How to Use 12-Volt Circuit From Trailer Connector to Maintain Trailer Mounted Battery

Updated 06/25/2016 | Published 06/24/2016

Products Featured in This Question

Curt T-Connector Vehicle Wiring Harness with 4-Pole Flat Trailer Connector

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$50.31

10 Gauge Wire - Blue Primary - Per Foot

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40 Amp In-Line Circuit Breaker - Perpendicular Mount Bracket

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7-Way Round RV Style Trailer Connector - Trailer Side

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

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

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Question:

Hello and thanks for taking my question! I installed this part on my 2008 Toyota Sienna in order to tow a pop-up camper. I didnt realize I needed separate 12V power to charge the camper battery while towing while the battery runs the fridge in the camper. The camper has a 5 square to 4 flat pigtail from the hitch. Whats the best way to bring 12V to that pigtail? I saw the Tow Ready adapter 20036. Could I connect the blue wire to the power outlet in the trunk area of the Sienna? Im assuming it would be better to run a 10 gauge wire up to a fuseable link and the battery. Id also have to change the pigtail 4 flat end to a 5 flat end.

asked by: Kevin

Helpful Expert Reply:

In order to provide 12-volt power to maintain your trailer mounted battery, you'd need to install a 7-way trailer connector adapter, part # 37185 and a few other parts. The adapter would plug into the 4-pole provided by the # C55580, and would have 4 other loose wires. The blue and purple wires can be bundled up out of the way, as they won't be needed at this time. The white wire would be grounded to the vehicle frame and the black wire would be extended by a length of 10 gauge wire long enough to run under the vehicle and reach the battery. We sell 10 gauge wire by the foot as part # 10-1-1. You'd want to install a 40 amp circuit breaker, part # 9510 inline on that wire, placing the breaker near the battery.

On the trailer, you'd want to cut off the existing 4-pole and install a 7-way, part # A7WCB. The FAQ article I've linked to explains how the connector should be wired. You'd run a length of the 10 gauge wire from the 12-volt pin of the connector to the positive terminal of the battery.

This 12 volt circuit can MAINTAIN the battery but it cannot CHARGE it once it becomes discharged. The distance between the battery on the tow vehicle and the battery on the trailer, the small wire gauge used by the 12-volt circuit and the fact that the alternator isn't strong enough to push enough power to the trailer battery all mean that to charge the battery, you'll need to use a charger like the # CTEK56353 and shore power when it's available.

Additionally, installing a battery isolation solenoid like # PK5231201 might be a good idea. This isolator disables the 12-volt feed when the vehicle ignition is shut off. This protects the battery on the tow vehicle from getting depleted in case you forget to unplug the trailer.