best charging method?

[Buffalobillcody]

any photos of your rig? mebe some sort of support that lifts it over your RTT and hinges out of the way really easy..

otherwise your looking at a DC-DC charger back in the trailer to keep voltage from sagging and boosting how much charge it can give.. you could go up to ~10A DC-DC charger off stock 10awg wiring more than likely.. Something like a Victron Orion-Tr 12/12-9 as close to your trailer battery as you can install it.. you can adjust the pot on it so it outputs ~8A solid of 14.2-14.6v even if the input voltage drops down to ~11v over that length of wire.. you'll want your trailer plug 12v line to be switched with ignition so its not draining your starter battery when engine is not running.. Or you run a dedicated line/plug to the trailer for charging purposes with a heavier gauge wire and then you could get a 15-30A DC charger which would be considered ideal for that size battery to recharge it faster, but this is considerable work and expense. could serve dual purpose for a rear winch or something.

Solar would be more ideal because it can continue to absorb you back up to up after you are done driving, and will take fridge load off the battery when your not on the trail, but you seem to do enough driving most of the time you could fully charge a deeply discharged battery on the trail if you are consistently pounding that many hours behind the wheel.. for most that rarely works out because few are willing to spend more time on the road than camping and only get a partial charge from an engine.

So I'm starting to install the DC/DC. How does it up the voltage, maintaining 9 amps, when the input current is lower than 9 amps?

 

[DiploStrat]

It doesn’t. A DC-DC charger trades extra amps for higher voltage - that is, it will take in more amps than it produces, but it can produce those amps at a higher voltage. That is why DC-DC chargers have low(er) outputs. You must still wire properly, that it, heavy enough, for a DC-DC charger to work correctly. The idea that a DC-DC charger "magically" lets you use smaller wire is simply wrong.

It may allow you to use slightly smaller wire, but that is only because the output is reduced. For example:

On my Chevrolet based camper with a 200A alternator set and an intelligent relay, everything was sized to pass 150 - 200A with minimal voltage drop. On my Mercedes Benz, running a 40A DC-DC charger (to change from 24v to 12v), everything is sized to pass 60A, the recommended fuse value.

A DC-DC charger is wonderful if you use it as originally planned, that is, on a vehicle whose native charge voltage is typically too low to fully charge a large, deep cycle camper battery. In the case of Sterling Power, this was narrow boats that ran at 13.9v. The first Sterling Power DC-DC chargers raised this to 14.4v and the results were amazing. Got a Chevrolet that goes to 15.5v? Use a relay and a big wire and you will get even better results. Got a Toyota that runs at 13.9v? Get a DC-DC charger or you will kill a lot of camper batteries.

Other, odd uses:

-- Connect 12v camper battery to a 24v truck.
-- Protect a vehicle alternator from overload from a discharged lithium batter.

None of this is hard, you just have to go step by step.

 

[Buffalobillcody]

It doesn’t. A DC-DC charger trades extra amps for higher voltage. That is why DC-DC chargers have low outputs. You must wire properly, that it, heavy enough, for a DC-DC charger to work correctly.

That's how I understood it. So in this application, the charger I bought is rated to put out 9 amps at 11-15v adjustable. Does this mean it will draw more current from the 7 pin than it can handle?

 

[DiploStrat]

Look at the owner‘s manual. Typically you can expect a DC-DC charger to draw about 20% more than its output. My 40A charger fuses at 60A.

 

[dreadlocks]

Your tow vehicle should be able to handle 120W of load out that pin, its probably fused for like 30A.. it just wont charge a battery very well with voltage drop alone.. A static load of Watts, will pull more amps as the voltage goes down.. so if its getting 12.2V in your trailer at 10A, your turning that into a usable 14.6v charge and outputting 8A.. otherwise that same voltage drop would never charge a battery up.. but it would run a 10A static load just dandy at that voltage.. this is why a direct connection to alternator w/no booster requires such absurdly large wiring, cant have a 10th of a volt drop let alone a whole volt.

Trailer Brakes are wired through that plug, they pull far more amps than this DC charger will.. check your fusing but you should be fine.

I've got a Victron BMV-712 wired up to my battery in the trailer, through my factory plug I only get ~4A charge input.. a full 7h trip on the road barely put a few AH back in with the fridge also fighting for that power while driving down a hot interstate.. its adequate enough to maintain the charge I left with in the summer, but not much more.. If I leave home w/a full charge I get to camp with a full charge, no matter how far I go.. but if I leave camp w/a flat battery the factory wiring w/out a DC charger is not going to do anything but keep the fridge running.. this is why I added a 325W fixed solar panel on the roof, now I see my battery getting 15-22A going down the highways in good weather.. 120W wouldn't be enough for me but I dont drive as much and as frequently as OP does, I discharge my bank much deeper and drive less.

 

[Buffalobillcody]

Your tow vehicle should be able to handle 120W of load out that pin, its probably fused for like 30A.. it just wont charge a battery very well with voltage drop alone.. A static load of Watts, will pull more amps as the voltage goes down.. so if its getting 12.2V in your trailer at 10A, your turning that into a usable 14.6v charge and outputting 8A.. otherwise that same voltage drop would never charge a battery up.. but it would run a 10A static load just dandy at that voltage.. this is why a direct connection to alternator w/no booster requires such absurdly large wiring, cant have a 10th of a volt drop let alone a whole volt.

Trailer Brakes are wired through that plug, they pull far more amps than this DC charger will.. check your fusing but you should be fine.

I've got a Victron BMV-712 wired up to my battery in the trailer, through my factory plug I only get ~4A charge input.. a full 7h trip on the road barely put a few AH back in with the fridge also fighting for that power while driving down a hot interstate.. its adequate enough to maintain the charge I left with in the summer, but not much more.. If I leave home w/a full charge I get to camp with a full charge, no matter how far I go.. but if I leave camp w/a flat battery the factory wiring w/out a DC charger is not going to do anything but keep the fridge running.. this is why I added a 325W fixed solar panel on the roof, now I see my battery getting 15-22A going down the highways in good weather.

Thank you! I couldn't get my head wrapped around it. Was stuck. Thanks for explaining it.

 

[dreadlocks]

those orion's can be parallelized too, so if you have ambitions to run double the power through it you can just add another one.. just leave room for it when you install this one without a bunch of rewiring.. On my vehicle I checked the wiring and fuses and it should be capable of running 20A through it just fine.. any more than that and I'd risk popping fuses.