I have had this problem for the past year. I had the alternator rebuilt and every 2-3 months I either have the battery light, and/or the deadly triangle (abs/red brake light/dsc). It is the alternator. New alt is about $900 for the part or 450 for a rebuilt from a auto store that they have to order. At this is for a 2001 e53 water cooled. Easy to replace but extremely expensive.