Wiring a charge line to battery on utility trailer

~DJ~ wrote:
Hello. I've mounted a winch and battery on the front of my utility trailer. I would like to keep the battery topped off while driving down the road. The trailer is currently equipped with a flat 4 connector. No trailer brakes. Can I replace it with a 7 pin and just run the charge line to positive side of battery and ground pin to negative side? Or do I need some type of junction box? Thanks for your help. DJ

Put a fuse (20A ?) on the positive lead close to the battery.

There’s no need to change the trailer wiring, just make up a new pigtail using a 7 way plug, a 4 way vehicle end plug and a 2 way polarized plug set. The 2 way will be heavier gauge wire and give you a dedicated charge and ground wires.

Be sure to connect the 2 way such that the positive side of the trailer end is the shielded one.

If you only ever want a trickle charge and take longer drives you can always just run a jumper wire from the marker light circuit to the battery and drive with your lights on.

I would change the connection to a 7-pin standard RV plug. As was stated use a fuse on thge (+) line by the battery. Personally I would go with #10 wire and 30a fuse. Also consider a relay of some sort so that the charge line to the trailer only has voltage when the engine is running, otherwise you have the potential to drain the truck battery with winch usage while the truck is parked.

Going to a 7-pin now also makes it easier to add brakes in the future if you ever get a heavier trailer.

~DJ~ wrote:
Hello. I've mounted a winch and battery on the front of my utility trailer. I would like to keep the battery topped off while driving down the road. The trailer is currently equipped with a flat 4 connector. No trailer brakes. Can I replace it with a 7 pin and just run the charge line to positive side of battery and ground pin to negative side? Or do I need some type of junction box? Thanks for your help. DJ

You don't need anything else. Just use 10ga wire and be sure it has a fuse as close to the point you connect it to the pulling vehicle and another fuse as it meets the trailer battery. These are to be sure a short somewhere in your hookup will isolate your batteries and not overheat any wires. Amperage on the fuses could be from 20A up and doesn't even need to be the same on both ends. You can use any plug setup you want to and wire it to the standard you find with the plugs or not as you choose. If you want to possibly use it with another vehicle to pull it, the standard wiring diagram is probably best. Wiring a separate ground is a good idea as using the hitch as a ground sometimes doesn't give a good connection.

I used a 12V solar panel for my dump trailer battery when not in use.

Not to high jack this discussion but what should the voltage be on this charging wire?

Voltage will fluctuate. So instead of knowing a fixed number focus on the trailer should be within on to two tenths of a volt of the voltage at the tow vehicle battery.

opnspaces wrote:
Voltage will fluctuate. So instead of knowing a fixed number focus on the trailer should be within on to two tenths of a volt of the voltage at the tow vehicle battery.

Agree. It could be anywhere from 14.9 down to 12.7 depending on the state of charge of the trailer battery. As a battery charges, it's internal resistance increases. As the resistance increases, the voltage seen will decrease. Add to that the fact that the car/truck has a battery that is also in that charging loop, so battery will act as a modulator and you probably won't see the upper end voltage or the lower end, but will see something in between. If you have a meter on it, you want to see a change in the trailer battery voltage after you hook up and start the tow vehicle. This will tell you things are working and the regulator in the car/truck will determine the correct voltage in the line depending on the charge status of both batteries.

I want to thank you all who provided advice. I ended up ordering a pre-made 7 pin with junction box. Then I wired the trailer to the box with the charge line being a fused 10GA to the battery and the ground a 10GA to the battery. All the lights work correctly and the battery goes from 12.5 to 14 when TV engine started. Very happy. Thanks again for your help. DJ

~DJ~ wrote:
the battery goes from 12.5 to 14 when TV engine started. Very happy. Thanks again for your help. DJ

for bgum.... this is the correct voltage.
All appears to be working great.