We had that happen on a car years ago. During the week, when we were using it every day, the battery would stay up, but on the weekends it would discharge because we didn't use it Saturday or Sunday. Drove me nuts, but I finally replaced the alternator and that was it. The diode had gone bad letting current bleed off.

If you choose to, your local Autozone or Advance Discount will put a tester on it for you free of charge and let you know whats what.

Don