Charging trailer battery while driving - Fiberglass RV


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Old 05-28-2020, 12:48 PM   #1
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Name: Pawel
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Charging trailer battery while driving

Hi there,

I have a typical 4 prong connector going to my small trailer from the towing vehicle. I would love to be able to charge the house battery while I am driving. What would be the cheapest and most suited solution?

Thank you kindly for any help!
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Old 05-28-2020, 01:46 PM   #2
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You need to install 4 to 7 pin adapter and run a (fused!) charge line from TV battery to the adapter. I suggest to use 10 AWG wire for the line. https://www.amazon.com/Hopkins-Towin...694754&sr=8-36


I assume your Trillium has 7 pin connector.
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Old 05-28-2020, 02:12 PM   #3
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You need to install 4 to 7 pin adapter and run a (fused!) charge line from TV battery to the adapter. I suggest to use 10 AWG wire for the line. https://www.amazon.com/Hopkins-Towin...694754&sr=8-36


I assume your Trillium has 7 pin connector.
Thank you for your answer. My trillium also has 4 prong (hacked together) connection.
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Old 05-28-2020, 02:15 PM   #4
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The best way is to upgrade Trillium to 7 pin connector too. 4 pin connector has no free pin for a charge line. https://www.amazon.com/MICTUNING-Hea...s%2C165&sr=8-3
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Old 05-28-2020, 06:17 PM   #5
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If you don't want to mess with your existing wiring, the easiest way would probably be to add a second connector between car and trailer. To make sure you have enough capacity, run a 10 gauge wire (black or red) from your car battery positive to the back of the car. Connect another (white) wire to a convenient chassis point near the rear of the car (or run all the way from the battery) for the negative. Run another black (or red) and white pair from the trailer battery long enough to reach the car. The positive wire should have a 30 amp fuse at the trailer battery. Affix an Anderson or similar polarized pair of plugs at the connection point.

To be sure you don't discharge the car battery while camping, either unplug your new charging wire when camped or use a relay between the car battery and the trailer. If you're electrically knowledgeable and have easy access to to a signal voltage to turn the relay on only when the car is running, you can wire it that way. Or you can spend more and use an easy to wire inline voltage sensing relay. Good luck with your project.
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Old 05-28-2020, 09:24 PM   #6
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I agree with Lynn.

A second set of wires that go from the tow battery to an Anderson plug on the rear bumper. Then an Anderson Plug on wires that go directly to the trailer battery. This is not hard to rig up, bypasses all old or small wiring and can deliver 50 amps to the battery. A50 amp marine breaker is installed under the hood and #6 wires run the distance. The parts are readily available and easy to connect.

Here is mine at the rear bumper. I just plug it in when I plug the trailer in, and if I'm staying somewhere for a while, I unplug the truck. It also works well to charge the battery during your stay, by idling the truck for a while.
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Old 05-29-2020, 10:08 AM   #7
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Charging wire

I would change from 4-pin to 7-pin, you need an isolator, you run a wire from your ignition witch to the isolator, this is so when you stop somewhere you are not drawing power from your tow vehicle, then run a wire from the tow vehicle battery to one side of the isolator, it does not matter side you put it on the isolator is nothing more than an on and off switch. Then run another wire on the other side of the isolator to the 7-pin connector. I can't remember but I think it calls for a black wire. Some of the new trucks and vans that come with a tow package with the wiring already there have a built in isolator Good Luck.
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Old 05-29-2020, 11:59 AM   #8
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Randy,

That gets you to the tongue of the trailer, but not to the battery. So the next step is to get the charging wire to the battery, from the seven pin, and make sure the trailer is not wired with the hitch as the ground, or that will have to be upgraded too. Then the size of the wire has to be decided on. Then at least one fuse must be added. For simplicity, the isolator can be deleted, and the plug unplugged for extended stays of overnight or longer. Too often, the seven pin system is only a trickle charge and not enough to really make much difference at the campsite with a low battery. All of the above leads to why I recommend an Anderson plug and #6 wires straight from the tow battery with a circuit breaker. A simple and powerful system. Also, with no isolator, the trailer can charge the tow if the trailer has solar.
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Old 05-29-2020, 12:29 PM   #9
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I agree with Lynn.

A second set of wires that go from the tow battery to an Anderson plug on the rear bumper. Then an Anderson Plug on wires that go directly to the trailer battery. This is not hard to rig up, bypasses all old or small wiring and can deliver 50 amps to the battery. A50 amp marine breaker is installed under the hood and #6 wires run the distance. The parts are readily available and easy to connect.

