Why a battery dies when nothing is on
Every modern car draws a small amount of current around the clock: the clock, alarm, keyless entry receiver and computer memory. That's normal and tiny once the car's modules go to sleep. A parasitic drain is when something keeps drawing more than it should, often a glovebox or trunk light that stays on, a module that won't sleep, a stuck relay, or an aftermarket stereo, dash cam or remote start wired in wrong.
A drain like that will flatten a healthy battery in a day or two, and a new battery won't fix it. That's why a car whose battery keeps dying gets three checks: the battery's health, the alternator's output, and the current draw with the key off.
+ 01 The test
How a drain test works
With everything off and the doors closed, the tech measures current flowing out of the battery and waits for the car's modules to go to sleep, which can take a while on newer cars. If the draw stays high, fuses are checked circuit by circuit to narrow down where it's going.
Simple causes, like a light that won't switch off or a phone charger in an always-on socket, get pointed out on the spot. Deeper electrical faults inside a module or wiring harness are a job for a mechanic, and the tech will tell you which circuit is responsible so the repair starts in the right place.
+ 02 Other reasons
It isn't always a drain
A battery that keeps dying can also be:
- Worn out: a battery that can no longer hold a full charge
- Undercharged: short trips that never let the alternator catch up
- A charging problem: an alternator with weak output or a failing diode
- Corroded connections: dirty terminals or a bad ground
- Simply sitting: cars parked for weeks slowly drain even with a normal draw
Questions about battery keeps dying
How much drain is normal?
Once the modules are asleep, most cars draw a few dozen milliamps. Much more than that, steadily, will run a battery down over a few days.
Can a bad battery look like a drain?
Yes. A battery with a weak cell can lose charge on its own overnight. That's why the battery itself is tested before chasing a drain.
Will disconnecting the battery stop the drain?
It stops the battery going flat, but it also resets settings and doesn't fix the cause. It's fine as a stopgap if the car will sit.