Those members who work on their cars and have not had alternator problems, here is a tip that should save you the headache of a dead battery and money.
MOST alternators on BMWs fail due to brushes wearing down (normal wear) between 150k and 180K miles. Brushes are attached to a regulator; brush replacement is a fairly simple procedure.
The back of the alternator held in place with two screws. Also there is a round transistor mounted on the outside on Bosch alternators -- other type is similar but uses different brushes. After removing the screws, regulater should come out.
when fitting the regulator back you must install it 1st at an angle, to assure that the brushes compress when the regulator is flush with the alternator, tighten the screws.
Your alternator should be ready to go the second 180K miles of alternator life.
Note: It goes without saying before you start replacing electrical components, you need to check and be sure you don't have a short to ground when your car is off. To check this, you'll need a DC ammeter. With everything turned off, there should be no more than a few millis drain to keep the time /clock and other electronic components (remote entry, alarm, shifter lights and soforth)
The pros on this topic/forum may take this post to a higher level. Thanks