. Much more and it's overcharging, which cooks a battery from the inside.
A conductance or load test
This is what a garage tester does, and it's the closest thing to a verdict. It measures how well the battery can actually deliver current rather than how charged it is, and it gives a figure against the cold cranking amps the battery is rated for. A battery well down on its rating is failing even if it starts the car today.
A parasitic drain test
If the battery keeps going flat but tests fine, the battery may not be the problem. Something is drawing current with the car locked and asleep: a light staying on, an aftermarket alarm, a failing module. That's measured differently, in the circuit rather than across the terminals, and it's a fault-finding job.
You can drive on a battery that tests weak, but plan around it
A battery on its way out usually gives you a few weeks of warning: slower cranking, particularly first thing on a cold morning, and headlights that dim more than they used to at idle. It won't get better. The thing to avoid is finding out on a day when you actually need the car, or somewhere with nobody around to help. If it's testing badly, get it replaced before the first proper cold snap rather than after it.
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