Adding an inverter for one circuit

[Pinstriper]

The larger trailers are coming with 7cf, 8cf double door fridges - separate freezer from fridge. Trouble is, none of them are three-way (dc, ac, propane).

Our new trailer is coming with a 7cf 2-way (ac, propane) which creates a minor problem in that I can't bring myself to travel with the propane valves open and the fridge running on propane. The concern is any kind of crash that doesn't put the bottle on its side where the check valve would close, and now there's an open gas line feeding a fire. I am not so much worried about the propane fridge being the cause of a fire - trailer fires are generally electrical in nature, or start with overheated brakes or tires.

But an open supply of gas makes the whole thing go ka-flooey in a hurry.

Without a dc function, it occurs to me that the fridge would be happy plugged into an inverter fed off a dc circuit. The tow vehicle provides dc to keep the battery charged and the inverter supplies ac power.

Imagine, if you will, that my new trailer is a camplite, and said fridge lives in the space directly above the PDC. It seems to me there just has to be an open 12v slot to connect to.

So now I'm looking for an inverter that would either live in the PDC space, and I would move the wires for that circuit from the ac circuit breaker over to the inverter.

Or, I would just run a new pos/neg pair from the dc circuit into a circuit box into which the inverter was also wired, and the inverter would like in the space behind the fridge, and we would unplug from the ac outlet into an outlet on the inverter.

Specific questions:

1. Size the 12vdc fuse and wire. I am thinking 12/2 romex and a 15amp fuse.
2. Size the inverter - I am thinking as small as 100w may  suffice.
3. Recommend an inverter that would be amenable for this wiring scheme.

Feel free to disabuse me of any aspect of this notion.

Again, ignore the reality that this is a Venture Sonic 220vbh, and lets just pretend it is a CL that nobody has seen before.

David ? Merlin ? Charlie ? Tinkering Techie ? Anyone ?

[DavidM]

Well, I won't try to debate your propane concerns, as they are somewhat valid and if you want to be squeeky clean safe, that is your choice. But here are some facts, and observations about your scheme:

I had a 6 cu ft, one door three way fridge. When it was on AC power a few weeks ago I measured the AC power input. This was approximate but the batteries were well charged when I did it so I don't think that the converter was pulling much if any, so the total AC draw was a little over two amps. I assume that was almost all fridge. So maybe 200-250 watts.

So you need an inverter that will supply that AC load. The DC required to do that is roughly 15-20 amps. Can your TV keep up, well no. A few months ago I ran my trailer's batteries down to a bit below 50% due to leaving the breakaway switch tripped and that switch is wired directly to the trailer's battery. I measured 10 amps from the TV connection when I started the engine and revved it to 2,000 rpm. As the battery charged up to 80-90% it dropped to 5 amps.

I had one Group 24 battery. If you have a significantly bigger bank, the impedance will be lower and you will draw more, but I doubt you will ever get to 20 amps due to the voltage drop in the 20' of wire to the trailer.

So let's go with 10 amps as the most you will get from your TV and you need 20 amps to supply the inverter to  the fridge, so you have a 10 amp deficit. Do the math with your batteries. With my G24 at about 70 Ahs I could drive for 3.5 hours before going below the 50% limit. But then if I were dry camping I wouldn't have any DC power to use for camping when I got there.

Solar can fill the gap certainly and that would be my recommendation if you go this way. 200 watts of panels on a mostly sunny day can keep up at midday.

It is unfortunate that you don't have a three way fridge like most LLs. I understand that the DC input to a three way fridge is less than the AC input in watts to deal with this lack of TV power.

If you want to check these figures, get a clamp on DC and AC ammeter and do the same measurements I did and adjust the conclusions based on this data as well as your actual battery capacity.

Oh and wire the batteries to the inverter with 10 gauge and use a 30A fuse near the battery if it is within 10 feet, bigger wire if further.

