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user report on Dometic 35 compressor fridg

This is not mine but over on the expedition portal forum there's a lot of 12v and other nice to know info there too. Here's something that you might find interesting if you are wanting to know average power consumption of a mid-sized compressor fridg:

  1. 8-27-2017 
    Dometic CFX 35 fridge testing and dual battery system setup (VSR vs Ctek).

    Hey all,
    New to this forum but not new to forums in general, have used this place for a long time to help with research and information and decided it was finally time to give back.I have been using a Dometic CFX 35 for the past year now and wanted to give everyone the exact run cycles and consumption that it has been typically using. I did my testing using two different multi-meters for accuracy, and each has a clamp for measuring amperage as well. I do have an insulating/protective cover on my fridge. I live in Arizona so the normal temps throughout much of the year are pretty hot. In real conditions during the hot part of the day (90+ degrees) this fridge consistently runs for 7 minutes at 3.5 amps and then has a no-run period of 28 minutes. This is for a fridge setting of 34 degrees F. That puts the cycle at 35 minutes, so basically it runs for about 12 minutes an hour at 3.5 amps. This puts it's average hourly draw at 0.7 amps. That is in hot weather on a pretty low setting (I like my drinks COLD), so I would bet you could get that to 0.5-0.6 amps pretty easily in cooler temps. As far as I know that is lowest actual amp draw per hour that I have seen in a fridge this size. It goes without saying that those measurements are after the fridge has been brought to temp and contents are cold (like during your initial drive). The CFX line is a little nicer than the older Dometic fridges; better molding, better lid, easy control panel, normal low voltage settings, interior LED, exc. The only negative I have observed is a slight clunking from the compressor when you hit bumps. Apparently this is common because under their FAQ's they address it as normal. I find it mildly annoying. Otherwise fridge works great. 

    I have used a couple of different dual battery charging systems and my latest is the Ctek D250S DC to DC charger. For a while I used a typical VSR from TrueAM' and that worked fine, but of course didn't ever really want to charge my AGM battery much above 80% charge or so. My Frontier has a startup voltage of about 14.2, and then settles into around 13.8, so it doesn't really put out a high enough voltage for the VSR to give the battery that final charge. The Ctek works well, simple, 5 stage charger, delivers a max of 20 amps and boosts the voltage to around 14.4 depending on temperature and battery type. Also is an MPPT solar charge controller so all you need to do to use your solar panels is plug them onto the Ctek. Seems to work as advertised, typically delivers 19.7 amps to my aux battery in hot middle of the day conditions, and then slowly decreases amps as battery get's closer to full charge. Some users report that if you mount this in your engine bay it gets too hot and doesn't perform well; this seems about right as the manual says that it prefers temperatures under 120 degrees F, so something to think about. Well, thanks for reading this write up, hopefully it will help some folks down the road.

    I typically keep my battery in a Minnkota battery box (best $50 bucks ever), which I place in a rolling tool box along with the Ctek. The fridge is sometimes in the cab on a slider and sometimes in the bed of the truck, just depends on the trip. Thanks again!That average hourly draw rate is sure nice to see, especially in warmer climates.

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[-] The following 1 user says Thank You to TWIH for this post:
  • Ballenxj (12-20-2017)
The duty cycle and amp figure are always useful as they are indicative of electrical consumption, in a specific persons usage/ climate.

And the numbers given are well within what i would expect of a compressor size in that environment.

But it also has to be known that there are huge infuencing variables, Like the cabinet temperature, how often the door/lid is opened, and for how long, and what types of items are being placed within, and what are the temperatures and densities of those items when placed within.

The 7 minutes on 27 Minutes Off, indicates to me the unit is pretty well insulated. My front loader fridge is more like 5 minutes on 17 minutes off, in 75 F, but I have not really sat there since it was new taking note of when the compressor turns on and for how long.

The 3.5 amp draw figure, is at what voltage? We do not know and we do not know how much voltage drop there is on the wiring from battery of unknown voltage, to the compressor controller. A wattage figure would be more precise as it would vary little with voltage.

