Adding a 12 Volt Circuit to a Trailer Connector on a 1999 Toyota Sienna to Charge a Trailer Battery

Question:

I purchased this wiring harness for my 1999 mini van. Ill be towing a small pop up trailer with battery. I would love to charge the trailer batty while driving. Is there a way to run a 12v from the car battery to the trailer? What would I need?

asked by: Austin B

Expert Reply:

First, the 12 volt power from the vehicle battery will only provide a maintenance charger to a trailer mounted battery, it cannot charge it up. The wiring is too small and the battery and alternator on the vehicle are not strong enough to push back the appropriate voltage.

It is best to charge the battery using an AC battery charger such as # CTEK56353. Several of us here use this charger and it works beautifully. It will not overcharge a battery and it can revive worn (not dead) batteries.

If you wanted to provide a maintenance charge to the trailer battery you could do so by adding a 7-Way to the vehicle using # 37185. You would then need enough wire, # 10-1-1, to reach from the rear of the van to the van battery. The wire is sold in one foot increments. Then you would need to mount a 40 amp circuit breaker, # PK54540, under the hood. The wire would go to the AUX post on it. Then another length of wire would go from the BAT post to the positive battery terminal.

The other end of the wire will connect to the black wire on the # 37185. You will need to ground its white wire as well. The 4-Way trailer connector you have will plug right into the # 37185. The blue and purple wires will not be needed.

When routing the wiring be sure to avoid any areas that can pinch or burn the wire. You can use wire ties to hold the wire to the bottom of the vehicle.

On the trailer side you will need to replace its connector with a 7-Way, # PK12706. I have included a link to our help article on trailer wiring that includes diagrams of trailer connectors. Please note the wire colors are typical but not standard. Always wire by function.