If you drive mostly short trips in inclement weather, the short trips and the extra load of lights, fan, defroster, wipers, etc. can cause your battery slowly die, due to there not being enough time to put back what you're taking out. The situation is worse with smaller cars, which typically have smaller batteries that don't have as much capacity as the ones in land yachts.
If this describes you, try turning off all your accessories after you park, and just sitting there for a few extra minutes once in a while with the motor running, when you're not in a hurry. Listen to the radio (which doesn't use much juice) or just decompress from your day. This will help keep the battery charged.
But it would be good to have your alternator retested. It could be weak and not putting out as much as it should. Have the drive belt checked too to make sure it isn't slipping under load. Cigroller's advice is good. Make sure there isn't some light staying on like the one in your glovebox or trunk, or a forgotten map light.