RV Electrical Plug Question

[Motorman001]

My truck is a 2015 RAM 3500 DRW Cummins with dual 220A alternators. My question concerns what is commonly referred to as the 12V power or auxiliary charging connection on the 7-way RV receptacle. This terminal is always hot, even when my truck is parked and turned off. I suspect that is normal, and I confirmed it with a volt meter. The circuit is protected by a 30A fuse located under the hood in the main fuse box. My fifth wheel has two large 12V deep-cycle batteries. It also has a 120V residential refrigerator that runs off an inverter when towing (converting 12V DC to 120V AC). Assume I was camping without shore power and ran the fifth wheel batteries down to, say 50% charge, then hooked up the fifth wheel to the truck and took off driving down the road. The 12V auxiliary charging circuit is going charge the two fifth wheel batteries in addition to powering the inverter which is running the refrigerator. The fridge doesn't run constantly but let's assume that in hot weather it runs 50% of the time. Taken all together this load sounds like it might be more than 30A and enough to blow the fuse which protects the wiring - but in actual practice the fuse doesn't blow, the batteries stay charged and the fridge stays cold. So here's the question: Is there some sort of current-limiting circuitry in the truck, in addition to the fuse? Or is 30A more than enough to charge two deep-cycle batteries and run the inverter for the fridge?

 

[Travelin Ram]

You won’t get anything near 30 amps due to the resistance of the small wiring in both the truck and trailer. Some RV operators rewire everything with larger wiring. It’s doubtful that 7 blade connector could withstand 30 amps sustained without overheating too.

The best solution IMO is a DC to DC charger if it’s important to charge trailer batteries from the truck.
Here’s one source.

https://redarcelectronics.com/collections/in-vehicle-dual-battery-chargers

 

[Motorman001]

Yesterday I had the back off the 7-way receptacle so I could wire in auxiliary back-up lights. The 12V auxiliary circuit and the ground appear to be 12 gauge wire. The ground also carries the current for the electric brakes. The other wires look like 14 gauge. I would have expected 10 gauge for a 30A circuit; but apparently RAM thinks 12 gauge is sufficient. In any case, the batteries do stay charged and the fridge runs fine. No signs of overheated wiring or connectors. I'm just surprised it all works as well as it does.

 

[stevenP]

I dont think this circuit was ever designed to fully charge the batteries, more like help top them off after your converter running on AC, in the RV charged them up. I never even think that truck to RV connection is anything more than a trickle charge. The inverter that runs the residential fridges in the RV's are a power hogs. If you have two deep cycle 27 series batteries like I do in my rig, you might be able to run the fridge like 5-6 hours before the batteries are dead. And I do mean dead, which isnt good for them. I bypassed the fridge inverter system in my RV, its a waste of time for us as we usually only tow to sites 2 hours or less away.

 

[Ribtipram]

Yea im not sure its made for charging as much as its made for supplying 12v's to the camper for the electric jacks and such. Not that it dosen't somewhat charge in a way. But not like an alt or the campers charger. A set of jumpers from the truck to the camper batteries would be quicker i would think.

 

[sandawilliams]

So the residential fridges dont have an LP option?

 

[Ribtipram]

No they don't. They are nice and big. But if you don't have an auto inverter switch over and the power goes you are screwed. I went with a mid sized fridge with the propane option for that reason.if your going to be camping off grid you need a gen. I got a cheaper champion gen for that purpose. Its mid quite but was 250 bucks and works great.

 

[2003F350]

Guys...YES that 12 gauge wire will fully charge 2 batteries on a trailer while running everything else. A 6 hour drive is about all it takes. I know, because I've been in the hypothetical situation that OP presented, and everything was fine. Batteries charged, fridge stayed cold.

I know some people upgrade their wiring, and while it is perfectly fine as a safety precaution, it's not necessary. They've been building RVs like this for decades with few issues. About the only way you'll generate too much heat is if you have a short in the system.

 

[Motorman001]

I just want to clarify something in my original post. When I stated, "This terminal is always hot...", the word "hot" means there is always 12 volts present. I'm not saying that the temperature is "hot". I think this terminology might have been misleading. Thanks to everyone who responded.

 

[tron67j]

For the trailer, you would need to see what your inverter draws at rest and under load. The refrigerator and battery charger are possibly all run off the inverter, so your draw amperage maximum is driven by the inverter and not directly by all you have running. Look at the tag in it to find draw.