Hello, I have been trying research this on the web but have been having a hard time finding direct answers to my questions.

We have a 12ft trailer that has had all the wiring pulled for a remodel however we left the breaker box. I would like to add 2 6v batteries to the trailer to run the few lights, outlets and an electric burner for cooking.

So my question is, how do you wire the battery up to the breaker box and still leave a plug in for when there is shore power?

What I do know is that we will need and inverter from the battery to make it A/C but the invertors that I see on-line all have a cigarette lighter hook up. Is there one that can be connected to the battery cables? Then how do I wire the inverter to the breaker box? Or is there a special inverter for just this need?

Lastly, how will the batteries re-charge, if I am hooked up to shore power will they re-charge?

Thank you


Tags: a/c, battery, breaker, hook, inverter, power, shore, trailer, up, wire, More…wiring

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  Fun chattin with ya Tim, as a retired EE I love sparky chat...

  WOW a 300 amp alternator!!! Now that will top off those batteries in less then the 8 or so hours like a typical 80 amp and, of course, it still takes gasoline and HP (about 5 or so) to turn such a huge device. Id need access to the specs and efficiency (alternator and inverter) to see if running that alternator and using a converter is better then using say an onboard Onan 2500 or 4000 genset as its already 120 volts??? I think an Onan 4000 uses about 3/4 to a quart per hour of gas subject, of course, to the load while a rooftop AC is pretty big (1400 or more watts). BUT if you dont have an onboard genset THEN YOU NEED THAT BIG ALTERNATOR OF COURSE. My gut says (to run AC down the road) the Onan would be more efficient then running the big alternator and charging batteries and using an Inverter, the more conversions and inefficiences and heat losses you have the worse it gets ya know


 Im not convinced on all electric however and Im sure youre not convinced on my preferred COMBINATION (LP gas, Battery electric, Genset, Solar) BUT WE CAN STILL HAVE FUN CANT WE


 Heres my RV in its Florida winter home




 John T


Likewise John!  No doubt in my mind you have all the expertise anybody would ever need in the world of sparks!  :D

As for efficiency, I'm sure your correct again. Nothing is free and I think your right, a good generator might actually save more gas. 

wanna borrow my waders?

 nice rig great loaction,i used to have a class c they were fun, now i have a class a.. below is mine ever thought about coming down to cabo area in the winter? we are a group about 325 strong every winter.


below:notice how much room i have around my inverter transfer switch area? haven't figured out what to put in there but someday something.


I know for a fact that, depending on the load, running directly from the generator will consume considerably less energy than charging the battery with a generator powered battery charger and then using battery power to power loads through an inverter. A typical battery/inverter system - a really well designed one - is about 85% efficient at best, not including the charging source. So at the very best, the inverter system is losing 15% right off the bat through electrical inneficiencies and heat losses. A typical gasoline powerred generator will be about 75% efficient - again a really good one - so you can see where this is headed. :-)

I have to jump in here and say that I've never run into a 300 amp alternator before - an I deal with very large solar and wind energy devices. A 300 amp current would require at LEAST 4/0 cable to charge a flat battery - and that's @ 24 volts! How in the ever loving world is that much energy forced through a conductor without turning into a big space heater?? I'd like to see some pictures of this system...

Mathew, glad to see you jumping in!  I ran only 2/0 wire from my alternator to the battery bank, luckily for me they are close.  But honestly, the wires still get warm when I'm running the overhead AC and the fridge together.   Not sure what redwing used for his 300 amp, but 12v, 300 amp alternators are fairly common in medium sized medics.  I bought mine from nationsautoelectric.com.  I'll get pictures posted in my album when I get a chance.   

My biggest issue was actually keeping the belts tight on the alternator, we ended up going with dual belts to keep them from stretching.  Finding the pulleys alone was a challenge!

wait a minute you bragged you ran the ac all night and then heated water and cooked using 120volts, getting pretty deep in here. and the load a 300amp alternator which at idle wouldn't even get close to 30 amps, yeah my diesel rig has one, there is no way to charge those 8devils up without running a higher rpm and about 8 hours running at 1000rpm. if discharged all the way to the bottom like the ac would do to them.

Thats correct redwing, I also charged my laptop and cell phone as I do every night.  (And I didn't mean to come across bragging, just letting others know some possibilities) My rig is a 1972 Winnie Brave, nothing to brag about.

And thanks for the tip as I do appreciate the knowledge, however I would never try to charge my batteries by letting the RV idle, I have an old Dodge 727 transmission.  For those who don't know, the older 727s do not pump tranny fluid while in Park w/out a modification.   They sell a kit to fix that, but I have never done it.

Have you ever considered changing the pulley size on your alternator?  You might be able to squeeze more amps out of your alternator at idle!


