Charging your trailer with your tow vehicle alternator

Charging your trailer with your tow vehicle alternator

By Russ and Tiña De Maris 

You’ve boondocked (dry camped) for a couple of days now. You’ve taken showers, run the lights, maybe watched a little television. You know your trailer batteries need a little charge, so why not just fire up the truck engine and charge them up? Not a bad idea, but if you’re not properly equipped, you’ll find that you can charge like mad and not seem to make headway. What’s the problem?

While your trailer “umbilical” cord does have a “charge wire” in the connector, typically the size of the wire is quite small. To adequately charge an RV “house” battery, a heavy gauge wire is required. Think of it this way: Your rig is on fire, and you’ve got to put it out. The firemen arrive, but the hose they connect to their truck is only about as big around as a soda straw. Bye, bye RV!

What complicates the situation is that the farther your trailer batteries are from the engine compartment of your tow vehicle, the larger the diameter of the charge wire needs to be. On some trailers the manufacturer actually puts the batteries clear at the rear of the rig, compounding the problem.

In our rig, we calculated the need of 40 amps of charge power for the batteries, and ran the appropriate heavy wiring from the battery isolator under the hood of the truck back to a heavy-duty fitting on the rear bumper. We then ran heavy gauge wire from a corresponding receptacle on the trailer to the RV house batteries — a long lead on the “trailer” side attaches to the truck side. Now whenever the truck is running, a good amount of current gets to the house batteries.

If you do this, you can use the truck frame and RV frame as one “side” of your connection (negative ground in our case). If you use a positive wire and negative wire for the connection, be sure to take the entire length of the wire run (both positive and negative) into consideration when you calculate your wire gauge needs.

Here are a couple more considerations: You may find it beneficial to upgrade your tow vehicle alternator to enable pumping more juice back to the trailer batteries. And on connectors between the tow vehicle and the trailer wiring: You see in the illustration we used an off-the-shelf electrical connector system. It has its limits in the amount of power that can be transferred. If you have more dollars, here’s a link to an outfit that can set you up with a connector system rated at 350 amps.

In “a pinch” situation with the truck disconnected from the fifth wheel, move it around to where you can reach the fifth wheel batteries with a good heavy gauge battery jumper cable. Be sure to observe polarity!

photos: top: Russ and Tiña De Maris; bottom: centurytool.net

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