I have found with my E53 is that after the car has been sitting overnight or even shorter intervals is that the alternator will not start charging the battery until the voltage gets down to around 10.
This could explain why there are quite a few problems with short trips etc, I have been caught just by fuelling up soon after I leave home, because the battery is already down a bit just by driving a short distance with a/c, lights radio etc on you can return to your car and it wont start.
Im going to replace the voltage regulator but Im not convinced this is going to solve everything - until then I will drive with the voltage display on the OBC just to be sure there is enough charge for a re-start when I stop - these cars are to heavy to push.....

cheers

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X5 4.4i 2006 sports package, Titanium silver, Sports exhaust with Xpipe, Limo tint, 20" wheels 168's (Black), 25mm H&R wheel spacers& arch flares all round - Loud........

Work in progress -:
Replace BMW Ipod system with DICE - installed but no sound yet??