I don't know what's going on but my car is going through batteries like underwear. The battery is new and it won't hold a charge already. I took it out and changed the connectors didn't work, then I looked at the wire and had to cut like 2 inches off and redo it because there was corrosion working it's way down the wire. I then put it all back together jumped it , let it run for awhile, shut it off after like 20. Went out the next morning and again DEAD would not start. Please if anyone has had this problem or knows anything it's a 07 GT.

 

Sounds like the battery (or batteries) are fine and your system is just not charging, carry alternator and battery and have autozone/advance/napa/ect.... test them both. If both are fine I would suspect some screwy electrical issues.

 

Don't know that I'd bother with having autozone/pepboys/etc "test" the alternator. Their testing is about as reliable for the charging system as a politician's promise.

If after a few seconds from start-up, it's holding around 14v, I'd go with the assumption the alternator is doing it's job until you can rule out a parasitic drain issue. That said, 20mins really isn't going to recharge a fully discharged battery for us. The OEM alternator is pretty crappy even fully functional.
Charging system workshop manual:
http://iihs.net/fsm/?dir=361&viewfil...g%20System.pdf


This is actually an extremely common issue due to the OEM stereo/amps. So common, as to have a few TSBs. So, check there first.
https://mustangforums.com/mustang-ts...TED-USAGE.html

 

Yeah it holds good when it's running so I don't freaking know what to do

 

If a regular automotive battery is over drained, it will damage it and no longer be able to be charged (like you describe).

If the alternator is fine, it sounds like something is drawing power when it's shut off.

 

i would get the alternator replaced..