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

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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

Hey Jack, If you're talking about a transition plate then you'll want the etrailer Enclosed Trailer Transition Plate #e32JR. This can be installed on either end of the ramp to help make the transition onto easier or at the hinge of the ramp…

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Answer 3

First it is important to know that the battery on the Engager break away kit #20099 will only be charging as long as the 12 volt circuit is providing power. If you shut off the vehicle and disconnect the trailer it will not be charging. When there is…

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Answer 4

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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Answer 5

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 6

For a 12,000 lb. electric car trailer winch like the Bulldog Winch Trailer Winch part #BDW10040, I recommend a standard 12 volt lead acid battery rated up to at least 650 cold cranking amps (CCA). As long as the battery is rated up to 650 CCA then…

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Answer 7

Hi Walter, For the Deka Battery Isolator #DW08770 you can go with 8 gauge wire. Deka actually makes an installation kit with this wire, the Deka Battery Isolator Installation Kit for Standard Alternators #DW08769.

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Answer 8

The auxiliary 12-volt wire that runs from the 7-way connection to 7-way trailer plug will supply 12-volt power from the vehicle to the trailer. The 12 -volt power will allow interior or exterior lights to run that are not wired in to the…

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Answer 9

Hi Rick; the right weight distribution hitch will depend on the loaded tongue weight of your setup. The best way to find this is with a tongue weight scale like the #BTBRV44FR. Once you have this measurement you can find a system with an appropriate…

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Answer 10

First, the wiring on a trailer connector can provide a maintenance charge to a 12 volt battery on a trailer but it will not be able to fully charge it. The 12 volt circuit on a 7-Way is already circuit protected so a separate fuse or breaker should…

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Answer 11

If I'm understanding you correctly the starter solenoid #PK5231201 is not the solution to your problem. Based on what you have described, it sounds like both the breakaway battery and winch battery are hooked to the 12V power coming from your…

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Answer 12

Since you've already got a 4 pole on your trailer, you'd just need to add a 7-way with #ETBC7 so that you can run a charge line using #10-1-1 from the accessory circuit to your battery. This circuit is located at the 1'oclock position on…

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

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