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Do I Need an Isolating Solenoid to Wire Aux Battery on My Trailer

Question:

Can you tell me how to exactly wire a 12v battery to power my interior lights to my trailer and to my breakaway battery without an isolator. Do you recommend an isolator?

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Expert Reply:

There are a few different ways you can wire a 12v battery to power your interior lights and maintain your breakaway battery on your trailer.

You can find the 12v circuit on your trailer's 7 way connector, it's usually a black wire at the 11:00 o'clock position (if you are looking at the face of the plug). Connect that wire to the positive post of the new aux battery, and ground the negative post to the frame. Then you can connect to the aux's positive post to the positive post on the breakaway battery. This will give a maintenance charge on both the aux and breakaway battery when hooked up and towing, but will not charge the battery from dead.

You want to protect the breakaway battery from being drained while using the interior lights, so I do recommend an isolation solenoid like part # TR118665.

The solenoid will wire in between the aux/new battery and the breakaway battery, with a trigger power source, in this case it can be the running light circuit on your trailer. That way, when your trailer is connected, you can charge the break away and the auxiliary trailer battery, but when not connected the break away battery will not be drained by the auxiliary trailer battery, because the trigger is not completing the circuit inside the solenoid.

If you don't already have a junction box on your trailer I recommend adding one like part # 3375601101, along with ring terminals and wire to make all your connections. This will give you a cleaner look in the end, and make it easier to add any other components or repairs in the future.

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