Digital Thermostat for camping fridge.

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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Mon Sep 10, 2012 7:49 pm

Yes part of the reason for trying this modification is to avoid frozen food over night when the ambient temperature drops.

My understanding of how it works was a little off in my last post. If I set the trigger temp at +4 deg C and the hysteresis at 3 the compressor will run until the temp drops to +4 deg C then the relay switches off until the temp rises (3 deg C) to 7 deg C before the relay activates again.

You get a little over run on the set temperature when the relay switches off at +4 deg C and the temp continues to fall to around +3 deg C before slowly rising.

At the moment I am trying hysteresis of 2 and fridge setting of +3 deg C. My thinking this will give a +2 to +5 deg c range.

I can complicate things more with another adjustment that activates a time delay on triggering . Adjustable from 0 - 10 min . I tryed it at 10min but the small problem was when I turned the fridge on I had to weight 10 min for the fridge to start the first time. I don't think this setting will help me much as it usually takes more then 10min for the temp to rise so I don't think rapid cycling( switching) is a issue .

At the moment I have the probe in my house fridge door to try and get some idea of the temperature range a modern fridge works within.i will also test the freezer as I have another unit I want to set up for my 2nd Engel that runs as a freezer.

At the moment the house fridge is only showing +7.5 deg c with probe in the door where milk sits. This fridge had been running fine for year's. though that seems a little high, i will keep A check on it and see what the minimum is over the next few hrs. Perhaps I am being to tight on my temp range with my Engel?

The Engels use a large exposed evaporator plate with press formed coil that covers walls all the way around and spans from top to bottom on the 35lt and bottom to 80% up on the 40lt.


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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Tue Sep 11, 2012 9:28 am

After longer testing of the house fridge the internal temps at the rear of the middle shelf hold around 3.6 - 4.4 Deg C . Although when the probe is in the bottom of the door shelf it ranges from 4.4 - 5.5 Deg C.

The freezer section gives -15 to -17 deg C in the center of the freezer.

This is interesting because the Engel when set on max for deep freezer shows down to -23 deg c and the compressor cycles around 80 - 90% of the time I might try setting this at -18 deg c like the home fridge and see if there is any efficiency to be gained. I'm not trying to freeze food for max life - 6 months in the Engel as the longest camping trips are usually one or two months

Real world testing on the two fridges in the heat and small air space of a car . With 35lt Engel set at setting 2 fridge and 40lt Engel set at max freeze setting consumed 75ah in a 24hr period. Using the Engels internal mechanical thermostat dial.

I will try the same test using the two digital control units set at +3 deg c and -18 deg C and see what the 24hr consumption is.

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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Tue Sep 11, 2012 1:21 pm

I just tried the first unit on the 35lt Engel . Setting the temp at 2 Deg C and the hysteresis at 2. This works great :D . Compressor runs until temp reaches 2 deg C then switches off. There is a over run after the compressor is off and the temp continues to fall to around 1.4 Deg C. with a empty fridge it takes around 18 min for the temp to rise to 4 deg C again and around 6min run time to bring it back down to 2 deg C.Although i feel the run time will become slightly less each time to a point. But I will use 30% or 6min as a average.

So we are looking at a 30% duty cycle for the fridge maintaining 1.4 deg c to 4 deg C. 30% of 2.5A is .75A x 24hrs = 18ah per day.

I used a large plastic component box with enough room for two units side by side and genuine surface mount Engel screw sockets on top. The 30A (overkill) plug is mounted flush with the box so the relay its self is external . I did this for two reasons one being a small amount of heat a relay gives off and the second is the relays I purchased have a built in fuse that's easy to replace externally.

Kurt.

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Re: Digital Thermostat for camping fridge.

Postby Gordon-Loomberah » Tue Sep 11, 2012 1:47 pm

offgridQLD wrote: with a empty fridge it takes around 18 min for the temp to rise to 4 deg C again and around 6min run time to bring it back down to 2 deg C.Although i feel the run time will become slightly less each time to a point. But I will use 30% or 6min as a average.