Here is mine at the rear bumper. I just plug it in when I plug the trailer in, and if I'm staying somewhere for a while, I unplug the truck. It also works well to charge the battery during your stay, by idling the truck for a while.
Thank you, that makes sense. I would only have to remember to switch it off when parked. Or some sort of diode would do...
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Old 05-29-2020, 12:39 PM   #10
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Diode isolators are available but they drop the charging voltage by about .6 volts. In the old days, when alternators used external regulators, you could easily work around that. Now, use a relay or unplug. A voltage sensing relay is the easiest, but they're more expensive.
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Old 05-29-2020, 12:58 PM   #11
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Forgot to mention.

If you get set up with an Anderson plug wired to the battery from the tongue, it is also a good place to plug in a suitcase solar. Whereas the seven pin is not. There should be an additional circuit breaker or fuse right next to the battery, on the leads. One at the tow battery, and one at the trailer battery. They are simple things, cheap, also act as switches, and require no power. And remember, if you have solar, and an Anderson setup, you can charge the tow too. The solar could plug into the trailer plug, or the tow plug, with no adapters. Anderson plugs are not male/female, they are one piece that turns over to plug into another of the same thing.
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Old 05-30-2020, 04:15 AM   #12
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One possibility is to leave the 4 pin wiring on the tow vehicle intact and use a 4 pin to 7 pin adapter. This has the added benefit of allowing you to add brakes in the future. Finally, for a group 24 or group 27 battery, 10 gauge or even 12 gauge wire will work fine.

On the trailer, I would use a molded 7 pin cable and make all connections inside the trailer.


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Old 05-30-2020, 07:05 AM   #13
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Originally Posted by Raz View Post
One possibility is to leave the 4 pin wiring on the tow vehicle intact and use a 4 pin to 7 pin adapter. This has the added benefit of allowing you to add brakes in the future. Finally, for a group 24 or group 27 battery, 10 gauge or even 12 gauge wire will work fine.

On the trailer, I would use a molded 7 pin cable and make all connections inside the trailer.

Attachment 135057
Yes this is the way to go. 4 pin to 7 pin converter like the one pictured and run added wire(s) for charging and / or brakes from the battery to the 7-way. Except I would use a socket that has both a 7-way AND a 4-way so you can tow the camper with a 7 way as well as a utility trailer, etc. with a 4 way connection.
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Old 05-30-2020, 08:06 AM   #14
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Originally Posted by mlequo View Post
Hi there,

I have a typical 4 prong connector going to my small trailer from the towing vehicle. I would love to be able to charge the house battery while I am driving. What would be the cheapest and most suited solution?

Thank you kindly for any help!
Do you want to charge or maintain the battery. If maintaining the battery is all you need all of the advice you have gotten is great. If however you want to truly charge your trailer battery while driving you will need a battery to battery charger and heavier gauge wire.

Go to this calculator and plug 13.6 volts, 30 amps, 20 foot run of wire, and 10 ga wire you will have a 1.2 volt drop or only 12.4 volts for your trailer battery. This will maintain but it won't charge it very well.

https://www.calculator.net/voltage-d...s=30&x=47&y=17
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Old 05-30-2020, 08:26 AM   #15
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What is often forgotten is that the load is a battery which has a chemical reaction that produces a voltage.

The charge line circuit is two batteries connected by a resistance. Voltage divider rule does not apply. The voltage drop on the charge line is the difference between the two batteries voltages. If you draw the circuit and apply Kirchhoffs Voltage Law you'll see it. Larger wire increases the charging current thus shortening the charging time but has no effect on the battery voltage. No matter what size wire you use, sooner or later the battery will charge.
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Old 05-30-2020, 08:48 AM   #16
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I don't have CAD software so paper and pencil is the best I can do.

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Old 05-30-2020, 10:46 AM   #17
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"What would be the cheapest and most suited solution?"

That was the question. You've certainly gotten enough responses to find an answer in there somewhere. Good luck.
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Old 05-30-2020, 12:10 PM   #18
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Originally Posted by Lynn Eberhardt View Post
"What would be the cheapest and most suited solution?"

That was the question. You've certainly gotten enough responses to find an answer in there somewhere. Good luck.
but of course the cheapest is not the most suited, nor the best.
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Old 05-30-2020, 03:39 PM   #19
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I don't have CAD software so paper and pencil is the best I can do.

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Thank you very much for your help. I have a plan now and the parts are coming!
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Old 05-30-2020, 03:41 PM   #20
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Originally Posted by Lynn Eberhardt View Post
"What would be the cheapest and most suited solution?"

That was the question. You've certainly gotten enough responses to find an answer in there somewhere. Good luck.
Thank you, much appreciated
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