How to Charge a Trailer Battery Through a 7-Way Connector

Question

Question:

I need to charge a marine 12v battery that powers a winch and tongue jack in a 24inch enclosed race car trailer. The tow vehicle is a Toyota Tundra and has 7 pin round connector. I want the trailer battery to charge from the Tundra when the 7 pin is connected.

asked by: Joseph F

0

Expert Reply:

You can put a maintenance charge on your trailer battery with the 12 volt accessory circuit of the 7-way. The 1:00 position of your 7-way carries this circuit. You would just need to run a length of 10 gauge wire like part # 10-1-1 (sold by the foot), from this pin to the positive terminal of the battery.

You should note that this won't charge a dead battery, but it will keep the battery topped off when you are towing the trailer.

expert reply by: Jameson C

0

Mark E.

3/2/2020

Would you not need to run a ground wire also.

Reply

Chris R.

3/3/2020

The 7-Way harness itself will be grounded to the chassis with its white ground wire, so an additional ground isn't needed when running wiring up to the battery to activate that 12V circuit.

Reply

Mike C.

9/24/2020

@ChrisR can the isolation solenoid # TR118665, be mounted near the battery in the trailer?

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Jon G.

9/24/2020

@MikeC You will need to install it near the start battery in your engine bay.

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See All (5) Replies to Mark E. ∨

Mike

6/21/2020

@ChrisR Once I've connected the trailer battery for charging, will the battery charge when truck is running only, or continue to charge when the trailer plug is connected even though the vehicle is not running?

Reply

Chris R.

6/24/2020

@Mike If you leave the trailer hooked up it will continue to draw power from the truck's battery even when the ignition is off. You'll need to either remember to unplug the trailer from the truck when parked or add an isolator like # TR118665 on the 12V circuit. I've linked an answer page below that explains that install process a little further.

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question-60644.html

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Steve

7/30/2020

I don't understand why you don't think trailer battery will not be charged while in transit. Alternator will raise voltage to 14.2 V to run what ever electrical load vehicle and trailer are using. Alternator will produce what ever amp is required to maintain 14.2 amps. I am sure trailer battery will charge as long as wiring to trailer is sufficient to carry the load.

Reply

Jon G.

8/9/2020

You are more than welcome to try to charge your trailer battery straight off of the 12V power from your vehicle, but in our experience there is enough power lost that you'll only get a trickle charge on the trailer battery. You're going to need to install a battery charger like Redarc In-Vehicle BCDC Battery Charger # 331-BCDC1225D in order to actually charge your trailer battery.

Reply

Rolle

7/12/2022

I get maximum 5A charge from the trailer connection. It is a brand new RAM and Lithium batteries in the trailer. So the batteries are not the restriction. They take whatever the tiny cable can supply.

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Jim

8/11/2020

Would a diode be recommended in order to prevent the trailer battery from discharging the vehicle battery?

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Jon G.

8/24/2020

A diode wouldn't do that - they are just used to keep the trailer wiring from backfeeding to the tow vehicle wiring (as in if the trailer wiring shorts out it won't damage the vehicle). You'll either need a smart charger (make sure it cuts off the battery) or a battery isolation solenoid # TR118665.

Reply

Gregd

12/27/2024

@JonG WRONG! A diode will definitely stop the vehicle from draining. The only problem is that you will drop .7 volts across the diode and the diode will need to be big enough to carry the rated current

Reply

Paul

7/15/2024

Would I need a fuse on this wire as well? I have a 2018 Honda Odyssey I'm looking to run a wire from the car battery back to the 7 way I recently installed. Thanks!

Reply

Jameson C.

7/15/2024

@Paul You'd want to use a breaker like the part # 9510.

Reply

Answer 1

You would not be able to charge your battery through the wiring on your 4-way trailer connector. Since your truck has a 7-way connector, you could maintain the power to the battery but not charge it. To begin, you would need to cut off your 4-way…

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Answer 2

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Answer 3

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Answer 4

Yes, you will want to use a single function diode, like part #RM-690. You will need to wire it in between the breakaway battery and the trailer battery to stop the backfeeding from the battery to the breakaway system.

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Answer 5

If all you want is to prevent the 12 volt power supply of your trailer battery wiring from backfeeding onto your vehicle wiring or vice versa what you need is a one-way diode like part #RM-690. This only allows current to flow in one direction so…

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Answer 6

Since wire color codes are NOT universal, we always recommend wiring by function, rather than wire color. If you look at the Trailer Wiring FAQ article I have linked you to, you can see some diagrams showing the proper way to wire a vehicle and…

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Answer 7

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Answer 8

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Answer 9

A diode like the part #RM-690 is a great idea but the only issue is that is has a max load of 85 amps. A standard 5,000 lb winch, like the ComeUp DV-5000si #CU645010, will greatly exceed the 85 amp rating of the diode so it would be useless. What you…

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How to Charge a Trailer Battery Through a 7-Way Connector

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