A couple weeks ago I drove to a park about 5 mins away and went for a walk. It was about 15 degrees F outside and when I got back to my car an hour later the battery was very low. A lot of electrical functions like interior lights and the radio would work but the battery did not have enough power to turn the engine. After a jump start all was fine and when I got it home I put the battery charger on it and ended up leaving it to trickle charge for a few days. I have not had another instance of this happening. As far as I can tell, the battery is now fine. I usually only use the car for short trips and am wondering if multiple short trips (spread out over a few days/weeks) in a row depleted the battery without allowing the alternator long enough to charge it. Anyone have any idea roughly how much battery it takes to start the car vs. how long it would need to be running to recharge the battery?

2010 A6 Avant 3.0t The battery was purchased from the Audi dealer in March (~9 months ago) and I installed it and coded it with VCDS.