It takes 25.3 volts to swing the meter to 100%.... Fully charged may show 25.1 or 29.9, depending on the type of battery. This is common in 8400's to show 0% even when a charged battery is plugged in. The battery wont maintain it's voltage after a charge and drops slowly. You can jumper the the truck with a good battery then plug in the weaker one. But you need to change the Bsoc parameter 24 by lowering it to 66 % from 75%. An 8400 really monitors the battery condition and flags code C41 if it sees the battery go below 17 volts for even a nanosecond.