I have an 02 A6 Avant. The battery wasn't holding a good charge, it would start from day to day but you could tell it was not good. I decided it was time to replace it when the air conditioner started shutting off when I turned the lights on. I'm assuming that is something the computer is doing when there's not enough juice for everything? Prior to putting in the new battery when driven at speed with nothing else on, the gauge was at 14 volts. I put a new battery in and it's barely showing 12 when the key is turned to on despite the battery being fully charged and takes some work to get it to start. When run at speed it's about 13 volts and when idling it really wants to die. If I had to guess I would say a broken wire or loose plug but I can't see anything I could have done pulling out the old battery and putting in a new one. Connections were all cleaned, battery has more CCA than the one that came out of it. Any ideas, Thanks.

 

I'd replace the voltage regulator on the alternator. It's a fairly easy swap out provided you can get to the back of the alternator. Bosch alternators are pretty much bulletproof. The thing that wears out is the brushes on the voltage regulator. When they get worn down, they will not make good (or intermittent) contact. The car is trying to run all the electronics from the battery at times and it simply cannot keep up. The other battery may have been ok but just not being fully charged when driven.

I had a similar thing happen on my wife's 5000 a few years ago. The battery was a little over 5 years old and it acted like it was going down. Not keeping a charge. It died on her on the road one evening. I stopped and picked her up, pulled the battery and went and got a new one. Car cranked and ran fine until it died again on the road just a half mile from the house. I had another 5000 I'd been robbing parts from and got the regulator off it. When I pulled the regulator from the wife's alternator, the brushes were worn down to near nothing. They would have been barely making contact. Put the other regulator in and fixed the issue. That had to have been at least 5 years ago.