So mine decided it was going to take a little longer to charge back to normal after the grids shut off. I took it off after it was decided the battery terminals weren't the problem. NAPA tested it and it failed the voltage test. I looked at the brushes, and they passed a visual inspection (didn't pull the cover off to look at the wires).

So is it still a chance the brushes are bad, or is it a little further into the alternator?

 

What was the alternator putting out? It is controlled by the ECM. Make sure the cross-over postive wire from the passenger side battery to the drivers side battery is good. The ECM senses the load on the drivers side battery. If NAPA did the test properly, then the internal regulator may have a diode going out.