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Disconnect 7-PIN for charging

horseplay

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Aug 19, 2021
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My camper has the WFCO 8735 converter. The 7 pin is connected to the camper for trickle charging and signal and brakes. Since I will be replacing the Lead Acid battery with Li, I want to disconnect the 7 pin from trickle charging the battery. I do not want to install a DC/DC charge, just want to disconnect. How do I do this?

I guess there is a positive wire going into the WFCO DC side and I can just disconnect it. Question it will it also disconnect the brakes?
 
If you disconnect the correct wire, it will not disconnect the brakes. Two different wires. And the emergency brake actuator should still work as well.

Why do you want to disconnect the trickle charge from the tow vehicle? I had to do that for a while, too after I converted to LFP. My 2015 Chevy 2500HD was not isolated from the trailer. It charged fine when the truck was running, but then it discharged the trailer batteries when I turned the truck off. I ended up putting in a smart DC to DC Victron charger.

If you decide you really want to disconnect the charge wire, make sure you hook it back up before selling the trailer.
 
If you disconnect the correct wire, it will not disconnect the brakes. Two different wires. And the emergency brake actuator should still work as well.

Why do you want to disconnect the trickle charge from the tow vehicle? I had to do that for a while, too after I converted to LFP. My 2015 Chevy 2500HD was not isolated from the trailer. It charged fine when the truck was running, but then it discharged the trailer batteries when I turned the truck off. I ended up putting in a smart DC to DC Victron charger.

If you decide you really want to disconnect the charge wire, make sure you hook it back up before selling the trailer.
Just don't want the complexity and I will not run a dedicated wire from the alternator anyway. So would rather spend money on getting solar if there is a need.
 
Then I would say the easiest solution is to just pull the fuse in your tow vehicle.

Why would you need a dedicated wire from the alternator? If I did nothing, I would get a nice 4.7A going into my LFP batteries while towing. No added complexity. You don't need to add a DC to DC charger. Just leave all the wiring just as is and let it trickle charge the LFP going down the road. At that low amperage, you don't need a charge controller.
 
Then I would say the easiest solution is to just pull the fuse in your tow vehicle.

Why would you need a dedicated wire from the alternator? If I did nothing, I would get a nice 4.7A going into my LFP batteries while towing. No added complexity. You don't need to add a DC to DC charger. Just leave all the wiring just as is and let it trickle charge the LFP going down the road. At that low amperage, you don't need a charge controller.
But you're saying the battery is drained when TV is not running? I am not concerned the LFP not getting charged just don't want the battery and alternator killing each other.
 
I guess it doesn't matter since when the vehicle is not running the 7 pin is disconnected anyways.
 
I guess it doesn't matter since when the vehicle is not running the 7 pin is disconnected anyways.
If that is the case, then it doesn't matter. On some vehicles (like mine), if the 7 pin is hooked up, it is connected to both batteries. Most new vehicles disconnect the 7 pin connector when the vehicle is turned off.
 
There are two brake "systems" on an RV trailer. The normal brakes are actuated by the tow vehicle. You shouldn't need to worry about this. The breakaway system gets power from your trailer's battery. You really don't want to disconnect this!

On my trailer I took a hybrid approach. The 7-pin circuit does not charge the LiFePO4 battery bank. I disconnected the wires coming up from the tongue to the main distribution center and that took care of that. Pretty easy. Here's where I cheated. I retained a small lead acid battery on the tongue to start the generator, run the tongue jack and most importantly power the breakaway system. The 7-pin circuit still charges this small battery.
 
There are two brake "systems" on an RV trailer. The normal brakes are actuated by the tow vehicle. You shouldn't need to worry about this. The breakaway system gets power from your trailer's battery. You really don't want to disconnect this!

On my trailer I took a hybrid approach. The 7-pin circuit does not charge the LiFePO4 battery bank. I disconnected the wires coming up from the tongue to the main distribution center and that took care of that. Pretty easy. Here's where I cheated. I retained a small lead acid battery on the tongue to start the generator, run the tongue jack and most importantly power the breakaway system. The 7-pin circuit still charges this small battery.
Thanks. I took a look at the wiring today and this confirms my findings. I do not have access to all the wiring so to do what you do is a much bigger job. so here is what I plan to do.

The existing battery positive goes to a switch. On the other side of the switch are 3 wires. One thicker one (wire 1) goes to the main DC panel. another thinner one is for the emergency breakaway. Another one I have no idea. What I will do is to setup the new battery inside and have it connected to the main panel. Disconnect the old battery and leave everything else in place. So this way there is still power at "the other side" of the switch from "wire 1", which powers the breakaway system and the unknown wire. Is there any issue with this?
 
I would want to know where that unknown wire #3 goes. If you don't disconnect anything then you'll likely still get power from the 7-pin to the new battery bank.
 
If that is the case, then it doesn't matter. On some vehicles (like mine), if the 7 pin is hooked up, it is connected to both batteries. Most new vehicles disconnect the 7 pin connector when the vehicle is turned off.
Unless it is a GM vehicle where it's hot with the engine turned off.
 
Unless it is a GM vehicle where it's hot with the engine turned off.
They still do that on new trucks? I have a 2015 Chevy 2500HD and it stays connected all the time, too. I ended up putting in a Victron 18A DC-DC smart charger. It senses when the truck is off and cuts off the 7 pin connection. Pricy solution to something GM should include.
 
What vehicle because I've never seen a relay activated trailer power circuit?
My 2018 F-150 turns it off. In fact, it is even more complicated since it needs to see a load before it will turn it on. Unfortunately with the F-150, it also has a tiny wire going to the 7 pin so its pretty much useless to give more than a couple amps. The Ford has a special trailering module that controls it
 
2017+ Ford Super Duty won't power the 12v trailer charge circuit unless the following conditions are met:
1. Engine must be running
2. Brake pedal must have been pressed, after #1
3. It must sense that the trailer is actually there. The draw from the brake system is key here I believe.
4. Wave a dead chicken over the dashboard, while mumbling suitable voodoo mumbo jumbo

This functionality came up from the F-150 which has had it for a few years. It's a 30 amp circuit. 30 amp relay, 30 amp fuse. But you're unlikely to see too much more than about 20 amps.
 
One thing I am happy about with my 2015 Chevy 2500HD is that it uses 10ga wire on a 30A fuse for the +12V accessory line. That puts 13.6V at the connector when the truck is running. The DC-DC 18A charger can get the full 18A out of the line @ 14.4V for charging my battery bank going down the road. More than enough to run the residential fridge and even put a little charge into the batteries.
 
On my F-150, I put in 6AWG from the battery to an Anderson connector to run my 30A Victron DC to DC.
 
I have a T@B teardrop trailer withe the same WFCO converter and wanted to disconnect charging also. I pulled the fuse in the tow vehicle which was the easiest solution.

Another option for me was to disconnect and tape off the charge wire in the 7 pin distribution box under the front of the trailer. Regardless of what you do, you should confirm there is power to the breakaway brakes when not connected to your tow vehicle.

Side notes on my WFCO 8735…I’ve seen instances, while on shore power, where power is drawn from the battery instead of the converter. It only happens when the battery is fully charged (from my tow vehicle’s dc to dc charger). I’d never would have known without my BMS App or a shunt. Also, I don’t get much more than 80% charge as it’s not a lithium charger model. I have to disconnect power to the converter and wait 10 minutes then turn off the battery and restore power to the converter, then turn on the battery. The charger will kick back in and charge the battery some more.
 
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