Digital Thermostat for camping fridge.

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Digital Thermostat for camping fridge.

Postby offgridQLD » Thu Aug 09, 2012 3:48 pm

I have been using a 2nd gen Engel 32lt refrigerator when camping. It's efficient but I feel some improvements could be made to make it even better. To adjust temperature you have a simple dial knob at the top with numbers 0 - 5 . I find setting 2 is a good for refrigerator (0 - 4 deg C) setting 4 or 5 is if I want to use it as a freezer (-12 - 20deg c).
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In general this works fine but I do find there isn't any compensation for ambient temperature. For example on setting 2 I get between 0- 4 deg during the day but over night when the ambient temp drops I end up with 1/2 frozen milk. But If I was to set the nob at 1 or 1.5 and its hot during the day it might not be enough to keep things below 4 deg. Anyhow its manageable but you have to keep an check on it and fine tune the knob to make sure I am not over cooling then under cooling depending on ambient temp. This has to be wasting some energy to. I don't think the 0 - 5 settings on the nob relate to any actual thermostat setting or if the unit actually has one at all. Perhaps just a compressor cycle time ratio every hr or something like that.

The digital readout is nothing more that a simple a battery powered temperature gauge showing inside cabinet and has no control over the motor.



Just recently Engel came out with a 50th anniversary model fridge. It has all same features specs of the normal model other than the fancy gold paint. There was one notable addition..... (digital temperature control) I feel this is a valuable upgrade. Set the thermostat at 3 deg C and it will maintain it :D

I wont take credit for the following retrofit upgrade Idea as I did see an example selling online.

If I was to purchase a 12v digital thermostat unit . One with inbuilt relay trigger output. Then wire the 12v relays output to my refrigerator and place the temp probe in the fridge. Effectively interrupting the refrigerators 12v power cord. By setting the fridges knob to setting 5 (run compressor 100% of time) I would then Have digital control of temp and compressor cycling via the digital thermostat.

I can purchase the unit temp control unit for $15 online (That one is very popular unit and received a great review in a automotive magazine) .
http://www.ebay.com.au/itm/DC-12V-LCD-Digital-Thermostat-Temperature-Control-Regulator-Controller-Aquarium-/290686773509?pt=AU_B_I_Electrical_Test_Equipment&hash=item43ae482905

The unit is programmable for set point delay's so the compressor doesn't cycle on,off,on,off around the set point.

Below is a link to the guy who is selling the units marketed towards eagle fridge users for $120 programmed and set in a nice component box with two plugs. I'm sure the components would be lucky to cost me $35 all up but I'm not 100% sure how much work he has put into programing the unit to cycle correctly - efficiently.

Link to ready made units being sold
http://www.ebay.com.au/itm/Digital-Temperature-Control-Unit-12-volt-suit-Engel-Waeco-Camping-Fridge-Plug-CO-/221089974224?pt=AU_Car_Parts_Accessories&hash=item3379fd47d0

Anyhow I thought it might be worth me giving it a crack at building my own if I can save $90. I'm planing a 10,000km trip soon and hope to spend more time having fun than fiddling with the knobs to avoid frozen milk :D .

Kurt
Offgrid 2008, Selectronic PS1 6kw/48v inverter, x 2 Midnite solar classic 150 MPPT, 3960w NE PV 24 x 165w BP panels, 4200w NW PV 21 x 200w DAQO PV, 16x400ah lithium.
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Re: Digital Thermostat for camping fridge.

Postby debe » Thu Aug 09, 2012 10:54 pm

Have used one simmilar to the $16 one on Ebay on a Evakool fridge. You just program them to the temp you want & the temp diferential (difference betwean cut in cut out). I fitted mine in a small plasic box from Jaycar. Works a lot beter than the mecanical thermostat. For reliability use an external automotive relay if you are switching the 12V dc. The internal relay is only rated at 5A @ 220V AC, which is not 5A & 12VDCV. In the Evacool the switching current on the compressor control is only a few milliamps.
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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Thu Aug 09, 2012 11:38 pm

Thanks for that. Yes I was intending to Just have it switching a 10 or 20a relay. But now you mention it I could just have it switch the Engels mechanical thermostat.

I really don't think the knob is even a mechanical thermostat in a true sense . I don't think it relates to any set temperature or uses temperature as a reference , even mechanically. I don't see any area on the inside of the engel for pick to a mechanical temp sensor. I haven't played with to many mechanical thermostats. Aren't they usually just a bimetallic strips or , wax pellets or bulbs filled with some kind of gas. I could be wrong but I get the feeling the knob on the engel is just adjusting the run (on) time % . Say knob set on 7 is 100% run time. knob on 3.5 is 50% cycle time. And it just happens so that knob on 2 is a reasonable run time to maintain fridge temps but its a bit hit and miss -1 ..5deg c anything above 2 is starting to freeze.

Kurt
Offgrid 2008, Selectronic PS1 6kw/48v inverter, x 2 Midnite solar classic 150 MPPT, 3960w NE PV 24 x 165w BP panels, 4200w NW PV 21 x 200w DAQO PV, 16x400ah lithium.
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Re: Digital Thermostat for camping fridge.

