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.