2007 5.9 Cummins Diesel voltage
 

My voltage goes from 10v to 15v when I start my truck. My dodge dealer said this was a bad thing. So I let them "fix" it and it still dose the same thing. When I drive it 15mph or higher it stays at 15v. I thought 15v was bad dose anybody know? Thanks

Thats your grid heaters cycling. Totally normal.

15 volts is too high, your over charging the batteries at that point. 13.6 - 14.4 is a safe range. Over charging will cause the acid in your batteries to evaporate, and lead to battery failure. I'd go back and tell them to "fix" it again.