Postby debe » Thu Aug 09, 2012 11:51 pm

Proper mechanical thermostats have a capiliary tube which is usualy atached to the evaporator plate.
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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Fri Aug 10, 2012 9:15 am

I will have a good look and see what I can find inside. I have ordered two of the $16 units and will pop into Jaycar on the weekend for some component boxes.The ready made units that are for sale on Ebay just use two merit sockets (in - out) I prefer the genuine engel plugs that actually screw in for a more secure connection.

I plan on running two fridges this trip 32lt as a fridge and my 40lt as a freezer.(wife loves cooking) Going to do some testing this weekend with a watt meter over 24hrs on both units and see what my 24hr AH consumption is. My guess is the fridge will use around 25ah and the freezer around 40ah. Total 65ah. I have two 100ah agms so a total 200ah of battery and 120w of solar but I am tempted to mount another 120w considering how cheep they are now .

Will be interesting to see what i come up with as most online tests i could find tend to be a little bias or tweaked to the particular manufacturer that is publishing them

Kurt
Offgrid 2008, Selectronic PS1 6kw/48v inverter, x 2 Midnite solar classic 150 MPPT, 3960w NE PV 24 x 165w BP panels, 4200w NW PV 21 x 200w DAQO PV, 16x400ah lithium.
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Re: Digital Thermostat for camping fridge.

Postby debe » Fri Aug 10, 2012 10:42 am

Be interesting to see how it all works out.
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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Sat Aug 11, 2012 11:18 am

Just started testing. 200ah agm being charged from 4ah charger. 32lt fridge running through watt meter. Fridge at setting 5 - 100% no post cooling. First hr temp dropping rapidly and pump running 100% . Interesting that its only consuming 25w - 2A , specs for the fridge list 2.5A my guess is that is when the fan is also running. fan only comes on when things get real hot in your car or you have mad ventilation. So it doesn't come on much at all.

I have a 2lt ice cream and a tub of water in there. It will be interesting to see tomorrow if the compressor runs 100% or cycles a little . Even at 100% 48ah isn't bad for freezer.

Kurt
Offgrid 2008, Selectronic PS1 6kw/48v inverter, x 2 Midnite solar classic 150 MPPT, 3960w NE PV 24 x 165w BP panels, 4200w NW PV 21 x 200w DAQO PV, 16x400ah lithium.
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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Thu Aug 16, 2012 2:04 pm

Test results.

Battery was being charged while test was conducted and held over 12.6v through most of the test.

Daytime temp 27 deg C night time temp 12 deg C

Freezer. Setting five average temp -19.5 deg c over 48hrs = 43.8ah

Refrigerator. Setting two average temp 1 deg = 14.9 ah


Interesting on setting two the consumption is way lower than expected. Setting 5 I found the compressor was consuming around 2A but as the fridge reached -19 deg C the compressor consumed around 2.3A with around 80% run time.

All up I can run 32lt fridge and 40lt freezer for just under 60AH each day . I am guessing conservatively my 120w panel will produce around 30ah each day . At this stage I am short 30ah but driving each day for a minimum of 3hr and some days 10hrs with a 90A alternator things should work out ok. Though another 120w panel would be perfect.

I will test again with digital thermostat's connected to compare.

Kurt
Offgrid 2008, Selectronic PS1 6kw/48v inverter, x 2 Midnite solar classic 150 MPPT, 3960w NE PV 24 x 165w BP panels, 4200w NW PV 21 x 200w DAQO PV, 16x400ah lithium.
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Re: Digital Thermostat for camping fridge.

Postby offgridQLD » Mon Sep 10, 2012 6:45 pm

My digital temperature controllers came in the post today . i have wired one up to control a relay that provides power to my Engel fridge.

I am playing with the setting now. First off I wanted to get some idea of how calibrated the unit was (It's user adjustable calibration)I didn't have anything to calibrate it with. So I sat the probe under my tong and got 37 dec C. I then boiled water and got 100 - 103 C and with the probe sitting on a block of ice it was showing - 0.5 deg C . So for now I will say its close enough for what I want.

I just need to set the hysteresis or the difference in temp for switch on and switch off.

I have set the hysteresis at +3 deg C difference so when set in fridge mode at +4 deg C trigger point the compressor will run until the the temp reaches + 1 deg then turn off.

Is this a good starting point or should I go lower 0 deg c ?

Kurt
Last edited by offgridQLD on Mon Sep 10, 2012 8:28 pm, edited 1 time in total.
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Re: Digital Thermostat for camping fridge.

Postby Gordon-Loomberah » Mon Sep 10, 2012 7:28 pm

I reckon 0C is a bit risky if you have salad type foods in it- lettuce doesn't do too well at freezing temps!
Maybe 2-5C would be a safer range, especially in hotter weather when the heat gain through the walls is greater and the temperature at the thermostat may be a bit higher than closer to the evaporator coils, or whatever they use in Engels. My old egg and beer fridge sometimes freezes eggs in hotter weather, even when turned to its warmest setting.
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