I have 2004 Mustang GT and the battery light appeared on dashboard. It comes and goes and also notice when it does pop on the car feels like it is surging.. took it in for test and the battery is good and they tested the alternator and that was good also. they did have me replace the main belt..but after that is when the light would come on. the little box on dash shows its where it should be. doesnt change if i turn on lights or radio it never drops unless i turn the car off..

any suggestions?????

 

I would change both. My father had a similar situation, especially at higher rpms, but it turned out to be the alternator. Now, I suggest the battery as well since a car battery will be on borrowed time after 3 years, especially if you live in an area that has winter cold and summer heat.

 

Its generally the alternator diodes going bad. It sounds like its switching. My recent alternator did the exact same and I had to replace it. It would function fine unless it was a really cold day out and i would drive it before the car warmed up.