Re: What is the purpose of the 2nd battery in a 5.4?
The van's accessory circuit will also run off the aux battery, so you can have key-on, engine off, and not drain the start battery (ditto the SMB aux switch that some vans get). It will also provide trailer power when the van is off.
Chances are, your dealer may not have actually connected the aux battery circuit (comes disconnected from factory). When looking under the hood, at the left side just below the windshield cowl there is a 4-way round connector. Typically, this connector has a yellow wire jumper plugged in. If that jumper is in place, you aux batter isn't doing anything, and is not getting charge. To connect he aux battery circuit, you remove the jumper, and plug in the other round connector hanging below, with matching wire colors.
2000 E450 dually V10 wagon