How to Wire 12V Battery in Enclosed Trailer to Power Trailer Lights Independent of 7-Way Connection

Updated 10/16/2024 | Published 10/11/2014

Products Featured in This Question

Starter Solenoid - SPST - 12 Volt - 100 Amp - Continuous Duty - Grounded

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$34.34

10 Gauge Wire - Blue Primary - Per Foot

(102)

$1.86

CTEK MULTI US 7002 12-Volt Universal Battery Charger w/ Pulse Maintenance and Backup Power

(91)

$157.99

Question:

I have an enclosed trailer that currently powers the interior lights from the 12v black power wire on the seven way connected to the vehicle. I would like to add an auxiliary battery. Do I simply tap into the existing black power wire coming from the seven way and into the trailer into the new auxiliary battery and ground the battery to the frame. I know that the black power wire coming from the seven way to the auxiliary battery will keep and maintain the battery charge on the auxiliary battery but will this maintain the circuit and allow the interior lights to run even when the seven way is disconnected from the vehicle. Should there be any type of isolator between the batteries. Or would I be better to have the black power coming from the 12 V on the seven way go directly to the auxiliary battery to maintain the charge during travel and then when I arrived at my destination disconnect that wire coming from the seven way and connect the trailer black power wire to the auxiliary battery to power the interior lights to keep everything separate. Thanks

asked by: Jay C

Expert Reply:

You are correct in your description of how to wire in a new auxiliary battery in your enclosed trailer. You will take the 7-Way's 12V power feed and route this to the positive terminal on the auxiliary battery, and you will ground the second battery to the trailer frame, as you described. You might refer to the linked article on trailer wiring to see the usual wiring configurations on trailer connectors.

If you want to protect your vehicle battery from accidental discharge (such as if you left the 7-way plugged in without the engine operating) then you might consider an isolation solenoid, part # PK5231201. This provides a gate to control power flow from the tow vehicle's battery and is triggered by any ignition-controlled circuit. The linked photo shows how this simple device wires into your charging circuit. Either side terminal can be used as input or output, but the photo shows the left as input.

When the trailer is connected to the tow vehicle and you are running the engine a 12V maintenance charge will be delivered to the trailer's battery. This will maintain it but will not charge it if it is depleted. For a depleted battery you will need shore power and a charger like the CTEK # CTEK56353.

For your trailer's hot and ground battery wiring I suggest you use 10-gauge wire such as # 10-1-1 from Deka.

expert reply by: Adam R

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Ross M.

10/16/2024

Adam R., So to install the isolation solenoid, is there an "ignition controlled circuit" going to my trailer from the 7-way plug that I can access in my 7-way juntion box in my trailer? Or do I have to run a new wire from the engine bay back to the trailer? Thanks, Ross

Jameson C.

10/16/2024

@RossM If you wanted to install the solenoid on your trailer and have that circuit only active when the vehicle is on then yes you'd need to run a keyed power source back to it so that it would function like you want it to. Otherwise you could run the middle pin directly to the battery's positive post but put a switch on the wire so that you could manually turn it off and on. But at that point you may as well just bypass the solenoid and use a switch.