[klhansen]

the cable between the two batteries, must be big and Fat, with large brass terminals.

That shouldn't be a problem with a diesel van, as both batteries are paralleled at all times for starting capacity. Same as a diesel pickup except for the under body location of the second battery. But corrosion at the terminals and under the insulation is something that has to be eliminated.

 

[klhansen]

@James-Bob
Good info. Unfortunately on a van, the second battery is mounted under the chassis and a PITA to get at. That balanced charging diagram is like reverse-return piping for an HVAC hydronic system. Good if you can do it (long enough leads on the charger to reach both batteries at the same time. I'm questioning how much imbalance they're stating on the unbalanced charging. Assuming that the connecting cables have minimal resistance (are adequately sized) and the internal resistance of the batteries is the same, they should each draw very close to the same amperage on charging. But if the cable connections are corroded, then all bets are off.

 

[klhansen]

You can locate high resistance connections using an infrared thermometer.

Or you could look for the smoke drifting up like I've seen on a loose GPR connection. 😕 (not my truck.)