I have a 98 C1500 that's been showing voltages around 10v while running. Two batteries were installed when i got the truck and i replaced both batteries and cleaned and checked all grounds the batteries were attached to about a week ago. It ran at around 14v before I replaced the batteries and for a few days after the batteries were replaced. In the last few days its been showing around 12-13v while running and was at around 10v when I drove home from school today, by the time I got home the radio was fading and turning on and off. I hooked a multimeter to the battery once I got it home and it showed 11.8-12.2v while it was off. It starts up just fine and the belt is tight and looks fairly new. What could this be?