I've had your problem and the same non-existant warranty on the Motorcraft battery wasn't worth the paper they wrote it on. Believe it or not, your car is designed to "go to sleep", just like your computer. If you have anything plugged into your power points or cigarette lighter socket (other than the lighter unit), then your car detects this power drain and remains active pulling a lot more juice than when it sleeps. I started unplugging my power points and have no more problems. This is the info I got at the dealership and it appears to work for me. I had a radar detector and GPS and MP3 player all plugged into 2 power points and it was always going dead. It finally ruined the battery which I replaced and no more problems.