sounds like you have a live wire or a bad ground. for whatever reason you have current flowing when it shouldnt be. usually a wire is stripped and touching the chassis. have yuou installed anything lately?
sounds like you have a live wire or a bad ground. for whatever reason you have current flowing when it shouldnt be. usually a wire is stripped and touching the chassis. have yuou installed anything lately?
One thing to check for that I see quite a bit is a bad and/or intermittent diode in the alternator. The battery will charge like normal when the car is running but try to act like a motor when th car is off.
The other thing you can do is start pulling fuses, one by one until you get up one morning and the car is not dead anymore.
For further and more specific trouble-shooting, do a search of JRichker's posts and threads. He's got a step by step method for using a multi-meter to track down current drains.
Typically it is something draining the battery. Small things like glove box or courtesy lights are often the culprits. If you have an aftermarket stereo or alarm system, it is also suspect.
The ideal method is to disconnect the positive terminal, and connect a Digital Multimeter (DVM) between the positive terminal on the battery and the positive cable. Set the DVM on a low current scale of 2-5 amps if it doesn't auto range. Watch the current draw, and then start pulling out fuses. When you see a sudden drop in the current, that circuit is the likely culprit. Note that the computer, radio & clock will draw less than 1/10 amp to keep the settings alive.
I had this happen to me recently with a Lincoln Mark VII, which is very similar to a Mustang in the engine bay. Although my battery would be dead within about 8 few hours of being parked.
I checked the battery...it was OK. I checked the alternator...it was OK. I checked the voltage regulator...BINGO!