Fifth Wheelin'
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Wednesday, August 22, 2007

Charging Your Fifth Wheel Battery With Your Truck Alternator

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 fifth wheel 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 fifth wheel "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 fifth wheel!

What complicates the situation is that the farther your fifth wheel batteries are from the engine compartment of your tow vehicle, the larger the diameter of the charge wire needs to be. On some fifth wheels the manufacturer actually puts the batteries clear at the rear of the fiver, 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 is 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.

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!

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  • Nice article. An excellent resources for all things 12volt DC is "managing 12v systems" by Harrold Barre.

    I like the idea of the heavy duty connector, but I would think about a different style if you can. The connector you show in the photo appears to be a 30 amp RV connector, and although you know what it is for, it opens the possibility that an error could be made by the next owner or someone who is helping set up. Most Battery Stores (places that sell mostly Battery stuff not a regular auto parts store) have very nice high amperage connectors that will move tons of power to your RV and close with a spring loaded cover when disconnected. You see them on Tow Trucks for the Jumper cables to plug into the truck itself.

    A good point about the battery isolator. Many people might not think about it but without the isolator, you can drain your trucks battery if you turn it off and forget to disconect.

    By Anonymous Matt Elyash, at 7:02 AM  

  • The heavier wire will work fine but check to make sure that your alternator is big enough.
    Most vehicles use up to 85% of the power put out by the factory alternator just to run all the electronics that the vehicle needs to run.

    By Anonymous Anonymous, at 4:34 PM  

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