Why Your Car Battery Keeps Draining Overnight

Normal versus abnormal drain

Every modern car draws a small amount of current when parked — the alarm, the clock, the immobiliser and the body control module all stay partly awake. A healthy figure is very small. When a circuit fails to go to sleep, the draw can be many times that and will flatten a good battery in a night or two.

The usual culprits

In order of how often we find them:

• A boot, glovebox or vanity light that never switches off.
• A hardwired dash cam left in parking mode.
• Aftermarket alarms, trackers and audio installed without a proper ignition feed.
• A faulty door or bonnet switch that keeps the car thinking a door is open.
• A failing alternator diode letting current flow backwards.
• An infotainment or telematics module that will not shut down.

How it gets diagnosed

A technician measures current draw at the battery once the car has gone to sleep, then pulls fuses one at a time until the reading drops. The fuse that causes the drop identifies the circuit. It is methodical rather than difficult, but it takes patience — the car must be left to settle first.

In the meantime

If you need to keep the car usable while you sort it, disconnecting the negative terminal overnight will stop the drain, or a smart trickle charger will keep the battery topped up. Neither is a fix, but both stop you being stranded. Once the battery has been run flat repeatedly, it will usually need replacing even after the drain is cured.