Alright I haven't run into a power problem for like 15 years so please excuse the dumb question. Scenario:
Car has been running great. Recently serviced with all green checks. Car starts in the morning as usual, drive cat to the vet 3 miles away. 10 minutes later try to start the car, a bunch of system failure messages flip rapidly through the display (automatic headlights, Audi pre-sense, etc) while the mirrors aren't folding back into position like normal. windows don't go up. i shut it off (wasn't on but hit the button again so it's just in accessory mode). no dash lights. window goes down half an inch then runs out of power. screen says "very low battery, system will shut down soon" and then shuts itself off in like 20 seconds where it usually stays on for 10 minutes. So yeah, battery is dead.
call wife, she comes and jumps me (seemed to take a little long but then again i haven't had a power issue for years and years) . let it run a couple minutes, drive the three miles back home. Turn it off. Try to turn it on again, met with same problem. okay maybe the alternator?
Jump it again, let it run, rev it up for a few minutes, drive it around a couple blocks. Come back and this time it has retained a little energy. keyless works, no warning messages turning accessory power on. try to start it and it starts with one push but almost doesn't. no warning lights, no battery light. rev it up, drive it around the block a couple times.
Same as above when i tried after letting it sit an hour (but keyless entry wouldn't work--the fob did though). rough start after one push. drove it around the block again.
Same as above when i tried after letting it sit two hours.
So what is it? Audi can't see it for almost two weeks. The first trip back didn't seem to charge the battery at all but after the jump at home it retained a little at least. didn't even check the battery..just assumed that was part of the annual service. looks brand new. didn't want to unscrew anything to lift it out of the encasement.