Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM. This week I discuss running a 2-way fridge on battery power alone.
Dear Mike,
Since I’m doing a lot of 6-hour drives across the desert, is there any way I can add an inverter to the 2-way refrigerator in my trailer and power it from 120 volts instead of propane?
How long would my 100 amp-hr battery last doing it? Can my truck alternator power this through the 7-pin trailer connector? —Toni
Dear Toni,
Great questions… and it shows that you’re thinking this through before plunging in and trying it. So let’s run a few numbers on what the fridge requires and see how you can power it.
A tale of electrons and propane…
The problem with a 2-way refrigerator is that it needs a source of heat to make things cold. When in propane mode, this is using a flame to boil ammonia. But in 120-volt mode, it’s using a heating element very similar to an electric space heater.
In your case, it needs 400 watts (edited) of electric power at 120 volts to heat the element that keeps your beer cold in your fridge. Yes, it does seem like a long way around to keep things cold, doesn’t it?
How much energy is that?
Since we already have your power requirements in watts, all we have to do is multiply the watts times the length of time in hours to find watt-hrs. (See how easy this is gonna be?)
In this case it works out to 400 watts times 1 hour equals 400 watt-hrs per hr (400 watts x 1 hr = 400 watt-hrs).
How much energy does my battery have?
A 100 amp-hr battery at 12 volts works out to 100 amps x 12 volts = 1,200 watt-hrs of energy. Of course if you have two batteries you can double this, or four batteries would quadruple the total power available.
How much can I pull from this battery?
In your case, this is an AGM battery rated for a minimum SOC (State of Charge) of 50%. So there’s 600 watt-hrs of available energy that can be used to power your refrigerator. And, of course you’ll need an inverter to step-up the 12-volts DC to 120-volts AC. So throw away maybe another 10% in conversion losses.
A simple calculation…
If you have 600 watt-hrs of available energy from your battery, and your fridge uses 400 watts to heat the coil, that’s only 90 minutes of running time at 100% duty cycle before you’re out of battery power. Even if your fridge only runs 50% of the time (a 50% duty cycle) that’s still just 3 hours of available battery power. So, no good if you need it to run for at least 6 hours.
What about 7-pin trailer plug power for my 2-way fridge?
If you want to know how much energy your 7-pin plug can supply between the tow vehicle alternator and the trailer, it generally maxes out at around 5 amps or so, which would be 60 watts (12 volts x 5 amps = 60 watts), so that’s not going to extend your cold beer and ice cream time by much.
Remember, its total current is limited by the size of the conductor feeding that plug from your alternator, as well as the current carrying ability of those contacts, which I believe are rated at 10 amps max.
You could get a DC-DC charger
REDARC, and others, make a variety of these DC to DC chargers that would do the job. But you’ll need heavy wire and a special connector to connect it.
So you could get a 50-amp DC to DC charger and run a husky (4 or 6 gauge) cable between your tow vehicle alternator and your RV battery, since that would supply maybe 600 watts of power (enough for your refrigerator, but not an air conditioner). However, that seems like a lot of work just to run a refrigerator on DC power alone.
Danfoss compressor to the rescue
I think you should consider replacing your 2-way refrigerator with a 12-volt DC compressor refrigerator that only uses around 80 watts of power while running. In fact, I’ve done tests where I was able to run a 10 cu. ft. refrigerator for 36 hours on a single charge of a 100 amp-hr lithium battery. I’m guessing a refrigerator upgrade may be in your future.
Future Shock…
More on this later. How about an Ask the Expert video on this topic? I know a few smart people that might be willing to do an hour interview about 12-volt DC refrigerators.
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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Further to your comment on the DC to DC charger. You mentioned 600 watts from a 50 amp DC to DC charger. If I connect it to my alternator, ( 220 amp ) will it recharge my 100 amp hr AGM battery in an hour? Not too good at the ‘electro-math”. Thanks
No, but you’re on the right path, sorta. Your AGM (lead) battery won’t like being hit with 50A – you can’t charge that fast without damage. I don’t charge lead fast than “1/10 C” which just means LET it take 10 hours. So, 10A per 100AH capacity at the start of bulk charging..
