On my 2010 F150 There was a RELAY and FUSE in an unmarked envelope in the glovebox of the truck. The FORD manual told me where these two things were to be installed in the front fuse panel.
GOOGLE says "These are the fuse and the relay that powers the 'constant power' or 'battery charging' circuit on your 7-pin trailer connector. The relay goes in position #9 and the fuse goes in position #21 in the power distribution box (fuse box) in the engine compartment." This is for a F150 FORD so don't know if it will be the same for the F350 or not. I think the magic words in the manual is to look for TRAILER TOW FUSE and TRAILER TOW RELAY.
This becomes activated when the ignition key is turned ON and will go OFF a few seconds after you turn the ignition key OFF.
One post on the F150 forum said the fuse and relay kit p/n is AL3T-15A416-AA but mine was in an unmarked baggie. I think you can buy a fuse and relay kit from etrailer. I would just go to Ford Parts dept and pick them up.
Like others have said here this should not be use as anything PRIMARY as the small wiring to the 7-way trailer connector is small. Should only be used as a trickle charge PLAN B for the trailer batteries. You DO NOT want to mess with RULE #1 and that is to mess with the truck START battery system as this is the way you get back home.
For camping off the power grid I have installed a smart mode converter/charger on my trailer and have increased the trailer battery power to run all the things we want to run from 120VAC using an inverter and 12VDC things directly connected to the battery bank. When the battery bank gets down to around 12.0VDC (appx 50% charge state) we will run our 2KW generator to re-charge the batteries. The game plan of course is to plan for this to happen around 8AM in the morning when you are allowed to run your generator at most camp grounds for a quick three hour generator run time.
My truck START battery stays fresh, my trailer batteries all get immediately charged back to their 90% charge state for the next day/night camping run, the camping world is running at normal.
Roy Ken
