I have a trailer I haul around for my business. It has a 12v marine battery that is constantly getting 1/2 as*ed charged from my Honda engine that has a 18a charging system on it. It’s a pita to hook it up to the garage charger every single day.

Is it possible and a good idea to run some wires from my truck batteries back to the trailer battery so when the trailer is hooked up, the trailer battery is charging?
Obviously I would have a connection at the back of the truck to plug into.

 

Quite common to charge a trailer battery from the tow vehicle. I would venture that 99% of RV trailers are having the batteries charged as they are pulled down the road.

 

Does the trailer use the flat 4-pin plug or the round 7-pin RV style plug?
Round 7-pin has 12v hot to it already, charge battery off of that.
If the trailer is a 4-pin flat plug, change it over to the 7-pin.

 

I have the round 7 pin. Wouldn’t I need to have a bigger wire for charging that battery?

 

Assuming the trailer is a cargo trailer or a flat bed style and not an RV, the only thing that battery is for is to power the electric brakes in the event of a break away. Most such batteries I've seen are about like lawn mower or motorcycle batteries. Is your battery like a Group 27 marine battery and if it is do you really need all that.

If you tow the trailer almost daily, and you have a 7 pin connector, the tow vehicle should be keeping the battery charged, assuming all the wiring is intact. The wire gauge is surprisingly small as the charge current isn't all that great. It can take as much as 8 hours of towing to recharge a depleted Group 27 marine.

You might have the trailer looked at by a good trailer and hitch shop to make sure everything is working like it should.

 

this is a flat bed trailer. The battery is a 27 series.
the battery starts my engines on my trailer,
it powers my 12v hose reels as well. Its
completely different from the electric brake battery that is the size of a lawn mower, maybe smaller.

 

OK, all that is good to know. If your brake battery is getting charged via the 7 pin connector I would leave that alone and find some way to get one of the engines to charge the battery. Maybe even a solar trickle charger? Google for a site called "the 12 volt side of life" a lot of RV people reference that site for such things.

And maybe have the battery tested?

 

Solar charger sounds like it would be super simple.

 

Frank,

The 12 volt output of the Honda is terrible. Buy a 45 to 90 watt smart charger

https://www.pplmotorhomes.com/parts/...er-55a_55-0030
but these all seem to be out of stock…

Or


Or better



that is standard equipment in a camper trailer, plug it into the Hondas 120volt ac and connect the charger’s output to your batteries. That is your easiest, fastest and cheapest solution.

Connect truck batteries…

The trailer wiring in both the truck and trailer is woefully inadequate for more than a trickle charge. Too many feet, wire too small. You can install heavy gauge (6 or larger) wire between truck battery and truck bumper. You need a heavy fuse close to the battery connection. You can and should install a relay (like a winch relay) at the truck bumper that connects to that heavy line from battery - and a smaller trigger wire from an upfitter or other 12 volt source to turn it on. That prevents a big spark every time you connect/disconnect so long as the upfitter is off.

That heavy wire from the battery tapped into a stock trailer connection at the bumper will help a bit if you also enhance the ground wire size too at that point to the truck frame- No need to run ground all the way back to the battery.

To get full throughput you really need is a supplemental heavy gauge wire from truck battery terminus at truck bumper to the trailer and also a heavy gauge wire for ground running to trailer. NAPA sells such quick disconnect “pairs” without wire that easily connect to the wire size of your choice. Many of the pre-built with wire setups have crappy wire ( not thick enough or flexible enough, or both).

 

I still maintain that if all that is needed is to keep the starter batteries hot while towing, then the hot lead from the truck's trailer plug will be adequate, that is how my trailer battery gets charged. And, that battery gets used pretty hard for running the trailer slide outs and jacks, truck hot lead is plenty to keep that battery charged back up.

 

I would be more worried about it charging back into the truck and screwing something up? Once you address that you should be able to charge the battery enough to crank your honda motor.

I would also question the battery if thats all it has to do and wont hold a charge.

 

I presumed, perhaps incorrectly, that original post reference to “ getting 1/2 as*ed charged from my Honda engine” meant a Honda generator on board the trailer or truck and that owner was using the stupid 12-volt output to charge his trailer battery, and the trailer used for work had lights and/or fans running for hours while working off grid.

 

If you have the upfitter switches buy this and wire it to #5 or #6. Wire the output with at least 8AWG to the trailer.

https://www.victronenergy.com/dc-dc-...a#pd-nav-image

 

The 12 volt output at the 7 pin outlet will never produce more than 30 amps. A forum member tested the circuit and the output was often less than 30 amps. Still, even 15 amps is a good amount of current and given enough time (and not too much load) it would charge the trailer battery.

The system is designed to not back feed. A fully charged trailer battery will not charge the truck battery. I suspect there are diodes in the system. Further, once the ignition is off, the truck should no longer be providing 12 volt current at the 7 pin.

A totally separate circuit with wires coming off of my truck battery, to the bumper and then to the trailer battery can provide whatever amperage you want. However, the more amps you expect to push to the trailer battery, the bigger your cabling needs to be. That can be quite expensive. To control the amperage (and therefore the size of the cabling) consider using a DC-DC charger rated for no more than 18 amps output. A good quality DC-DC charger will have a voltage sense. This will make it so that it tries to charge the trailer battery only when it senses that the truck is running.

 

bingo

i use a set of diodes to prevent a HOT line on the 7 pin pig tail once its unplugged from the truck.
if trailer is plugged into truck and truck is OFF the power is removed from the 12v pin in the connection and will not back feed to trucks system, at least thats how its showed to be wired in the electrical book.