Second, charge tapers off at the end near full, or absorption and float stages (think of the waitress trickling the end of your coffee refill so she doesn’t splash you).
…but YES, a good DC>DC charger will do massively better than the 5A (usually less) charge wire in the pigtail!
Lithium batteries are getting to be dirt cheap. Over 3000 Watt-hours can be had for about $500, and it only weighs about 60 lbs. Aliexpress has 280Ah cells using the safe LiFePO4 chemistry used in batteries like Battle Born. That much power would require 5 AGMs and more than 300 lbs. And even cheap AGMs would likely cost more. Not to mention the lithium battery should last 5-10 times as long.
Howdy!
Having just purchased and installed a Isotherm fridge/freezer to replace my cursed Dometic, I would love to know more about Danfoss compressors!
Thanks!
I read and understand your calculations on the DC refrigerator discussion. Although I always put my refrigerator (Dometic 3way -not sure of the Cubic feet, but it is in my 17 foot Casita fiberglass trailer) on DC while I’m on the road plugged into my tow Vehicle. I check the battery voltage meter I have permanently installed in the trailer and it always shows full charge and have never had a problem with a warm refrigerator or low battery. I have always assumed that the tow vehicle generator can keep the two batteries charged even with the refrigerator draw. I do have a high capacity battery SUV tow package, will that make the difference? I thought it wasn’t recommended to travel with your Propane tanks open to device. So what gives please?
There’s a lot of conflicting opinions about the safety of traveling with your propane tank and the refrigerator on. I do know of a few instances when a tire shredded and took out the propane lines that were in the path of the tire pieces. In those cases having the propane turned on could have led to a disaster. But there’s probably millions of miles driven safely every year with the LP and refrigerator on.
I myself drive with the propane on for the fridge, however I will say that having blown out a tire that shredded the lower WALL of my trailer, it would be a complete jerk to design an RV with propane lines anywhere near the tires. Shame on dangerous routing!!
I installed the “safety fans” behind the 2-way fridge in our old coach to keep things a little cooler back there. The kit also came with 2 tiny recirculation fans for inside the fridge. I always ran it on propane when traveling. Used very little.
New coach has Whirlpool residential fridge, my inverter display tells me it draws 105-110 watts. Battery bank is twice as big as old coach.
As a plumbing contractor very familiar with gas systems and a RVer of over 30 years need to chime in. Your refrigerator on propane uses so little propane that it’s not worth even thinking about let alone coming up with a solution to a non existent problem. I calculated 1 gallon of propane in our rig will last over 4 months at $3.00 per gallon VS an electrical solution some of you are suggesting that will run 5 to 10 thousand to buy (solar, lithium batteries, convert to 12v compressor). As for fires I see more of them caused and reported on here as electrical fires with electric heaters and undersized cords than a refrigerators propane flame.
Crowman, what is your opinion and experience with RV Propane refrigerators having fires caused by the “propane/ammonia boiler” overheating and starting the fire. I have read about this type of RV disaster and spoken with others that believe it is a risk.
https://www.arprv.com/ Has developed a device called fringe defend that claims to eliminate the boiler over temp problems.
That’s the way I see it. One thing that would make using propane a little safer while traveling would be a flow sensing shut off valve on the tank if a line should break open.
There already is a flow sensing shutoff as part of the OPD valve on your propane tank. A sudden drop in pressure will trigger the valve and stop the gas flow. You may have already experienced this when opening the tank valve too quickly on an empty line and having to wait few minutes for the valve to reset and then gently reopening the tank valve.
Its there! Part of every OPD tank is a max flow clampdown. My home grill activates it with all burners on. Annoying!
Most propane devices are rated in BTU use per hour. There are 430,270 BTU in a 20 # tank. A more accurate way to determine how long a device will run is to divide one by the other. A 6 cubic foot refer should run 286 hours, wide open. That’s only12 days. The refer probably won’t run continually but I doubt it will run 120 days.
I question running the fridge from the engine at all compared to propane. An ICE is 25% efficient at converting heat to rotary motion, alternator is 90% converting from rotary motion to 12VDC, inverter is 90% converting 12VDC to 120VAC which is converted back to heat. Propane is directly converted to heat.
