#152245  by Neddy
 Tue Jun 18, 2019 7:37 pm
My experience with “under-bench” 12v compressor fridges has been that “small” models of around 50L generally draw an average current of about 1 amp, “medium” sized models of around 85L generally draw about 1.2 Amps while “large” under-bench fridges of around 130L generally have an average power consumption of about 1.5 Amps.

Having found that our new Juntek battery monitor was able to measure and record total power consumption over a period of time, I was keen to evaluate the 130L Isotherm fridge that came with our van.

My test regime was very simple.
(1) Charge the House battery.
(2) Disconnect the DVSR.
(3) Disconnect the solar panels.
(4) Turn the fridge to its mid-range setting (equivalent to about 4°C).
(5) Using the meter CLR button, re-set the AmpHour readout and Timer to zero
(6) Press OK to start the test.
The Ah and Time readings can be seen climbing. After an hour or so you get a very good idea of the fridges power consumption, but I figured that a longer period must give a more precise result.

Over 16 hours, the fridges total power consumption was measured at 8.81Ah. That’s an average current draw of just 0.55 Amps. Such an incredibly low figure must surely be met with widespread disbelief, so first off, I checked the meter. Running a known load over a known period of time, its Ah measurements proved to be absolutely spot-on.
The 16 hour power consumption test was performed overnight when the vans interior temperature was about 7°C. We all know that the lower the ambient temperature, the less power a fridge consumes, so I needed to measure how much current the fridge would draw in the heat of Summer. I repeated the test with the van heated to 28°C. Sure enough, fridge power consumption was much higher. Total consumption over 16 hours was 13.3Ah, giving an average current draw of 0.83 Amps.
This is 20Ah/24hr day.

Samsung quote 269kWh per year for their 255L inverter fridge. That’s 61Ah/24hr day.
Two forum users cite their 24hr Samsung Inverter fridge power consumption as 60Ah and 65Ah.
T1 Terry reckons 70Ah, but that is in the heat of Australia.

To make a fair comparison, keep in mind that at 130 liters, this fridge is just over half the size of the Samsung, so its power consumption figures should be doubled if you want to compare them directly with the Samsung results.

What can explain such low power consumption? Turns out that the Isotherm CR 130 EL fridge is equipped with a new model Danfoss/Secop BD35F compressor. These are made specifically for 12 volt DC use and they vary the compressor speed electronically – just as Samsung Inverter fridges do, but without incurring inverter losses.

It is easy to see that such a fridge would have quite modest battery capacity requirements and we find that a single 100Ah AGM battery is more than adequate for our needs. Not so long ago I was staunchly advocating a minimum of 200Ah storage to run such a fridge. Not any more!

Neville.
 #152247  by scubadoo
 Tue Jun 18, 2019 8:39 pm
Thank you for the report Neville. Impressive indeed!

I wonder how much extra battery effort is involved in maintaining our 53 litre freezer compartment at around
IMG_20190618_201332a.jpg
Photo taken a few minutes ago.

Quite a bit I expect. 20 or 30Ah would not surprise me.
 #152248  by Nut17
 Tue Jun 18, 2019 8:41 pm
Wow Neville ! That is really good information. In our first RV - A Mazda Parkway I fitted a Vitrifrigo 85lt under bench 12 volt compressor fridge. This had a current draw of 3 amps, but averaged about a 30% duty cycle which equated to around 25 AH / 24 Hrs. I had a 220 AH battery bank and one measly 80 watt panel. Most of our battery charging was accomplished via a dedicated 12 volt 50 Amp alternator on the bus engine.
The other extreme is the Dometic 12 volt compressor 215 lt fridge / freezer originally fitted to our last van. This consumed around 85 Ah / 24 hours - a little more over the summer, and little different over the winter months as we kept the van nice and warm.
The Samsung was 20 % bigger and much more efficient - Freezer at minus 19 deg and fridge at 3 degrees and was more frugal with around 60 - 65 Ah / 24 hrs.
 #152270  by Nomadic
 Wed Jun 19, 2019 5:09 pm
Wow, thank you for all your hard work very useful information.
 #152379  by Neddy
 Mon Jun 24, 2019 1:12 pm
In practical terms, running our 130 litre fridge for 16 hours from a 100Ah AGM battery takes the battery charge from 100% down to 90%. This leaves plenty of spare capacity for lighting, running our TV and diesel heater etc. Could be even better yet, though.

Our new van came with the fridge already installed. To my mind, the job had not been done very well. While air venting was provided at the top of the fridge, there were no vents at all at the bottom. Without a "cool air" inlet at the bottom, there was very little warm air coming out the top. I cut two oblong holes in the fridge "baseboard" and fitted these with anodised aluminium louvres from AliExpress. These cost NZ$1.50 each. Freight free. (You can see how reckless I am with money. Devil-may-care, even.)
The net result has been a 10% reduction in fridge power consumption. Running the fridge all night now reduces the battery SOC to 91%. I reckon there is still room for further improvement. The next project is to fit a couple of 12v computer fans to assist with the fridge cooling air circulation. In my experience, such fans save far more power than they use and can have a marked effect on fridge efficiency and power consumption. Being wired in series and therefore running at half throttle, they are inaudible. I will wire them to the compressor circuit so they will only turn on when the compressor is running.
P1010032.JPG
 #152383  by MartyB
 Mon Jun 24, 2019 5:27 pm
Neddy wrote:
Tue Jun 18, 2019 7:37 pm


The 16 hour power consumption test was performed overnight when the vans interior temperature was about 7°C. We all know that the lower the ambient temperature, the less power a fridge consumes, so I needed to measure how much current the fridge would draw in the heat of Summer. I repeated the test with the van heated to 28°C. Sure enough, fridge power consumption was much higher. Total consumption over 16 hours was 13.3Ah, giving an average current draw of 0.83 Amps.
This is 20Ah/24hr day.
Neville.
This is really interesting Neville, clearly these new fridges are far better than mine!
Question for you though - Is your fridge vented externally?
Bare in mind that if it is externally vented then raising the compartment (room) temp to 28 would most likely effect the length of 'on' time rather than the 'load' on the refrig system as that 28 heat will not be raising the condensing temp much at all. Conversely, in summer a 28 ambient will have a more dramatic effect.
Does your model have a condenser fan as standard?
Cheers, Marty
 #152388  by Neddy
 Mon Jun 24, 2019 7:26 pm
"Is your fridge vented externally?"
No. It wasn't even properly vented internally, in my opinion.

"Does your model have a condenser fan as standard?"
Yes it does. Nevertheless I have found that other 12v fan assisted fridges I have had were invariably improved by adding a couple of supplementary fans. I fit two 12v fans in series with one "pushing" cold air in and one "pulling" warm air out.

See viewtopic.php?f=102&t=2836
and quite a few other similar threads.

Neville.
 #152687  by Neddy
 Thu Jul 04, 2019 12:56 pm
"Barneykiwigirl, here is a quick comparison of the two fridge models that you have so far investigated. It is only when you stack options side-by-side like this that the exceptional performance of the new Danfoss compressor becomes fully evident.
FridgeCF.JPG
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