Forget electric burners and use propane. Resistive loads (like hotplate coils) will nail even large capacity batteries quickly and propane is a much faster heat source. tritt

yes even with a monaco and 2 4d batteries it is nigh impossible to efficiently run a electric burner for cooking.

it would be a worse case senario, it is cost prohibitive to run a electric burner type element off batteries, i have a monaco which has a 2000watt inverter, and even with my 2 /4d batteries i would not be able to run an electric burner for more than an hour because of the load it would generate.

I will try to just answer your specific questions:

So my question is, how do you wire the battery up to the breaker box and still leave a plug in for when there is shore power?

The 12 VDC battery power and its distribution (lights, vent fans, water pump etc) is NOT a part of the 120 VAC breaker box. Usually the 12 VDC distribution consists of an automoutive type fuzed distribution panel with several 12 VDC plug in fuzes that feed a few circuits like 12 VDC lighting or 12VDC water pump or 12 VDC vent fans etc.

The 12 VDC distribution panels input is fed directly by the house battery or batteries so they all work regardless if youre plugged into shore power or not.

BUTTTTTTTTT the batteries will eventually run down ya know SOOOOOOOOOOO you need to keep them charged and thst by one of two sources......When youre driving down the road the tow vehicles 12 VDC (powered by its alternator) can be connected to the trailers house batteries (to also charge them) via the 6 or 7 pole RV plug and connectors hot auxiliary terminal which is a protected circuit fed off the tow vehicles battery

NEXT if youre plugged into 120 VAC shore power at an RV park and want to keep the trailer batteries charged you need EITHER a Converter/Charger orrrrrrrrrrrr an Inverter/Charger which takes the 120 VAC and converts it to 12 VDC (actually 13 to 14 VDC or so). Think of it as a battey charger just like at home you plug to 120 volts but it charges a 12 volt battery.

NOTE typical run of the mill 20 or 30 amp converter charger systems are cheap n easy on Flea bay and they contain BOTH the 120 VAC and the 12 VDC systems and charger and both distribution panels (12 VDC and 120 VAC) all in one single fan air cooled ready to wire and use package system.

NEXT if you want to use the battery bank to power low power 12 VDC devices (like if dry camping not hooked to shore power) like cell phone chargers or computers or small TV;'s etc then you need an INVERTER that changes 12 VDC to 120 VAC. The small units (100 to 400 watts or so) may plug to a 12 VDC cigarette lighter type outlet and have 2 onboard 120 VAC outlets. Thats the cheap easy n dirty way to power smalll devices and with only 2 batteries and a 12 foot trailer Id say a 400 watt inverter is all you need to power cell phones and laptops and a small TV HOWEVER id dont like to feed them with a wimpy 300 watt rated cigarette lighter type of outlet I prefer to hard wire them direct to the batteries
with a fuzed circuit of 8 or 10 gauge wire.

NEXT I mentioned an Inverter/Charger versus a Converter Charger and they are combination Inverters and chargers DUH and is a different system then the cheaper tyypical Converter/Charger systems above. I see them more on high end bigger coaches that may have a 1000 or even 2000 watt inverter and may have 300 to 500 amp hours of battery storage.

With a 12 ft trailer and a couple house batteries Id go with a combination Converter/Charger with built in 12 VDC and 120 VAC distribution and a seperate 400 watt Inverter.

Battery capacity. You mentioned two 6 volt batteries and typical may be two Trojan T 105's in series so do the math and see what loads you use dry camping and you can see how long and how well that will run say a vent fan and lights at night etc. Im an engineer and could do all that but you need to do it yourself since youre the user

BOTTOM LINE with a 12 ft trailer and two golf cart batteries if you use a 20 or 30 amp converter/charger with built in 12 and 120 distribution and charge the batteries while driving off the tow vehicle and use a 400 watt inverter you will get by just fine. Dont even think about trying to power big loads like microwave and AC or heaters or fridge etc with only two golf cart batteries and a small inverter DUH

What I do know is that we will need and inverter from the battery to make it A/C but the invertors that I see on-line all have a cigarette lighter hook up. Is there one that can be connected to the battery cables? Then how do I wire the inverter to the breaker box? Or is there a special inverter for just this need?

Even if they connect with a plug you can still hard wire them yourself which I recommend unless its only say a 100 to 200 watt unit but if a 400 Id hard wire it wiht 8 or 10 gauge fuzed wire. Use the on board outlets to charge your phones or laptop

Lastly, how will the batteries re-charge, if I am hooked up to shore power will they re-charge?

See above, tow vehicle charges them while driving via the plug while a converter/charger or inverter/charger charges them when plugged in

John T retired electrical engineer

i pulled out all my gas stuff because i plan to camp only where there is electricity.i would have left it in if i was planning to dry camp.i think an electric cooktop is going to kill your batteries quick.




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