Help P2502 code

[MLimpert]

Driving along the interstate tonight at a steady speed, I noticed my dash lights were unusually dim. A few seconds later a the check gauges light came on and the volt meter pegged all the way to the left to 8 volts. I pulled over and checked codes and I had a P2502, which seems to be "Charging System Output Voltage." I got the code cleared at autozone, and headed home. The headlights and dash lights were bright and I had no check gauges light. Midway back, it tripped the code again, and the lights dimmed back down.

What is going on with my truck? What causes this code? Any suggestions of things to check?

I think I'm heading to the dealer tomorrow morning - how long can I drive with 2 fully recharged batteries? Its about 45 minutes to my favorite dealer.