2007 mustang 4.0 Battery light

[Jinjy]

Question for everyone. I just purchased and installed a agm battery about a week ago and everything was fine for the first few days. I have noticed about three days ago my battery light come on at idle. As soon as I bring the rpm’s up the light goes off. Car is running fine no issues when the light comes on. Just replaced the serpentine belt and alternator 2 years ago. What could be causing the light to come on?

 

[Dino Dino Bambino]

Evidently the alternator isn't charging the battery sufficiently at idle, and there are two possible reasons:

1. The alternator is weak.
2. The serpentine belt doesn't maintain sufficient tension and is slipping on the pulleys.

If the belt is old I'd advise you to replace it as it's probably stretched. You might also want to check that the belt tensioner is working properly, and check if any of the idler pulleys has a wobble caused by worn bearings.

 

[Juice]

Was that alternator "reman" that you put on 2 years ago?

 

[Iceman62]

Run electrical diagnostic...also betting the alternator's part/mostly the issue. Are all your cables/connections/grounds tight & clean?

 

[Juice]

I am willing to bet that the alternator put on two years ago, the battery was not charged up before use.

 

[ElGuapoK20]

I also have the battery light on from a relocated battery that's in the trunk, hmmm could be alternator then.