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DIY: How to make your own 12V Fridge
#1
I'm breaking my current "pseudo 12 volt fridge" discussion to this new thread from my build log ( ViaVacavi Build Log )

The purpose of this thread is to discuss DIY fridge conversions, essentially converting inexpensive and readily available mini refrigerators designed for 120 VAC use, to run off of our 12 VDC electrical systems as efficiently as possible. I would also like to gather information on energy use of popular 12 volt fridges for a comparison.  I will detail cost breakdowns, energy use and efficiency and details to do your own conversion.

Why would someone want to convert a 120 volt fridge to run on 12 volt?
  • Cost.  Using all new high end parts, a converted fridge can be made for around 1/4 the cost of a similarly sized 12 volt fridge.  Using used, free and cheaper parts, a converted fridge can easily be made for less than a tenth the cost of a true 12 volt fridge of similar size.
  • Layout.  Some people prefer a vertical layout vs. the standard "cooler" layout of most 12 volt fridges.
  • Size.  Many mini fridges are available in the 3 to 4 cubic foot range - this is equivalent to a 90 to 120 quart 12 volt fridge in size.
  • Separate Freezer.  There are many mini fridges with separate freezer compartments.  Only a few 12 volt refrigerators have this option and they are at the high end of the price range.
To be fair, there are also some drawbacks:
  • A "pseudo conversion" will nearly always be at least a little less efficient than a true 12 volt fridge.
  • There is a bit of added complexity.  There will be added components which could fail
  • You must be at least a little familiar with wiring
  • Nobody really knows anything about longevity or reliability, nor how well a converted fridge will stand up to life on the road.
Stay tuned!
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#2
Why not just get an inverter, and run your 120 volt fridge off of it from your battery bank?

This certainly will work, and many people have done it.  However, it is the least efficient option.
Inverters are not 100% efficient, so power will be lost during the conversion from 12 volts DC to 120 volts AC.  This power loss is present whether the compressor in the fridge is running or not.  On most fridges, the compressor only has to run roughly 25% of the time (or even less), so it makes no sense to suffer this power loss 24 hours a day.  On a positive note, no special skills are needed and you just plug the refrigerator into the inverter at one end, and plug the inverter to a 12 volt socket at the other end.

Here is an example of the power needs for such a setup. 
This example will assume a fridge that uses 60 watts while running, where the compressor runs 25% of the time.  This will also assume that the inverter consumes 0.5 amps just being plugged in. (Of course in the real world, there are many factors that will influence efficiency)
  • Running 6 hours per day, the compressor will consume 30 amp-hours from your battery bank (60 watts divided by 12 volts = 5 amps, multiplied by 6 hours equals 30 amp-hours)
  • Running 24 hours per day, the inverter will consume 12 amp-hours from your battery bank (0.5 amps multiplied by 24 hours equals 12 amp-hours)
  • This adds up to 42 amp-hours per day to run the fridge
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#3
Conversion Plan

I still like to call this a "pseudo-12 volt conversion", as it will still use the 12o volt compressor of the refrigerator.
The plan is to convert the control system to 12 volts, and use that to turn an inverter on and off ONLY when the refrigerator calls for the compressor to run.  To explain this a little better, I've made a couple simple diagrams of a standard mini fridge

[Image: 120v%20fridge_zpsiqg5ljc7.jpg]
Above is a typical diagram of a simple mini fridge.  As you can see, the thermostat of the fridge is a simple switch mechanism that closes to turn on the 120 volt compressor whenever the internal temperature goes above a certain value.  Most refrigerators have a "coldness" dial that allows this temperature value to be changed.


[Image: 12v%20fridge_zpsmuztnprr.jpg~original]
Above is the proposed conversion, which I will complete on my own mini-fridge.  While I changed the color of the wires, you should notice that the main control loop is pretty much the same, except that it is now being run on 12 volts DC.  Since the thermostat is a simple switch, it really doesn't care whether the electricity is AC or DC - it will simply open or close as designed based on temperature.
When the thermostat closes, it will energize a simple relay, which will send 12 volts to a small and efficient inverter, which will then run the compressor.  Aside from that, the light bulb may also need changed to a 12 volt bulb, but most incandescent bulbs work fine on DC. Another thing to point out is that the switching mechanism in the thermostat has a limited current capacity, and in the case of my mini-fridge handles about 0.5 amps.  Since this will equate to about 5 amps now on a DC circuit if we were to try to use the thermostat switch to directly power the inverter, we use a simple automotive style relay to do the switching to avoid damaging the thermostat.

The advantage to this conversion over just plugging the fridge into an inverter, is that the parasitic losses from the inverter will ONLY be present when the compressor is actually running, which should be 6 hours or less per day.

