As you probably know the trailer brake controller (TBC), if you have one, controls the charging of the trailer or camper batteries. You must hook up to the trailer or camper, the truck must sence that there is a battery, and the trucks brakes must be depressed to start charging.
Well, it didn't seem to be working on my dump trailer, and it is really hard to trouble shoot becase as soon as you unplug thr trailer cord the power shuts off. So I bought a Bluetooth volt meter to put on the trailer battery so I could see what was happening from the truck while I was driving. It also has a graph that came in very useful.
I only did one test so far and it was interesting. The trailer battery showed 12.54 volts before hooking up to the truck. After hookup voltage stayed very close to the same maybe .02v change. What was interesting after shutting down an re-starting the truck I could see a pattern in the graph. Each re-start had 4 very slight voltage dips evenly spaced, like the truck was putting a very slight draw on the battery. After about 10 miles the battery was only about 12.56, only .02 volt increase, and not near what I would like to see. I then used the electric pump in the trailer to dump the load. Voltage went down quickly to about 11.5v, the after to pump shut down I was soon up to over 13v at the trailer battery. It seems that the truck didn't want to charge the battery that was at about 80% charge, but after it got below that it kicked in. I do not know if there is something in my trailer or the trailer battery that is causing this or the truck itself. Next time I hook up to it I will run the pump as soon as I hook up the truck a hit the brakes to see if it starts charging sooner.
I will do more tests and report back.
Some of you will post that you put the fuse and relay in you truck to get the trailer charging. My truck does not have anything to plug into at those position in the fuse block, so that will not make a difference in my appliation.

 

What year is your truck? I just bought a 2021. Had a '15 before and it always charged when the key was in the run position. I guess I didn't know that it won't charge unless it senses trailer brakes?? I haven't pulled yet and now i'm wondering......

 

it's a 2018.
It does not need to see trailer brakes, just a battery connected to the connector..
For the charge circuit to work thr truck must be running and the trucks brakes applied once..
This is for 2017 and newer with the factory trailer brake controller

 

I have a '19 and I have the same problem. Hopefully someone will post up some answers or solution.

 

Trucks with the factory brake controller don’t need the “missing” fuse and relay.

The truck doesn’t need to sense trailer brakes, but it does need to sense trailer brake lamps.

IIRC, in order to activate the trailer battery circuit on a 2017+ with the factory trailer brake controller, you need to connect the trailer, sit in the driver seat with the driver door closed, start the engine, and press the brake pedal. Opening / closing the driver door will interrupt / reconnect the circuit.

If your trailer doesn’t have brake lamps, then an LED trailer lamp adapter plugged in between the truck and trailer will do the job.

Or, something close to this.

HTH,
Jim / crewzer

 

My truck recognizes the trailer every time I plug in, I get the msg about the cross traffic alert turned off and the screen goes to the towing page.
Part of the reason for the Bluetooth voltmeter is I can read the trailer battery voltage from the driver seat, that is what my post was about.
it did start to change, but not untill the trailer battery got below 12 volts.

 

This is a very interesting post I just got a F-350 super duty Loaded and have a 12' MSI Trailer my wife uses for Craft Shows and the trailer does have electric brakes. I would like to put a Battery And Electric winch. THe Pug was Funcked Up on the trailer so I purchased a wnew One From O riley Auto Parts and it was too short! The Wiring on this trailer looks like a bunch of kids were playing under it my granddaughter could do a better Job. I have ordered a 11" plug and junction boc and will need a couple more I plan to run all wires in seal tight like it should be done instead running the wire main trailer connector wires with dangling wires through the connector. I plan on mounting the Battery Box inside the front of the trailer and put a 110V battery maintainer on the Battery because I Have power for an RV outlet (we plan on be getting a fiver this spring. Thanks!

 

Interesting! I’ve been involved in many 2017+ trailer battery charging discussions, and this is a new “feature” for me.

As best as I can tell, our 2017- and 2019 F350s operate as I described above, and we are generally able to run our camper fridge and sort-of charge the camper batteries while underway.

However, there’s still not enough voltage and current to run our fridge (up to ~21A) and charge the two Group 24 camper batteries, even with the 200 W of solar array on the camper roof. One of my Spring projects will be to run a higher-capacity charging circuit from the truck to the camper.

HTH,
Jim / crewzer

 

Given that observation, I would have to conclude that something is wrong with the trailer power circuit on your truck. I get a constant voltage of at least 13 volts, no idea on how much current.

 

Jim, as I posted. I don't know if this is a quirk of my trailer or trailer battery or something else.
I need to do more testing for sure

 

Try it on another trailer to see if the same thing happens.

 

What you need to do is to plug in the trailer but have your trailer battery disconnected. See what the charge voltage (unloaded) is. Then reconnect the battery and see how much it drops. Since the charge circuit only supplies mA (I can't remember exactly how much but way less than even a 1/4 of an amp I don't expect the battery voltage to be very much higher than the unloaded voltage while it's charging.

It's a stupid design. Better off cutting the charge wire from the 7 pin and running a larger wire from the battery to there. At least you would then have enough current to operate a trailer jack

 

I do not think there is any voltage at the trucks charge terminal unless the truck sences that there is a battery connected to it.
There have been several posts about trailers with interior lights but no batteries that the truck will not send power to operate the light..
in my post I mentions some small evrn spaced voltage changes after the truck is running and the brakes have been applied.
I believe the truck is checking the trailer batteries

 

OMG guys! This is insane! I'm going to make a trip to our storage place and plug in to my 5th wheel. If all it gives you is 1/4 amp, that aint gonna work for me.....

 

Its definitely not limited to 1/4 amp. When I brought my camper to storage, I had to do a little bit of work to winterize it. I had already taken the batteries out. Powered only via the 7 way plug from the truck, I operated both slides simultaneously, tongue jack, lights, furnace, and water pump.