can the second OEM battery be separated?

Diesels with the glow plugs have a significant electrical drain prior to hitting the start switch. Remember, you wait for the glow plug light to extinguish before starting. A single battery might be to low of a voltage for the rest of the engine controls to even function. So the dual battery as standard is what many vehicles come with. Ford has not put large capacity batteries (series 27 and series 31) in vehicles for many years which is too bad. The standard batteries in Transits have a vent tube going to the underside of the vehicle to get rid of fumes.. The AGM batteries do not require the vent tube. I have installed an aux AGM battery in my base model T150 and have a relay that I control to connect to the electrical system in case my factory battery is low. After changing out some of the original lighting with LED's I now have much longer run time with the cargo and dome lights turned on.

I have the stock single battery under the seat and installed an AGM series 31 behind the seat. Connected ground to the chassis wall next to some of the factory grounds. Connected the positive through an 80 amp manual disconnect breaker and relay to a positive post under the seat. My rig has spare positive mounting bolts on a bus bar in the battery compartment. I control the relay with a switch in a panel that I installed in the overhead cubby. I am also controlling other items with the switch panel, as my van is used for communications maintenance and fire department traffic control. Haven't seen any problems with my installation and no factory wiring has to be touched. The CPP terminal on the side of the seat has 60 amp fusing, so I have a 50 amp manual disconnect breaker from that point to provide power to some of my accessories. I just received the Helm CD that has service and electrial wiring so now can really see what the factory did for this rolling computer system.