Sounds like either a battery or alternator issue. I had the same problems with my M6 and it was alternator.
2014 Carrera 2S
- White/2 Tone Espresso/Cognac - Aerokit - PSE - PDCC - SPASM - PDK - Sport Chrono - 20" wheels w/OEM Spacers - Glass Roof - Leather Dash - way too much more to list
2013 F12 M6
- Alpine White/Black Merino - Executive - LEDs - B&O - DA - Lots more
2013 E92 M3
- LeMans Blue/Black Novillo -(Lease ended need new signature pic)