Just solved a trailer battery charging problem (with the help of this forum!) and thought I'd share the results.. By way of intro, we realized that we had a problem when we awoke in our travel trailer at 4 o'clock in the morning to find the trailer freezing cold and the furnace not working. After doing some checks, turns out the battery was quite dead. We had originally thought the battery itself had cracked, but turns out that it was simply dead as a door nail. Seems it hadn't been getting charged during the 4 previous days of drivings & camping. The remainder of the trip wasn't so bad as we were able to charge the battery back to full using the on-board charger.

Fast forward to yesterday when we finally got back home and I had a chance to look into things. Turns out our truck was missing a relay under the hood (there are also a couple of required fuses, one under the dash and the other in the same fuse box as the relay, but these were already in place). Noticing that the missing relay looked exactly like several others, I stole the power window relay and plugged it into the slot. Voila, there was now voltage on the Bargman charging pin! Of course, now the driver side power window didn't work so a trip to NAPA was in order (also tried Schuck's, but they didn't have any of the right part on hand). Now everything works great!

For reference, NAPA part #AR272 is a perfect match for the Ford FOAB-14B192-AA full relay.

 

Some vehicles came with a pigtail and relay kit for adding trailer controller and relay(s) for charging the trailer battery. Lots of owners didnt use it and didnt pass it on to the new owners.

 

So I've been reading... I was told at the dealership that the previous owner had towed a horse trailer a couple of times with the truck before deciding that she needed a bigger one (truck, that is). Thankfully for us this meant that everything was already wired in properly, including an aftermarket electronic brake controller. I guess there probably wasn't a need for the charging system, though...

 

Not to be a hard a** but you always check all these things out everytime you hook up to tow and never trust what anyone else may have or not done. It's to important to safety.
Trailer tow battery means to me there has to be power to apply the trailer brakes in case of trailer break-away.
The battery for utility use is secondary.

 

Just note that it only has power when the key in on. This keeps it from draining the truck battery when it is not running.

 

Pet peeve of mine... it also prevents 12v lights on the trailer from working with the key off. I hate to say it, but my friend's C2500 powers lights when the key is off... and I don't think he's ever had a shot battery!

Horse trailers DO need a charger line, but they don't show it. People are terrible about checking their brake-away batteries, I betcha most are dead! But its not something you notice unless you look hard. Kinda like your problem

 

I wonder what the minimum voltage and/or amperage is in order for the emergency brakes to work... Guess it's time to flip through the stack of manuals that came with the trailer again, though I seem to recall that most were pretty skimpy on details such as this.

No, my bigger problem is how the emergency brake cord is currently hooked to the breakaway chain (this was done by the trailer dealership); as-is, the chains are so short and the cord is so long that the cord would never get pulled taught. I imagine it's a fine balance between having it short enough to do the job as intended versus having it too short so it sets the brakes off all the time....