Charging a Utility Trailer battery with truck's alternator?

[Artfuldodger]

Was in this discussion today with a mechanic and another guy who asked the question. He has a utility trailer or car hauler with a winch or something he uses to lift equipment.

He was looking for a way to charge his trailer's battery while going down the road. We discussed using the hot wire of the 7 pin connector but it doesn't have a very big wire. I did read this article where they ran a heavier hot wire from the truck's battery all the way to the rear bumper. We also discussed that this charging connection would not be used while using the trailer battery's winch/lift.

https://www.rvtravel.com/charging-your-trailer-with-your-tow-vehicle-alternator/

 

[Artfuldodger]

The mechanic was worried that any charging more than a small trickle from the 7 pin connector might mess up the tow vehicle's charging system.
He suggested installing a 110volt inverter on the truck, perhaps under the seat, and a 110volt to 12 volt home style 2 amp charger.

I can't find anyone on Google doing this. Plus going from 12 vdc to 110 volt ac and back to 12vdc seems like overkill to me.

 

[Artfuldodger]

We also discussed that certain trucks have or had built in inverters and Ford even has one powered by a motor-generator.
https://pickuptrucktalk.com/2020/10...-2021-ford-f-150-pro-power-onboard-explained/

The Ford one is on a Hybrid but I think GM made one a few years ago that used the Engine > Alternator > Battery > Inverter vs the hybrid
that uses Engine > Electric Machine (Motor/Generator) > Power Converter > HEV Battery > Inverter.

It was also suggest to just buy a gas generator and take it in the truck bed. This more for taking the place of an inverter once you get to your destination, not for going down the road.

 

[Gadestroyer74]

aslong as the battery on the trailer is 12 volts there is no issue using the truck alternator to keep the battery charged on the trailer. No matter which way you go you are going to have to run wires. If it where me I would run 8 or 10 gauge awg wire from the hot side of the alternator and negative side to the trailer. I would put a quick disconnect plug in the same tongue area on the trailer where you can just un plug it when you unhook the trailer. From the quick disconnect I would run a 50 amp in line fuse then run your wires to the trailer battery. This would be the best option in my opinion. I wouldn’t fool with all the trickle battery charger power inverter etc.. no more than that winch is gonna draw your vehicle will have no problem running that .

 

[transfixer]

years ago when I had a car trailer with a winch, I ran two 4 gauge wires from the battery down the frame all the way to the rear bumper, used an industrial connector there to terminate the wires, and the same connector on the wires coming off the winch . So when I disconnected the trailer it was simply like hooking up the normal trailer connection. That way I didn't even have a battery on the trailer, but just left the truck running whenever I was hooked up and using the winch.

I don't see a problem with using something like 10 gauge wires if he's wanting to simply charge the battery sitting on a trailer

 

[gma1320]

I don't see where it would be any different than some of the newer trucks and the diesels that have two batteries. The only difference would be most of those come with a higher amperage alternator. Which might not be a bad idea to upgrade to.

 

[transfixer]

I don't see where it would be any different than some of the newer trucks and the diesels that have two batteries. The only difference would be most of those come with a higher amperage alternator. Which might not be a bad idea to upgrade to.

to simply keep a battery charged wouldn't use many amps I don't believe, it might use a lot if the battery were discharged way down during use, but generally a winch isn't used for an extended period of time, so that shouldn't be much of an issue I don't think

 

[NE GA Pappy]

this is done with campers/RVs all the time. It is an auxillary connection on the plug for the towing vehicle. I can't remember which pin, but a quick google of the wiring diagram of a standard 7 pin connection will tell you how to wire it.

My 2018 Ram with the towing package came wired and ready to go with the charging circuit connected

 

[NE GA Pappy]

here ya go...

 

[Artfuldodger]

aslong as the battery on the trailer is 12 volts there is no issue using the truck alternator to keep the battery charged on the trailer. No matter which way you go you are going to have to run wires. If it where me I would run 8 or 10 gauge awg wire from the hot side of the alternator and negative side to the trailer. I would put a quick disconnect plug in the same tongue area on the trailer where you can just un plug it when you unhook the trailer. From the quick disconnect I would run a 50 amp in line fuse then run your wires to the trailer battery. This would be the best option in my opinion. I wouldn’t fool with all the trickle battery charger power inverter etc.. no more than that winch is gonna draw your vehicle will have no problem running that .

If the 7-pin would work, why run bigger wires from the alternator? This was also suggested in the link in post #1.

 

[Artfuldodger]

Reading that the 7-pin will only charge the battery to about 80% because of the wiring size and distance. That to charge it more and faster you need a DC to DC battery charger.
So can you actually charge your trailer battery with your vehicle's alternator? Yes—but you'll need more than a 7-way.To charge your trailer battery, you'll need a DC to DC battery charger. These chargers typically connect directly to your batteries (rather than your 7-way) to provide the power needed to charge your trailer battery from a depleted state.
Kind of expensive. But this sounds more logical than a 12 volt to 110volt inverter and back to a AC(110volt) to DC(12volt) charger.
https://www.etrailer.com/dept-pg-Ba...-sz-Vehicle_Battery_to_Auxiliary_Battery.aspx

 

[Artfuldodger]

Here someone who ran two #2 welding wires and installed a relay, etc. He also mentioned poor charging due to skinny wires.
http://www.damouth.org/RVStuff/TrCharge.shtml

Maybe these folks aren't going to a destination with shore power. Some of those DC to DC battery charges could also be switched to solar panels. Plus that 7 pin charging might not provide enough for winches and lifts
Meaning charging while towing might be their main way of charging their batteries.

 

[Gadestroyer74]

The bigger wires will carry more amps and voltage over a longer distance, where as the 7 pin will not have. Doing it the way we mentioned will keep that separate from your plug and to ensure the battery is getting its maximum potential. Any issues with the battery or winch will not affect your trailer lights and fuses. It will have its own fuse so it’s much easier to trouble shoot should there be an issue

 

[Gadestroyer74]

Also it best to keep things simple and not over complicate the process. A power, ground, and a fuse is all you need..

 

[K80Shooter]

1st Let's get this out of the way. An alternator is intended to "Maintain" a charge, not charge a battery. Charging a rundown battery will eventually cause damage to your alternator.

Now after saying that, we (I managed a tractor/trailer repair shop) have installed a 12-volt battery on a trailer and used the 7 pin connector to maintain the battery with no problem. This was a road tractor pulling a 42ft trailer with hydraulic ramps. I fused the positive wire at the trailer battery and the truck side of the plug with a 20 amp fuse just in case something shorted out. I used 12 gauge wires to do this. We never had any problems. If for some reason the battery happens to get run down just make sure to properly charge it with a battery charger.

 

[Artfuldodger]

I saw some article that mentioned using two alternators thus one for the trailer. Some trucks have two dual alternators in parallel. This same article also said you could buy a larger amp alternator or one that has two outputs.