For an example (and still theoretical), now consider the following:
  • Running 6 hours per day, the compressor will consume 30 amp-hours from your battery bank (60 watts divided by 12 volts = 5 amps, multiplied by 6 hours equals 30 amp-hours)
  • Running only 6 hours per day now, the inverter will consume 3 amp-hours from your battery bank (0.5 amps multiplied by 6 hours equals 3 amp-hours)
  • This adds up to 33 amp-hours per day to run the fridge.  This is nearly 30% more efficient than just plugging the fridge into an inverter.  It is also getting very close to numbers reported by users of true 12 volt fridges of the same size.
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#4
Have you tried starting an inverter with a motor connected directly like that? Might need a time delay relay between them to let the inverter charge up before applying the load. I have no experience with this though.

Guy
"We're all bozos on the bus, so might as well sit back and enjoy the ride."

Wavy Gravy

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#5
Starting Measurements Before Conversion

We've had our mini-fridge for about a week and a half now, and it has performed flawlessly on the 120 volt shore power available at the campground.  We have been monitoring power usage using a kill a watt meter to get a starting point so we can compare energy usage after the conversion.

Here are some facts about this fridge:
  • This is a Magic Chef 3.1 CuFt. Refrigerator/Freezer combo
  • The unit draws exactly 60 watts while the compressor is running as reported by the kill a watt meter
  • The compressor currently runs anywhere from 9 to 15 minutes per hour depending on ambient temperature




[Image: IMG_20160612_190058_kindlephoto-35915790...g~original]
Power usage has been very consistent day to day.  From the above picture, you can see that the power usage had been measured for 81 hours and 15 minutes (81.25 hours in decimal)

[Image: IMG_20160612_190033_kindlephoto-35925034...bswfcg.jpg]
During this 81.25 hours, the fridge consumed 1320 watt-hours of power.
1320/81.25=~16.25 watt-hours used each hour. 
16.25 * 24 = 390 watt-hours used per day
If this were on a 12 volt system at 100% efficiency, that would equate to about 32.5 amp-hours per day usage out of a 12 volt battery bank, which wouldn't bad at all for a refrigerator this size.
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#6
For this test conversion, I picked the following inverter:
https://www.amazon.com/gp/product/B00IXOWPQM/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

(note to admin: please feel free to replace this link with Bob's own affiliate link and delete this text)
The inverter cost me $101, and is rated at 120 watts. It is a Samlex PST-120-12 and is a pure sine wave inverter.  Please note that a much cheaper modified sine wave inverter could be used, but I want this to be a best-case scenario test.

Here are the reasons why I chose this inverter:
  • Very high quality and reputable manufacturer
  • 2 year warranty
  • Nice and small size
  • Very efficient
  • Pure sine wave
  • Power rating closely matches what is needed for the fridge.
  • Has high quality Anderson power pole connectors

[Image: IMG_20160615_131358_zps1h4knek6.jpg]
Here is the inverter right after being unboxed.  I placed a pen near it for size comparison.

[Image: IMG_20160615_131413_zpsthjm2j8q.jpg]
The power plug is very high quality, with the power pole connectors on one side and a standard 12 volt plug on the other.  For permanent installation, it will be easy to cut the 12v plug off of the end and hard wire to a 12 volt source.


Tomorrow I will do the actual conversion and take pictures during the process.
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#7
(06-15-2016, 08:21 PM)gsfish Wrote: Have you tried starting an inverter with a motor connected directly like that? Might need a time delay relay between them to let the inverter charge up before applying the load. I have no experience with this though.

Guy

No I have not, but I already have a time delay relay should I need it.  Part of the testing is going to be how the inverter handles starting up loaded, and use the time-delay relay if necessary.  Here is a link to the time-delay relay I will be using if it is determined that it's necessary:
https://www.amazon.com/gp/product/B0126WT5QO/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
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#8
I plugged in my kill a watt meter onto my small 120 volt fridge today, and I will run it for 4 days or 96 hrs.
I will then put that same meter on to my Engel 12 volt compressor fridge for the same amount of time, as it is currently hooked up to 120 volts.
Then I will run the Engel off of 12 volts, and use one of my RC watt meters for the 96 hours and get the results based on 12 volt input.

I'll report back when done.
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skyl4rk (06-16-2016), ViaVacavi (06-15-2016), highdesertranger (06-15-2016)
#9
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#10
(06-15-2016, 09:28 PM)ahh_me2 Wrote: I plugged in my kill a watt meter onto my small 120 volt fridge today, and I will run it for 4 days or 96 hrs.
I will then put that same meter on to my Engel 12 volt compressor fridge for the same amount of time, as it is currently hooked up to 120 volts.
Then I will run the Engel off of 12 volts, and use one of my RC watt meters for the 96 hours and get the results based on 12 volt input.

I'll report back when done.

Thanks!

What are the capacities of the two fridges?

I'm thinking we need a metric that takes capacity into consideration, as a smaller refrigerator is understandably going to use less power - both due to a smaller compressor and less surface area to exchange heat with the surroundings. How about watt-hours/day/CuFt? That will allow reasonable comparisons between different capacity refrigerators. Thoughts?
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