David

[DaveL]

I think that running the fridge on propane is safe while traveling (IMHO).  There are several features built into your propane regulator that perform a safety function:

"ACME fittings are equipped with two safety protocols. First, they have a heat sensitive thermal bushing that if overheated, shuts down the flow of gas. Great for barbeque grills. Second, there's an excess flow check valve. When the flow of gas is first allowed through the ACME fitting, the check valve closes, allowing just a small amount of gas through the fitting. This gas pressure builds up in the lines on the far side of the ACME fitting, and provided there aren't any major leaks or broken lines, pressure builds up in the line, backing up against the check valve. When that happens, the check valves open fully to allow the maximum flow of gas."

In this case the "ACME" fitting is part of your standard propane regulator.  The excess flow check valve is the reason you see a lot of sites advocate for "slowly" opening up your isolation valves when placing the cylinders in service.  Opening them fast to quickly pressurize a line makes the regulator sense a leak.  I have found these sometimes to be very sensitive (ask me how I know).

Unless you turn the gas valves off at the cylinder every time you travel, then the risk of running the fridge vs. just having the gas line break or rupture in an accident are probably almost the same (assuming an accident will generate some kind of heat or spark; electrical or mechanical). 

I do understand that technically the laws require you to "have no open flames" while near the gas pumps and refueling.  This appears to be 50 ft by fire codes.  So technically you would have to shut the fridge off while refueling.  In practice it appears that a lot of RV'ers may ignore this.......

[Pinstriper]

Thank you, David. I knew with your extensive background you would be giving the real world data.

I do think if we were boondocking I would bring along my 700watt generator to keep the batteries up - probably not need to run for very long in order to recharge the batteries from transit, as the fridge would be running propane at the campsite, and then precious little draw on the batteries really so an hour a day would probably maintain very well.

The new rig is wired for solar, and we were given a Zamp briefcase with our CL and we'd certainly bring that for boondocking. I think it's actually a 150watt panel iirc. I wonder where I put it ?

[DavidM]

Yes, I think a small inverter generator would be a good way to recharge your batteries once you get to a campsite. 700 watts may be a little small because your converter will probably need all of that output, at least for the first fifteen minutes or so if the batteries are down to 50%.

What size batteries do you have? I would upgrade to two to give you more time on the road. Another thing is to just turn your fridge off while on the road. It might warm up a little over several hours but not bad.

David

[nhlakes]

... I think it's actually a 150watt panel iirc. I wonder where I put it ?

In the sun. 

[Merlin]

I went out today and hooked up my Kilowatt meter to just the fridge and found it pulled 193 watts when on AC. It's the single door 3-way Dometic LL installed for a few years in their smaller Camplites. Along with a few watts to run the control circuit (which is DC), DavidM's estimate of power usage is right on. The AC heater is on a thermostat, so it doesn't not run continuously, but obviously more in hot weather.

The DC power wire from the TV has other uses while underway, including running the converter to keep the house battery on float (or even charging it if you've just been dry-camping), the propane leak detector, the fridge control circuit, radio clock and whatever other DC uses you might have in your camper. There would be amps leftover to power a stand-along inverter, but the inverter itself would need some juice too and small inverters tend not to be super efficient. I don't know what fridge you have in your pretend CL (that I have not seen before  ), so its power needs could be more or less. But in my Camplite, an inverter powering my fridge would probably draw at least 17A DC whenever the thermostat called for heat. So once again, DavidM's estimates are solid ballpark.

So, given what we know and what we don't know, I'll hazard a guess it would be doable to run my fridge, but maybe or maybe not your fridge, off of a small 300W inverter with 10AWG wire and a 30A fuse. However and but, ONLY if you are wiling to spend quite a bit of time and hassle on power management balancing shore power, battery power, solar power, and TV power.

Have you considered a safety shutoff device for the propane line so you can run the fridge more safely going down the road?

https://www.amazon.com/GasStop-Propane-Emergency-Safety-connections/dp/B07N3169CB