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.

TRY LOOKING AT HOW THE CUT OUT AND THE RESETS IS PROGRAMED IN THE BSOC OR DEFAULT IT BACK FACTORY SPEC.

There is a service bulletin RSI PLT -11 -002 that discusses this issue.

The assigned batteries and the "borrowed" batteries load test the same (lift and travel at the same time while checking)? Depending on the set up you might need a service key to see or adjust the BSOC settings but would be curious to see what they're set at.

I don't rely on what the gauge says. I always take meter readings wth a voltmeter on the battery. It tells me 25.something as a reading. Granted half of our chager are ferro 5 crap but it does this even on the exide chargers. btw both batteries voltage and s/g are fine.

Raymonds have to see a 50% change in battery voltage before the BSOC reading will reset. Check the 2 assigned batteries thoroughly. Electrolyte test and cell by cell voltage readings as well as load test. Just because the charger says "fully charged" doesn't mean it really is.