That makes sense. I have seen the charge voltage drop while driving, but this only happened when the 12v battery is already fully charged at ~12.8v. If the resting voltage of the 12v battery is lower, it continued to charge at around 14v until it is fully charged as long as the car is on READY regardless of shift being in D or P. Well, I don't think my PRIME will last 20 hours of continuous drive. I run out of gas before that. Better yet, I will need to take a break long before that 20 hours.