How to Charge Two 12 Volt Batteries on an RV Towed by a Ford F-150

Expert Reply:

The 12 volt circuit on a 7-Way trailer connector can provide a maintenance charge to a 12 volt battery but it will not be able to fully charge it because the wiring for a 7-Way is too small to carry the current required.

What you can do to charge the batteries on the RV is to first install a battery isolator, # TR118665, in the truck. You will need to attach one terminal to the positive on the truck battery and then connect the center terminal to a 12 volt ignition source (something that is only on when the key is turned on).

The last terminal, connect heavy gauge wire (up to 00 size), like battery cable, and route it all the way to the rear of the truck avoiding areas that may pinch or burn the wire. Attach the wire to the Pollak Single-Pole, Round Pin Trailer Wiring Socket, # PK11851. Then, on the trailer side, you will need a Pollak Single-Pole, Round Pin Trailer Wiring Connector, # PK11852, and the same heavy gauge wire and connect it to one of the RV batteries positive terminal.

The 2 RV batteries will need to be connected in parallel to charge both of them. To do this you will connect the positive of one battery to the positive of the other and do the same with the negatives. Since there are 2 batteries it will take longer to charge but the large gauge wire can handle the current. The isolator will only allow power to flow back to the RV batteries if the truck is on.

expert reply by: Michael H