Mine had a similar thing happen out of the blue. Indy diagnosed the alternator as providing a fluctuating charge. They said that it's possible that the alternator can reverse polarity, which could cause a lot of electrical damage.

The new alternator was about $900. Expensive little suckers...

__________________
2006 X5 4.4i
Sport package, summer package, cold weather package, DSP + Nav
Bought 2/14 with 73k miles