The car was probably trying to charge it back when you were driving it. That might indicate why your gauge took a while to read correctly and why your AC was weak at first, but thats just my guess. But for this very same reason, If I know I can't drive my X for a while, I at least start the car, (or have someone else do it if I'm gone) and leave it running for 20 to 30 mins just to make sure that the car is charging the battery (although it being minimal since the car is not driving). You're not really supposed to leave your car idling for that long but its better then having a dead battery i guess. But your case is obviously different.