What Car? Q&A - What's causing my battery to drain?

Q: I'm having a problem with the battery on my Citroen Saxo. When I leave it overnight, the battery drains completely, to the point where it doesn't even turn over in the morning when I go to start it.

Is there something that could be draining the battery? I only bought the battery new six months ago, so it shouldn't be worn out already.
Matt Simpson

A: Most batteries should be covered by a guarantee, so the first step is to check that the battery itself isn't at fault. Take it back, exchange it for another and give that a try.

There are lots of things that could be draining it slowly, but it would have to be something fairly power-hungry, such as your headlights, to completely exhaust it overnight. We're sure you'd notice if you left your headlights on.

We've experienced power steering pumps that wouldn't switch off and have drained batteries overnight as well, but these make a loud humming sound, so it would be an obvious fault if it's happening to your car.

Fans and pumps that don't switch off when they should are also likely to make themselves heard.

However, silent faults with radios and other electronics can consume enough current to flatten a battery. We think this is the most likely cause in your case.

It's possible that the alternator isn't charging the battery as you're driving, but this doesn't really explain why it's being drained overnight.

To find out for sure, take you car to a garage or auto electrician to get the fault diagnosed properly.