Having spent a few years here I often read about the concept that while a typical trailer wiring connection from the vehicle to the camper will supply 12V, it is of low amperage and as such the camper batteries are just being trickle charged. The reasoning being very long wire length of inadequate diameter between the alternator and trailer wiring harness. However, this wiring can be upgraded which results in better charging capacity.I've recently begun reading a solar panel forum and sticking to the RV section. I've noticed that every time a discussion comes up, one of the frequent posters immediately chimes in to first install a battery isolator to allow for alternator charging and that the alternator will provide more power in an hour than solar panels can in days. No mention of wiring upgrades, just this battery isolator concept. This poster clearly knows his solar but I don't get the impression of actual RV knowledge beyond carry over solar concepts.I recognize that alternators are rated to deliver a healthy supply of amps, but this concept from the solar panel forum doesn't mesh well with what seems to be the common thought here on rv.net. So can anyone enlighten me on how many amps one can expect for battery charging from an OEM trailer wiring arrangement and from an upgraded wiring arrangement, and what this battery isolator may do? Thank you.

I have a 2011 Silverado 1500 with one OEM battery and no wiring for a 2nd battery. I added a smart isolator and 2 gauge wire to the second battery, which I use in conjunction with a pure wave inverter. Both batteries charge from the truck altinator. Check the Samlex Acr-160 for isolator info and other battery charging.I would never want to run 2 gauge wire from my altinator to RV batteries.

So much of what works depends on the style of camping you like to do. Some folks will drive for 30 minutes to their favorite remote spot, and then spend a week there. Solar is the answer. Others will stay for a day or two, then drive on for a few hours. Even others won't bother with upgrading to solar, or to additional wiring. I like to carry my Honda 2K. If I need it, it is there. If I run it for an hour a day, it will keep me going through most of my camping. My 45 amp converter will put around 500 watts into the battery in an hour.But to directly answer your question, with a normal setup you might get 15 amps back to your battery. That is 15 times 12 volts, so 180 watts in an hour. With 280 watts of solar, if you were perfectly aligned to the sun, can you get that much in an hour? Will the sun last? So, upgrading to get 30 amps from your vehicle can be a good thing, or even heavier if everything else can handle it. But only you can determine how much you drive.

It is simple. I have dual charging paths with #8 wire. If my house batteries are hungry I do see up to 70 amps--but only for a brief period of time. As the starter battery reaches fully charged the amps taper. If I turn the fridge on 120 volt and power it with the inverter from the house jars, then I see between 25 and 30 amps of charging--but that is a short fall on what the fridge is drawing.

I'm not so interested in this upgrade for myself. I just want to know more. We are the park for days type of campers and the solar keeps up just fine.

pianotuna wrote:It is simple. I have dual charging paths with #8 wire. If my house batteries are hungry I do see up to 70 amps--but only for a brief period of time. As the starter battery reaches fully charged the amps taper. If I turn the fridge on 120 volt and power it with the inverter from the house jars, then I see between 25 and 30 amps of charging--but that is a short fall on what the fridge is drawing.Would the battery isolator separate your starter battery from the circuit once full and allow the alternator to continue to deliver higher amps to the hungry house battery bank?

How much battery charging from alternator? | Good Sam Community

31 Replies

pianotuna
Nomad III
Aug 11, 2016
beemerphile1 wrote:
pianotuna wrote:
The trailer emergency brakes use the house battery as a source of power and are wired past any battery disconnect switches.

I think a better choice of words would be 'prior' to any disconnect.

Good point bemmerphile1. English can be so slippery.

I think "around" any disconnect is even better.
beemerphile1
Explorer
Aug 11, 2016
pianotuna wrote:
The trailer emergency brakes use the house battery as a source of power and are wired past any battery disconnect switches.

I think a better choice of words would be 'prior' to any disconnect.
ewarnerusa
Nomad
Aug 11, 2016
Thanks for confirming what I thought to be the case.
westend
Explorer
Aug 11, 2016
The trailer brakes, while underway, use the hauling vehicle's battery through the brake controller. The emergency brake-away power through the break-away switch is delivered by a battery on the trailer.
pianotuna
Nomad III
Aug 11, 2016
The trailer emergency brakes use the house battery as a source of power and are wired past any battery disconnect switches.
ewarnerusa
Nomad
Aug 11, 2016
OP revisit.
So I have backed off from regularly visiting that RV Solar forum because I really can't believe the advice I read on it based on my own experience and what I've learned on these forums. The latest eye roll was a recommendation of 500 watts of solar for someone's 2x6V GC batteries and a follow up post of needing more like 900 watts due to the poster's latitude in NY.
But one topic that I contributed to but am now uncertain of is the battery power supply for trailer brakes when break away is engaged. I said the camper battery supplies this power and is one of the reasons that a battery is required in an RV trailer. A response was that trailer brake system needs to be completely isolated from rest of camper system. I know that the wiring for the braking system is not part of the RV power distribution panel but it does use the same battery, right? There isn't another small battery for this purpose hidden away somewhere? I know that cargo trailers need a small battery for this purpose, but I've been under the impression that in a camper it just uses the main 12V camper battery.
time2roll
Nomad
Jul 04, 2016
pnichols wrote:
Why is an isolator needed? How about just a manual switch on the dash
isolator never forgets, chassis battery stays charged.
PaulJ2
Explorer
Jul 04, 2016
My older truck had a factory installed (camper package) isolation relay and #10 charge wire back to the 7 pin trailer connector. I had istalled an analog amp gauge in the circuit. The most I ever saw with a nearly dead trailer battery was 15 amp.
My newer vehicle has no isolation relay, power there all the time. I think I like that better as I can rob power from either battery if one is dead, (truck or trailer). Also when camping with hook ups converter keeps trailer battery and truck battery topped up. If you don't want that simply unplug.
pnichols
Explorer II
Jul 04, 2016
Why is an isolator needed? How about just a manual switch on the dash (and heavy cabling between the towable and the alternator) that just directly connects the engine starting battery to the coach battery bank? You turn this switch ON whenever the engine is running and turn this switch OFF whenever the engine is not running.

My Class C motorhome came set up this way from the builder - except that the "switch" is a heavy duty solenoid that is held ON automatically whenever the engine is running. My wet cell engine starting battery is direct parallel connected to the two deep cycle 12V coach AGM batteries whenever the engine is running. This arrangement has worked fine for over 10 years using two 12 volt deep cycle AGM batteries wired in parallel in the coach. This means that whenever the motorhome's engine is running, I have three 12V batteries directed connected in parallel - one wet cell and two AGM.

This three battery combination does for short periods show the coach batteries as drawing 55-60 amps from the alternator going down the road after camping and running down the coach batteries.
pianotuna
Nomad III
Jul 03, 2016
Hi Mex,

The inverter route does work in a pinch. Honestly I'd rather beef up my solar but that means starting over because of one stupid mistake (mea culpa).