How important is a battery isolator?

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Don Peterson

Well-known member
Mar 9, 2005
St. Charles, IL
I am new to RV-ing in that I have done a lot of tent-camping and pop-up trailer camping, but I now have a new Jayco Jayfeather LGT 26S travel trailer. Going through the owner's manual they talk about a battery isolator. I called my RV dealer about it and was told that most people don't install one.

I am just wondering how important this device is. Do you recommned one? What can happen if I don't install one? Also, where can I get a battery isolator and, is it installed on the trailer or on the tow vehicle?

Don Peterson

They could be talking about two different kinds of battery isolator:

  • A switch located on the trailer that disconnects the trailer's house battery while in storage (can help prevent the battery being drained).
  • A device that sits under the hood of your car/truck and allows both the car battery and trailer battery to be charged from your engine alternator as you're driving down the road without one affecting the other.

The first kind can be found at RV supply stores, dealers, or marine/boat stores. The second kind can be bought at virtually any auto parts store. I'll try to find some photos to upload for you to see the two.
I was looking for the battery isolator that mounts on the tow vehicle and is used to charge the auxiliary battery and tow vehicle battery separately. It also keeps the trailer from using the tow vehicle battery power when the trailer battery runs low. As long as I unplug the power plug from the truck when I am stopped for any length of time, I should be OK. I think I could find other things to do with the $80 or so that it costs for the battery isolator and associated wiring kit. But if anyone has had any caveats about this, I am willing to listen.

This is an example of the battery isolator you're talking about. Since the battery in the tow vehicle and the one in the treler are probably different (one is a starting battery and the other a deep cycle), I think you'd definitely need one of these.
Thanks for the feedback Tom, and for the link to the Hellroaring Tech site. After reading their detailed descriptions, it looks like I'd better pick up one of these little puppies, just to be on the safe side.

You're welcome Don.

I have a new one I bought last year, but decided I didn't need it. Would sell it for a discount but, in the last few months we consolidtated two houses into one, and I can barely get into my garage let alone find anything.
I couldn't get a bicyle in the garage right now Ron. I'm extremely frustrated. I can't even find the BBQ I took off the coach before putting it back in storage. How can I lose a BBQ in my gararge  ???

I have one car in a neighbor's garage. We gave a bunch of good furniture to our daughter, a bunch to the Salvation Army, washer and dryer went to a neighbor, and I have a guy with a new house and no money coming by when we get home Sunday afternoon. Might be able to get one car in the garage again!
My $0.02...
I see no need for an isolator on a trailer. Just unplug from the tow vehicle (which you would normally do anyway on most campsites) and you will be fine.

If you bought even half the stuff that somebody or other says you need to have on your RV you would need to win the lotto regularly to keep up.  By all means ask questions and take advice, but take it slowly and figure out what is important to you before getting out the credit card.
Thanks for the advice, Gary. I know that if you are not careful, you can get nickel-and-dimed to death with "necessities," especially someone who is new to RV-ing as I am. But I thought some of my travels might take me to places where I would end up sleeping in the trailer without disconnecting from the truck, so thinking it's better to be safe than sorry I went ahead and ordered one from J.C. Whitney at I asked them to size it for my rig and the bottom line price was about $70.00, which included a connection kit.


I agree with Gary - there's no need for a battery isolator on a trailer and having one may actually do more harm than good.

On a motorhome, an isolator is needed because both the house and chassis batteries are hard wired to the electrical system.  You need to be able to seperate the two systems when you stop, then re-combine them to charge from the alternator while you're driving down the road.  That's what the isolator does.  In a trailer, it's easy enough to just unplug the trailer's electrical connector from the tow vehicle when you stop for the night.  So there's no real advantage to installing an isolator.  Just run a hot wire from the output of the alternator (or the + terminal of the vehicle's battery) back to the connector.

The downside of using an isolator for trailers is the additional voltage drop through it.  Solid state (diode) isolators lose about a half volt between the alternator input and the battery outputs.  The charging rate to the trailer battery is already limited by the length of wire feeding the trailer battery, so unless you can raise the voltage of the alternator to compensate for the isolator's loss, your charging rate will drop even lower.

In other words, installing an isolator for a trailer is more trouble than it's worth.

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