How to Install 12V Battery Isolator to Prevent Vehicle Battery Drainage by Trailer Battery

Updated 08/02/2017 | Published 07/31/2017

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

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Question:

I recently purchased Part Number TWSP12BP with intentions of installing a Group 24, 12v battery on the tongue of my enclosed trailer. Currently, the trailer has interior lights that only work when the 7-way plug is connected to my truck. The purpose of the battery install is to run the interior lights on the trailer when not plugged into the truck. The trailer has a small battery for the break-away trailer brakes. From the new 12v group 24 battery positive post, I was going to just splice into the 12v positive wire on the trailer that goes to the 7-way 12v wire. I watched some youtube videos on how to do this and found conflicting ways. The question being, should I install fuses, isolators, or anything else with this set-up as to not mess anything up? Please explain the proper wiring install. I also have a battery maintainer that I intend to hook up to the battery to keep it charged when the trailer is parked and disconnected from a running truck.

asked by: Corey

Expert Reply:

If the vehicle 7-way's 12V power circuit remains active when the ignition is off, you will want to install an isolation solenoid # PK5231201 to prevent the trailer battery from depleting the vehicle battery. You can test the vehicle 7-way pin at the 1-o-clock position (please see the linked photo) with a tester like # PTW2993 to determine if this 12V circuit is ignition-controlled or always active. If it stays active all the time, then the isolation solenoid will prevent the trailer battery from depleting the vehicle battery.

You will use any ignition-key-controlled circuit in the vehicle, such as for a power antenna, along with a fuse tap # F2526, to give you an ignition key signal. This goes to the center post on the isolator to act like a valve. The 12V feed from the vehicle battery goes to the left-hand (input) post and the right-hand (output) post is routed to the 7-way's 12V pin. The solenoid will pass the 12V feed only when the key signal is present on the center post.

If the 12V circuit is already controlled by the ignition key, you do not need anything else.

expert reply by: Adam R