Tow vehicle battery isolator - Fiberglass RV


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Old 06-12-2013, 11:58 PM   #1
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Tow vehicle battery isolator

I was told I need an isolator device in the AUX power line from my vehicle battery to the camper battery/rear plug on the car. Any suggestions on what to get and where? The local parts store wanted to sell me a big honkin multi-battery charging device.
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Old 06-13-2013, 12:40 AM   #2
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You really don't need an isolator. I don't have one. If I'm going to be parked overnight while hitched up I simply unplug the trailer from the tow. If it's just for lunch I don't worry about it. There was a big discussion about it a few days ago.

In my humble opinion an isolator of any kind is a failure point, which when it fails you'll only find out when the trailer battery goes dead the first night out when the outside temperature is very cold. Don't ask how I know.
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Old 06-13-2013, 12:50 AM   #3
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Like Bryan, I just unplug the TV overnight.
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Old 06-13-2013, 01:05 AM   #4
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plus 3 on it's really not needed.

My power supply to the trailer battery is controlled by a switch. Once parked for the night the switch disconnects the two batteries. Or, has been said, simply disconnect the trailer plug.
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Old 06-13-2013, 06:05 AM   #5
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I agree. Keep it simple. My umbilical kept ending up on the gound so I made a holder out of PVC pipe. It keeps the dirt and water out. Raz
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Old 06-13-2013, 07:36 AM   #6
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The black wire of my trailer 7-pin connector when comes in the floor, I just leave it a long enough length for future use and cover it securely with an anti-vibration plug and label it with tape: +12VDC aux. from Tow Veh. and tuck in the trailer connector box without using it at all....
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Old 06-13-2013, 07:53 AM   #7
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If you do install an isolation relay get a modular relay They make a sealed ,plug in relay rated for 30 amps ,parallel the contacts to assure good conductivity and it will work fine, Personally I just unplug the trailer from the tow vehicle which accomplishes the same goal
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Old 06-13-2013, 08:11 AM   #8
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Whether you put in a relay/ isolator or not is your choice. The downside without one is that you can accidentally run down your tow vehicle battery.

I would definitely put in a circuit breaker on both the tow vehicle and trailer battery ends to protect against a short circuit and possible fire. I also used a small 30 amp relay and wired it to turn on with the ignition. I mounted the relay and the circuit breaker on the same board next to the TV battery.

Some of the cheap solid state isolators have a significant voltage drop that can affect how well the trailer battery charges, so I'd avoid those.
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Old 06-13-2013, 09:05 AM   #9
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I made a holder out of PVC pipe. It keeps the dirt and water out. Raz
Sounds interesting, can't really see the details in the picture. Do you have a bigger or better pictue RAZ?
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Old 06-13-2013, 09:24 AM   #10
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An isolation relay does add some complexity, could fail, and is not necessary; the same could be said for most of the features of the tow vehicle, including just about everything Henry Ford didn't put on a Model T. The day you special order a new tug without power steering, air conditioning, power windows, power door locks, and radio, I suggest also stripping it down minimalist wiring.

If you just don't find any value in the isolation relay, of course there's no need to put one in.

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Some of the cheap solid state isolators have a significant voltage drop that can affect how well the trailer battery charges, so I'd avoid those.
Right, I wouldn't use any of the diode-type isolators for this reason... and more importantly because they also interpose a diode between the tug's charging system and the tug's own battery.
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Old 06-13-2013, 09:53 AM   #11
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A simple 3 wire RV solenoid type battery isolator is easy to install and should last at least 10 years. Although they look the same, DO NOT confuse them with a starter solenoid. A starter solenoid will overheat and fail in short order if used as an isolator.

While unplugging is always an option it is sometimes forgotten, especially the time you overnight in a WalMart and it is raining. These are easy to install and provide cheap battery insurance.

Ferget the solid state type, they cost more and do nothing extra for you, and are more complicated to install.
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Old 06-13-2013, 10:44 AM   #12
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Sounds interesting, can't really see the details in the picture. Do you have a bigger or better pictue RAZ?
Just a short length (7") of 2" PVC pipe with an end cap. An offset hole lets the hitch pin be used to keep the plug in place. Joy A. did it first. Hers is attached to her front jack. My jack swivels up so I made a plate and used a muffler clamp. Raz
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Old 06-13-2013, 11:02 AM   #13
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A simple 3 wire RV solenoid type battery isolator is easy to install and should last at least 10 years. Although they look the same, DO NOT confuse them with a starter solenoid. A starter solenoid will overheat and fail in short order if used as an isolator.

While unplugging is always an option it is sometimes forgotten, especially the time you overnight in a WalMart and it is raining. These are easy to install and provide cheap battery insurance.

Ferget the solid state type, they cost more and do nothing extra for you, and are more complicated to install.
The question? How long does your trailer battery last vs how long to you plan on staying hitched up? I don't about you my trailer battery will last about 4 to 20 days depending on furnace use. With the two connected you have two batteries in parallel, the length of time before both batteries are run down is extended. I only know of one person that stays hitched up.
The conclusion is an overnight anyplace isn't going to run down your tow battery if left connected to the trailer.

One other note, I've seen at least one trailer that the owner connected the umbilical at night because there was no trailer battery.


I think a lot of the fuss and misunderstanding comes from motor homes and camper inserts. Those have the house battery connected to the cranking battery all the time unless there's an isolator. Different circumstances than a trailer which is disconnected so the tow can be used without the trailer.
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Old 06-13-2013, 11:08 AM   #14
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The question? How long does your trailer battery last vs how long to you plan on staying hitched up......? .
I think that it might depend on whether you run your refrigerator on 12 volts while towing. Of course, you would want to switch over to propane anyway when camped, but if you forgot you would have two dead batteries.
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