buckman98

So today I accidentally left my lights on and it run my battery dead . It took a lot to get it started ! But I went to advanced auto just to see what they would say about seeing if my battery was bad . But instead the battery was good and the computer said that's the alternater was bad . Although the battery gauge says it is charging up ! Do u think the alternater is Bad or still good ? Cause I think it's still good cause it isn't even a year old .

buckman98

This is off a 91 f-150 302

Iron Chief

If you had only run it for a short while on the way to the store, the alternator may have still been struggling to recharge the battery at the time of testing. The tester doesn't know that the battery was dead just a few minutes ago. It is working off of set parameters in the programming that says if the truck is running it should be reading this many amps and if the truck is off it should be reading this many amps.

Your alternator is probably still fine. One way to tell for sure is if you are able to start the truck with no issues and begin to lose more and more electrical power the longer it runs. OR, you can just test the alternator with a multimeter.

Manuellabour247

Put the battery on trickle charge to completely recharge it. Then take it back and retest it. A low battery can give false readings on the electrical system. Just like bad grounds.