Why is the battery repeatedly going flat on our BMW X5?

We have a BMW X5 2008 model 3.0 litre diesel SE. Ever since we bought it, if it was left for two weeks the battery would be dead, not very good when you return from holidays. The dealership did various checks on several occasions and concluded that we must deadlock the car even in the garage. In the end we got used to the problem and used a battery conditioner when the display clock failed. Also we used to use the car more often than we do now, which brings the problem back more frequently and is a real pain. We have done less than 30,000 miles and do not want to replace the car just yet, but need to sort this out if we are going to keep it. Many thanks if you can point us in the right direction to get it fixed.

Asked on 19 January 2013 by DE, Ballafesson, Isle of Man

Answered by Honest John
You always have to deadlock a modern car with Bluetooth when you leave it, otherwise the Bluetooth will continually search for a paired phone and will run down the battery. Another possible culprit is the self-dimming interior light. That's best switched off completely because if it fails it will discharge the battery in a night. A battery conditioning trickle charger is a good idea. Some longer runs will help because Efficient Dynamics means that short runs will not fully charge the battery, and a weakened battery that can no longer hold a full 13.5 volts will always be a problem.
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