Things You'll Need
2 lengths of 10-gauge or larger electrical wire
Crimp-on wire connectors
2 large alligator clips
You can maintain your trailer battery by charging it while you are towing. You can wire the trailer battery to the same seven-pin outlet on your towing vehicle that powers the brake lights and turn signals on your trailer. This method helps maintain the charge of batteries in travel trailers, horse trailers and other utility trailers. You cannot fully recharge your battery this way except on extended drives, and the method is not recommended for charging dead batteries.
Start your vehicle.
Use a voltmeter to check for current from the black wire in the seven-pin electrical outlet at the back of your towing vehicle. Use the white-wire connector for your ground. You should get a reading of about 12 volts.
Turn off your vehicle.
Install the correct fuses in both the vehicle cab and the engine compartment relay box if there is no current from the black wire. Check your vehicle owner's manual to find out the types of fuses to use for the charging/auxiliary power feature of your towing connector.
Detach the trailer's electrical plug from the vehicle's seven-pin outlet.
Run two wires from the trailer end of the seven-pin connector to the trailer battery. Use two different colors of wire, such as red and black, so you can easily identify positive and negative. You can use standard 10-gauge electrical wire, but thicker wire will carry more current and give a better charge.
Connect the end of one wire to the black outlet of the trailer's seven-pin connector. You may be able to take the connector apart and attach the wire inside. Otherwise, use crimp-on wire connectors to attach to existing wire.
Connect the other wire to the white outlet of the trailer's seven-pin connector.
Use the wire strippers to remove about a half-inch of insulation from the battery ends of the new wires. Attach the large alligator clips to the exposed ends of the wires.
Attach the alligator clips to the battery terminals. The wire running from the black wire connection should go on the positive terminal, and the wire running from the white wire connection should go on the negative terminal.
Plug the trailer into the seven-pin connector and start your vehicle to begin charging.
If you do not have a seven-pin outlet you can run a wire directly to your battery, but you should also install a fuse and a battery isolator to protect your vehicle's electrical system.
Attempting to charge an almost dead trailer battery from your vehicle may cause the battery to draw too much power and blow a fuse in your vehicle.