New to the forum. Posting an FYI. If you are having issues with a bad battery or alternator check your grounding strap / wire.

For the past few weeks my 2011 R60 S A4 had gremlins in the charging system. She seemed sluggish on start up. Then one day she did not start. Jump started her. I checked the alternator output at the battery terminals. It was 14 V. So I had the battery tested. It was okay. Put the fully charged battery in and she did not turn over. She would start after a jump. Then I checked the voltage at the battery. It was less than 12 when running. With my volt meter connected to the positive batter terminal I checked voltage at the engine side of the grounding strap. It was 14. I checked at the chassis side of the strap it was 12ish. If I wiggled the strap the voltage would go up but never all the way to 14. The strap looked ok but when I took it off it about crumbled in my hands from corrosion.

I bought a $6 12” battery cable to replace the grounding strap and now she starts and runs great. The MINI grounding strap was $50 from the local dealer - no thanks.