Also while Dometics now use a Waeco compressor, which I believe to be a danfoss clone. I have a danfoss BD-35f, running at minimum rpm(2000), and while it starts a compressor cycle at about 2.8 amps at 12,8v, 5 minutes later it is only drawing 2.2 amps at 12.8v right before it shuts off.

So 3.5 amps would indicate a faster rpm compressor. But does the amp draw decrease as the compressor runs during that on Cycle? My danfoss does, does the Waeco? If it does, then that 3.5 amp figure was it at the start of a compressor cycle or near the end? If we are going to multiply amp draw and time, well there is a farily significant difference in multiplying 7 minutes x 2.2 vs 2.8, when measured over a period of days.

When mine had the resistor installed in the Tstat circuit to tun at 2500 rpm, it would start at about 3.2 amps and then 4.5 minutes later be drawing about 2.65 amps.

Really, If one wanted to compare /measure fridge consumption, in their usage, then a wattmeter on the fridge over a period of a few days while the fridge is in actual use as it will be used, is much much more accurate than saying it runs 7 minues on 27 minutes off and draws 3.5 amps when it is running.

I personally have wired up most of my Devices with Anderson powerpoles. I use this wattmeter, which also has anderson powerpoles, so that i can put it inline on almost any device I wish to measure.

The last time i bothered to measure my fridge draw over a period of 3 days, it came out to 0.62 AH consumed per hour average as I used it. I would give this consumption figure odds of 90% accuracy.

This does seem pretty low, but my fridge does have extra insulation, I was not loading it with warm beer and drinking like a fish, and I would guess that max daytime temps inside the van approached 80f and dipped to 60 at night.

There are really SOOOO many variables that comparing one fridge's consumption to another based on a few reviews, when the variables are not known, is kind of useless. I could make my fridge look like the most efficient fridge every to move refrigerant around, or the biggest POS ever to leave a factory, by not including the influening variables in the data, and of course one can always simply lie. it is the internet afterall.

I would guess that most 12v every compressor fridge available out there, of the same internal volume, in the same environment, in the same usage, would not vary their electrical consumption more than 10%. I would also guess that number to hold whether the fridge has a Danfoss/secop compressor, a Sawafuji, a Waeco or some Chinese clone Knock off compressor.

Where one can greatly influence the actual electrical consumption lies in how well the condenser is ventilated, and how much insulation there is and ambient temps of the Vehicle that it is being used within.

One trying to be frugal with their fridge's electrical consumption would concentrate on minimizing door openings, limiting the time the door is opened, adding warm objects only in the coolest part of the morning, and not allowing the interior of the vehicle get much hotter than the exterior of the vehicle through proper and efficient ventilation.

but really, it is really best to simply have enough recharging ability to return to the battery that which the fridge removes, with a whole bunch extra so one does not have to waste fuel or spoil food.

I flew to the atlantic coast for Xmas for 2 weeks. I had actually forgot That I wanted to defrost the fridge and turn it off before leaving. I was running late, did not have time before my flight, still had a bunch of items that would not go bad in a 2 week absence. I said screw it, left it running. While my 200 watts of solar does not generate much over the winter solstice, with no door openings and relatively cool temps, its electrical consumption and the shallow yet unnecessary cycling of my old battery overnight, is also not a concern.

Any Vandweller or similar full time system set up with a compressor fridge, should really have enough overhead where the fridge consumption is not a concern in regular use.

But more data is always a great thing, and I urge the collecting nd sharing of it, with the influening variables both known and unknown measured and unmeasured, at least listed in that data.
[-] The following 2 users say Thank You to sternwake for this post:
  • Ballenxj (12-20-2017), rvpopeye (12-20-2017)
I do not know what most of that means, but I know we sure do love our cfx-40 whole bunches. It has a wifi thing so I can use it like a remote and check the temp from my phone, not terribly useful, but cool, I guess. Also like the charging portal and adjustable low voltage auto shut off. Great coolers!

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