2001 Alero charging problems
 

I have a 2001 Alero that died while my son was driving it. The battery light is on- I had the alternator checked, it is fine. The belt tensioner was weak- so I replaced it. The car will start, run only about 20 seconds and die. I changed out the battery with one from a working car, it starts fine, but the battery light will not go out. I checked the wire between the alternator output and the started solenoid- it is fine. Bought a new alternator just in case the one that tested good was actually bad- same problem. Measured the output of the alternator with the car running, only 11-12v. Any help would be appreciated..