Wednesday, September 22, 2021
Wednesday, September 22, 2021

RVelectricity – Just Ask Mike (J.A.M.): Dometic 12-volt DC fridge power usage tests – Part 1

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 is Part 1 of the test results to determine how long an RV fridge will run from a single 100 amp-hr lithium battery.


Dear Readers,

I’ve now tabulated some basic kWh usage numbers for my Dometic DMC4101 12-volt DC compressor refrigerator. This is a 10-cubic-foot RV fridge with a separate freezer and refrigerator section. It’s designed to be mounted in an RV in place of a standard 3-way fridge.

I’ll not go into all the various shelf and lighting features here. I’m mainly focused on just how long it will run on a single 100 amp-hr lithium battery before the SOC (State of Charge) hits rock bottom (0%). Remember, with lead-acid and AGM batteries you should only allow them to discharge down to 50% SOC for best life. But lithium batteries are rated to discharge down to 0% without damage.

Plan A: Failed

So I first thought that my trusty HP 6286A power supply with an inline kWh meter from Amazon was going to get me started. It all seemed to be working at first, but then I noticed that the battery usage meter showed 1.2 kWh of energy use in maybe 4 hours of run time.

Something was clearly wrong, so I reset everything and tried again the next day. This time I carefully monitored amp usage with a DC clamp meter on the power line. While the 5.59 Amp reading was pretty close, there was no way this fridge was using 1.2kWh of energy in 4 hours with the compressor drawing an average of 6 amperes or so. That 70 watts of power times 4 hours of time should have been around 280 watt-hrs, not 1,200 watt-hrs. That’s nearly a 250% error compared to my paper and pen measurements, so something was clearly wrong with this meter.

Plan B: Limited success

I repeated this experiment with a battery since I know that’s what the ultimate test is anyway. I happen to have a very nice Briter Products 100 amp-hr lithium battery with a built-in SOC monitor, and a Progressive Dynamics charger connected. So I was able to fully recharge the battery in a few hours and turn off the charger. I then monitored how long the battery took to discharge down to a few percent while powering the Dometic refrigerator. That’s a real-world test… 

Baby, it’s cold outside…

I also monitored the fridge/freezer temperatures with a logging inside/outside thermometer over the three operation modes of Performance, Silent and Eco.

I had plenty of frozen things in the freezer and water/beer/ribs in the fridge section, since that will affect how these refrigerators respond to temperature swings. Plus, I like beer and ribs….

It’s getting hot in here…

Air temp in my garage would drop down to the high 70s at night, and get as high as 98 degrees during the day. So I figured this was a pretty good test of these 12-volt DC compressor refrigerators in the wild. Yes, even Maryland is feeling the heat wave right now. While I know other states have topped 110 degrees in recent weeks, for Maryland to hit 100 degrees is almost unheard of this time of year.

What about the defrost cycle?

Hey, I think that having a defrost cycle with a 110-watt heater in an RV fridge that you’re boondocking with is a bad idea for energy usage. It’s a big waste of energy while you’re on battery power and can certainly wait until you get home.

However, there didn’t appear to be any way to shut off the defrost cycle while in Performance mode, but maybe I’m missing something. But, I did find a schematic inside of the door which shows there’s a fuse you should be able to pull to get rid of the defrost heater. I’ll look for that later.

Just the facts, please…

Here’s my quick table of how long this refrigerator will run from a single 100 amp-hr lithium battery in 95 degree ambient temp during the day. I need a better DC data logger if I want to correlate the air temp with energy usage per hour. But this is pretty good for a start.

Mike’s SWAG (Scientific Wild A$$ Guess)

I’ll need to do a few more test runs to be sure. But it looks like this fridge in Performance Mode would run around 20 hours on 100 amp-hrs of battery, 22 hours in Silent Mode, and 25.5 hours in Eco Mode. The run times could be longer once the defrost mode is disabled, but I don’t know that yet. But this is a good start on expected boondocking times.

I’ll follow up in a few weeks with more data once I get back from the FROG Rally in Goshen in a few days. Yes, I’m sure there will be a few FROG tales to tell from Goshen….

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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Al
27 days ago

Bottom line is: The fridge uses about 100AH to 150AH a day, depending on the ambient temp and how much you open and close the door. Very good info and testing.

Kevin
1 month ago

I look forward to each article, Thank You.

Clint
1 month ago

Mike: I have an unrelated question to this topic. I have a 44 ft. Newmar with a built in surge protector. I was advised to get a portable one (specifically the Hughes Watchdog with replaceable board) also. The reason is if there is a lightning strike or other power spike the portable will absorb the spike and the coach will be spared. Without the portable the protector built into the coach could be ruined and I will be stuck without power until it is replaced. Is this a good plan?? Thanks

L & G
1 month ago

Mike – thank you for running the tests. This information is very helpful!

Crowman
1 month ago

Why are you trying to reinvent the wheel? I’ve used the normal electric/propane refer for over 30 years without a single problem but looking at the comments there’s plenty of issues with the 12 volts.

