What Drains a Car Battery Overnight?

A healthy car battery should hold its charge for weeks. When one goes flat overnight, something is drawing power while the car is off, or the battery can no longer hold a charge. Both are common, and they are easy to confuse.

The usual causes

• Something left on: an interior light, a trunk or glovebox light that stays lit, or a phone charger and dash camera plugged in.
• A door, hood or trunk that is not fully latched, so the car thinks it is in use.
• An aftermarket accessory: an alarm, remote starter, dash camera or sound system wired in badly.
• A worn battery. An old battery can pass a quick look and still lose its charge in a day or two.
• A charging problem. If the alternator is not charging properly, the battery runs down while you drive.

What you can check yourself

1. Walk around the car and make sure every door, the hood and the trunk are fully closed and no lights stay on.
2. Unplug everything from the power outlets and the OBD port overnight and see whether the problem goes away.
3. Note how old the battery is. If it is more than four years old, a test is the first step.
4. Think about whether anything was added to the car recently, such as a dash camera or alarm.

When to get it tested

If the battery goes flat more than once, a test is worth doing. A load test shows whether the battery is worn out, and a charging-system test shows whether the alternator is doing its job. If both pass, the cause is a draw somewhere in the car, which is a separate diagnosis.