Sounds like either a battery or alternator issue. I had the same problems with my M6 and it was alternator.
2008 E63 M6
- Black Sapphire - Sepang Bronze Full Merino - Carbon Fiber Trim - Alcantera Headliner - SMG - HUD - Soft Close Doors - Comfort Access - Individual Audio w/Sat and HD Radio
2009 E92 335i
- Black Sapphire - Black Dakota - Burl Walnut - Sport - Premium - Cold Weather - Navigation - L7 - Comfort Access - Smoker's Package
Member: BMWCCA #471614