Help!!!!! Please!!!

I have a 2003 mustang...It was working just fine yesterday. Then we got into it and after driving about a mile the battery light turned on, on the instrument panel. It drove fine for about 20 minutes, then suddenly the battery gauge started going down and my instument panel and headlights were very dim. I went and go the battery tested and they said it was bad. So I got a new battery but on the way home it did it again. I turned the car off and now it won't turn on. Please help me what can this be???:dunno:

A bad alternator!

A bad battery could create a big demand for power, the alternator probably got fired.

A new battery wouldn't last long, because of the electronic fuel pump, computer, control modules, digital dash, and head lights.

Definitely a bad alternator. They're very easy to replace, though. I'm looking for a remanufactured one myself to replace mine which has bad bearings.