When my battery warning light came on, I figured it was time to get new batteries. I drove the truck for one or two days. Then when I tried to start it the next morning nothing...Dead battery, warning light lit & battery gauge reading low.

I bought two new replacement batteries & installed them.
The battery warning light still remains lit, but the battery gauge reads normal.

I am able to start, drive, use the headlights & normal features of the electrical system.

When the truck RPM pings above 2000, the warning light goes out.
From what I understand an alternator does not increase voltage as the RPMs increase, so I should notice no difference.

I checked the serpentine belt, it appears to be snug & in good shape.

Any suggestions before I visit the repair shop?

 

Take the alternator out and have it tested. Don't know if it has an internal or external voltage regulator but have it tested too.

 

Replace the alternator