Bill
1 month ago
Reply to  Crowman

My father-in-law had 12 volt compressor refrigerators beginning 50 years ago with no problems. Well, one time the door came open during a sudden stop and spilled spaghetti sauce all over two West Highland White Terriers, but the dogs enjoyed the experience. The point is that in RV applications the 12 volt refers potentially provide much better performance than either RV electric/propane refrigerators or residential refrigerators that are replacing them, with less fire hazard.

Larry Lee
11 days ago
Reply to  Crowman

In a word? FIRES!

Pat Belletto
1 month ago

Hopefully you can help us with our Dometic compressor refrigerator CRX110 SKU 9105306128.. We have had trouble with the unit shutting off at night. We have an 180 watts solar panel and have replaced the factory batteries with 2 6V AGM batteries as recommended by a local RV repair dealer. Solar is not a problem. Everything in the unit, including the fridge works perfectly except at night. The Dometic refrigerator shuts off as soon as the sun goes down. It is possible that this unit could be defective or is it just me?

Mike Sokol
1 month ago
Reply to  Pat Belletto

Have you metered this yet? Without more data anything I say would be a wild guess. DC clamp amp meters are really helpful for this sort of troubleshooting. I’ve written about them a number of times here so you should be able to do a search on them.

Pat Belletto
1 month ago
Reply to  Mike Sokol

Dear Mike,
Thank you for the response to our fridge question. My husband is going out to purchase a DC clamp.
Meanwhile, another question. We are thinking of installing an air conditioner on our Northstar 850 pop up. We are interested in an energy efficient unit and don’t want to overspend. Our concern is that the conditioner may well drain storage batteries during the evening. The unit came prewired to accept AC but hit the backorder problem.

Pat Belletto
1 month ago
Reply to  Mike Sokol

Mike, We now have a digital clamp meter. My husband is not familiar as to how to use this tester. We would appreciate if you would send a step by step set of directions. We’ve read your article but it’s Greek to us. You mentioned to put the tester over the power source. Would it be located off the batteries or is it somewhere else?
The fridge has always worked on shore power. We have a 180 watt solar panel with 2 6volt litium batteries. We were assured that these new batteries would provide between 12-20 hours of power. The fridge turns off at night and starts up again at daylight. Every other electrical device works through the night. Are we to then believe that the batteries are not at zero? The volt meter is consistently at 1.3 amps. At times it drops to 1.27 and then recharges.
Your advice and assistance is greatly appreciated. Many thanks!

Pat
1 month ago

I have a 12 volt DC. When the temperature hits 90, I need to find plug in connections. It will suck the batteries dry trying to keep cool. It does much better at lower temperatures. I seem to have similar usage to Mike. My 510 amps of lithium will not last 5 full days in 90 degrees. In all fairness, my previous propane fridge had the same problems in similar temperatures, burning through propane but not quite keeping up.

Mike Sokol
1 month ago
Reply to  Pat

It’s all about the ambient air temp which is why I thought my 90+ degree garage was a great real-world test.

Firefly
1 month ago

45 seems a bit warm to have your fridge set to. Do you think it was still in cooling down from the food you placed in it?

Mike Sokol
1 month ago
Reply to  Firefly

That was after 12 hours or so. When you change this fridge to eco mode it raises the temperature automatically.

Last edited 1 month ago by Mike Sokol
Tommy Molnar
1 month ago

I have an AM Watt Appliance Load Tester that measures watts and amps when something is plugged into it. I plugged my current RV fridge into it and turned it on in electric mode. It immediately showed 300 watts at 2.5 amps. Not bad, except that in real world use (going down the road for 300 miles) it drew my Lithium Ion batteries down to 80% before coming back up to almost 100% with the solar on the roof. I didn’t have my above mentioned tester with at the time.

Tommy Molnar
1 month ago

Thank you Mike, for this introductory testing of this fridge. I’m SERIOUSLY considering getting this fridge (or one like it) to replace my current gas/electric stock fridge. Since I have 700 watts of solar on the roof, and two Lithium Ion 100 amp/hr batts, I think this will be a viable option, especially when cruising down the road – and boondocking. My only issue will be getting it wired up properly with a direct line to the batteries and the appropriate fusing.

My other idea was to get a 120 volt RV size AC fridge and go that route. That would be a simple “plug in” option given that the stock fridge has a plug already in the fridge bay. Not so sure about that one though. I’ve heard good and bad about each.

Last edited 1 month ago by Tommy Molnar
Glenn
1 month ago

Defrost mode is a killer. That’s why 25 years ago I bought a freezer for our basement without auto defrost. Once a year everything goes in a cooler and the heat gun comes out. A lot of energy savings for 20 minutes of work.

Mike Sokol
1 month ago
Reply to  Glenn

I’ll locate and pull the defrost fuses once I get back from the FROG rally. Showing off my Rockwood GeoPro travel trailer.

Last edited 1 month ago by Mike Sokol
tom
1 month ago

Very interesting.

Donald N Wright
1 month ago

Thank you for testing the refrigerator.

Mike Sokol
1 month ago

You’re welcome… these 12-volt DC compressors are the most energy efficient way to cool food that I’ve found.

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