Sorry to mess up your numbers Kurt, but 6 min run time then 18 mins off is actually a 25% duty cycle - 6/24 = 0.25 ;)
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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Tue Sep 11, 2012 3:28 pm

oh now So see my mistake. I didn't account for the total time. Although I think I gave the wrong Data . It should have read total running time for a full on off cycle was 18min and 6min of that the compressor was on.

So I am still wrong as it should be 33.3% and 19.98ah over 24hrs


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Re: Digital Thermostat for camping fridge.

Postby Gordon-Loomberah » Tue Sep 11, 2012 3:35 pm

OK, that means it takes 12 mins to warm up rather than your originally posted 18 mins... so it is indeed a 33% duty cycle if it is running 1/3 of the time.

Maybe you can add some insulation in non-heat radiating places, such as the lid, to reduce the duty cycle somewhat. What does it have for condensation coils- are they built into the walls like newer household fridges, or is there some other method of removing heat?
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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Tue Sep 11, 2012 4:07 pm

The Engels are like the old refrigerators with the condenser coils mounted externally .Actually the majority of the c shaped coil unit is a good 6 inches away from the main insulated body.

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There is a pancake fan mounted at the base of the coils just under the compressor that kicks in if air circulation is low or ambient temps get to high.

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I have a insulated protective cover over the fridge but its just HD ripstop material with a thin foam and reflective foil backing it zips up over the sides and lid. This keeps the sun off the steel body. More insulation would be A idea but keeping the fridge compact has some value when camping.

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I do feel keeping the condenser coils cool would help potentially reduce the 6min run time. So my next step is some kind of ventilation on my Ute canopy as I noticed the air gets warm in there with two fridges running and it all locked up. i did see a little solar powered vent in a boat shop the other day.

This last pic shows how well the evaporator coils wrap around the entire content of the fridge from top to bottom. there is a basket that fits into the space to protect the evaporator coil. You do loose a little internal space with this style but I feel cooling is more even and faster.

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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Wed Sep 12, 2012 7:42 am

Freezer testing at -18 deg C. testing one cycle after the freezer had been running for several hrs. over 23:47 min it took 11:27 min for the temp to rise and trigger again with the compressor on for 12:19 min. Giving me 52% duty cycle.the compressor consumes 2.5A so 32.2Ah over 24hrs

So my total consumption for 40lt freezer at -18 deg C and 35lt fridge at +2 deg C is 52ah. That's a little better than 58.7ah I got testing inside the garage from 240v and very low ambient temperatures and its way better than the 75Ah I got with the real world test conditions in the car. It's a 30% reduction in consumption.

Now before I jump up and down to much. I want too to actually run both Engels for 24hrs with the AH meter connected to rather than just doing assumed consumption based on a limited testing time and multiplying it to get 24hr consumption.

that said I am sure there will be some worthwhile improvement in efficiency. I know for sure I now have a more stable temperature. Not a bad improvement to a $1000 fridge for under $25 and a little work soldering and setting it up.

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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Tue Oct 30, 2012 3:57 pm

I just wanted to say the two digital control system ran 24 /7 for 4 1/2 weeks 10,000km trip up north then down south at times 45 deg heat (more int he ute tray) Temperature was very stable for both the fridge and freezer. no frozen milk in the mornings :D or adjusting the dial up in the middle of the desert sun. Set and forget.

It was great to be able to peek into the canopy window and see the big red led Temp readouts .Our total of 310ah of AGM and 120w solar panel worked well together with the cars alternator . Most days we got a good 30ah from the PV and the remainder came from the alternator.

Pics of a few alternative energy examples I came across on the trip . Other than all the different water pumping wind mills (some are huge)

Comunications Northern territory

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Coober bedy wind turbine

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All i need is a big extension cord and I can charge my AGMS :lol:

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This give you some idea of the size of the thing

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This was a huge public building in Alice springs with one massing PV installation in progress :shock:

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Re: Digital Thermostat for camping fridge.

Postby Jimbo007 » Tue Aug 16, 2016 12:43 pm

I know this is an old thread but for people browsing as I am, this might be useful.
https://www.ledsales.com.au/index.php?m ... pg7u77rpe0
It is presently $10 plus freight and a very versatile electronic thermostat which can be used for cooling or heating with the range between on and off completely adjustable.
On onboard relay can switch 10 amps but recommended not for mains without a suitable secondary relay.
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