With my solar and lithiums, I run the fridge till the batteries are at 40%. My inverter shows 21A used running the absorption fridge.
I do the same. When the batteries drop to a certain voltage, the refer shuts off until the solar panels recharge them, then it comes back on.
I keep water bottles frozen in the frig. Works for me.
I agree Tom. We use those sealed ice packs to keep the fridge cold on long pulls. On short pulls too come to think about it. They just stay in there.
I alway freeze a few water bottles at home the night before a trip, then load them in an RV fridge or cooler for the road.
1200Watts is way too much. I think 400W is the most I have seen for an element and 325W is the average size.
I second that. I think my fridge heater draws 350 watts. You could probably get away with a 500 watt inverter. I run a 2k watt inverter so I can run the microwave if needed. The 100 amp alternator keeps the house batteries charged while the inverter is running.
That must be a motor home. This is a travel trailer with a 7-pin plug connection that will only pass around 5 amperes or so of charging current.
I’ve adjusted my post to 400 watts of 120-volt AC power since that seems to be the consensus.
Just thinking out loud here; is it feasible to install the inverter in the tow vehicle and run the AC power through the the 7-pin trailer plug? The AC line could be dedicated to the fridge & maybe the battery charger. A 5 amp connection should provide ~600 watts AC.
You misunderstood the article. Mike used 400W for the refrigerator draw in his calculation. 1200 watt-hours was for 3 hours of running time.
I just edited the article to calculate battery time with 400 watts of power draw. I originally wrote it was 1,200 watts because that’s what the OP told me, but I believe Toni must have been thinking about the electric water heater element.
Boiling ammonia is archaic. All RV fridges should be 12 volt compressors. Easy enough now to build in lithium and solar. End the fire risk. Propane cooktops should go away too, replaced by induction. Save the propane for heat and hot water, with those devices OUTSIDE the coach so any propane leaks fall harmlessly to the ground.
I agree completely. The old school propane/ac fridges belong in museums, right next to the 3 burner propane cook top. Does anyone actually use 3 burners on their cook top? Our old school fridge only works when it feels like it, so a new DC one is in the plan. Also thinking about an inductive cook top. Our water heater is propane or AC, so I can run the generator if needed for hot water. That just leaves the furnace, which we have never used. See ya later propane.
My son (the pastry chef) uses a portable induction hotplate and induction cooktop stove all the time for frying, sauces and making chocolate (yes, you can get a degree in chocolate making). And it makes the best onion rings you can imagine without burning the oil from hot spots in the pan when heating on a flame. I’m totally sold on induction cooktop technology.
It’s been my experience that those who use high power electrical devices are the people who find it necessary to run their generator a few hours a day or are dependent on RV hookups. While I’m encouraged by reports about 12 v compressor refrigerators and induction cooktops, it’s hard to beat propane for long term off grid.
Mike I don’t know what size refrigerator you’re talking about but I have a 8 cu ft fridge that I have connected through a watt meter in series that only draws 376w with it in initial cool down mode. My sil had a 42’ 5th wheel with the residential size fridge and ran his fridge all the time off a 650w full sine wave inverter.
Of course your mileage will vary. The OP told me his 2-way fridge used 1,200 watts of power while in 120-volt AC mode, and many of my readers tell me that their absorption refrigerators have a lot of trouble getting cold. But the point of the article is that absorption refrigerators are very power hungry when running on electricity, so if you want to run them from an RV’s 12-volt battery you’ll need to beef up your house battery and charging system. I think that a 12-volt DC Danfoss compressor refrigerator and several solar panels makes a lot of sense for boondocking.
Hi Mike. I didn’t read anywhere in your response to Toni about the refrigerators duty cycle of operation. The fridge wouldn’t be running for 6 hours constantly, but would cut in and out as needed right? I would think the biggest problem would be the efficiency of the inverter losing about 20% of the energy from the battery per hour.
Mike said: “Even if your fridge only runs 50% of the time (a 50% duty cycle) that’s still just 3 hours of available battery power”