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DIY: How to make your own 12V Fridge
#11
My thoughts

It takes some gear to hook up a compressor and such. Gauges, vacuum pump, brazing torch, and that's it you can just switch out the Compressor. Got the gear and knowledge and it might be a fun project for a donor mini fridge. I've seen 12v compressor kits for around $200 at some point.

My second thought is that by the time you get to the size 3.2 ft two door mini fridge, the 12v's can pull some power too. The Grape 12v, 5 ft upright is a pig. For a bit more than most two door minis the Edgestar 3.2 mini pulls .64a at 120v. Yes you need a inverter but for $200 more my Tripp-lite industrial inverter will run a lot more than the fridge and shuts itself off when not needed. That's the fridge and inverter for much less than a bigger 12v unit.

You will need to add insulation as the 12v's are better insulated. Even at that size they will be more efficient just for being a chest unit and may still use less power. The thing is by the time you have a system that can run a fridge of that size plus the rest of your van, the difference between the fridges and even the inverter losses are not going to matter. It should take days for the difference to make a noticeable impact on a bank big enough to dependably run either fridge.
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#12
(06-15-2016, 10:28 PM)jimindenver Wrote: My thoughts

It takes some gear to hook up a compressor and such. Gauges, vacuum pump, brazing torch, and that's it you can just switch out the Compressor. Got the gear and knowledge and it might be a fun project for a donor mini fridge. I've seen 12v compressor kits for around $200 at some point.
I've thought about this approach, but it isn't DIY friendly for 99% of people. I work with an HVAC tech that could evacuate the system, but I'm aiming more for a reasonably easy DIY solution. Definitely would be fun though

Quote:My second thought is that by the time you get to the size 3.2 ft two door mini fridge, the 12v's can pull some power too. The Grape 12v, 5 ft upright is a pig. For a bit more than most two door minis the Edgestar 3.2 mini pulls .64a at 120v. Yes you need a inverter but for $200 more my Tripp-lite industrial inverter will run a lot more than the fridge and shuts itself off when not needed. That's the fridge and inverter for much less than a bigger 12v unit.
I agree. I think that we are going to find that there really isn't a huge difference in efficiency between 12v and 120v fridges, with insulation probably being one of the bigger determining factors. The idea behind the dedicated inverter for the fridge is so that parasitic losses can be cut 100% when the compressor isn't running.

Quote:You will need to add insulation as the 12v's are better insulated. Even at that size they will be more efficient just for being a chest unit and may still use less power. The thing is by the time you have a system that can run a fridge of that size plus the rest of your van, the difference between the fridges and even the inverter losses are not going to matter. It should take days for the difference to make a noticeable impact on a bank big enough to dependably run either fridge.
Insulation is planned, but I want to do it after the first round of tests so that a comparison can be made before/after insulation
I agree, that the difference is going to be pretty minimal on any well designed 12 volt system - which just lends credence to the idea that doing a conversion like this can save a substantial amount of money over buying a 12 volt compressor fridge of similar size. I could easily put together a conversion like I'm proposing in this thread for under $200 with all new parts, and about half that if I shop around and use a cheaper inverter. To get a true 12 volt refrigerator of the same capacity easily goes into the $800-$1000 range. I'm hoping that I can publish actual figures so that others can judge for themselves which option may fit their needs.
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#13
Were I to convert anything, it would be a mini freezer. They are already better insulated. Also with the mini fridges you have to look for the draw. The same style two door pulls .8a if Galance and up to 1.5a with other brands. The edgestar is alone at .64a.
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#14
(06-15-2016, 11:19 PM)jimindenver Wrote: Were I to convert anything, it would be a mini freezer. They are already better insulated. Also with the mini fridges you have to look for the draw. The same style two door pulls .8a if Galance and up to 1.5a with other brands. The edgestar is alone at .64a.

The MagicChef 3.1 CuFt Refrigerator/Freezer combo I have draws between 0.48 and 0.51 amps @ 120VAC while running.  I was actually surprised with how low the draw was.  I'm sure there is a lot of variation between brands and depending on the age of the unit
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#15
That's great. Do you have a model number and what the tag on the back says the draw is?
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#16
I agree that brand of inverter is good quality. Did that small inverter handle the in-surge of current to start the compressor?
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#17
I'm keeping an eye on this one too. I have a friend who did something similar, but converted an oddball Norcold AC/DC fridge which had an AC "swing motor" compressor and 1970's era inverter. It worked well, but was an old unit. He removed the old inverter and wired in a current tech, albiet MSW inverter.

I really want to be able to swap out my gas fridge and just leave it on all the time so I can store my rig closer to where I use it. I have 390W solar and 225Ah of Battery. The prices for units like the Norcold DE0061 are just too high. I could do the compressor swap on a low cost magic chef, but then I have an oddball again. I'd rather just do the electrical conversion.

Thanks for your very detailed Posts. I like your paper cabinets.
-Zonie
Still Workin...
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#18
When comparing power use over time or duty cycles ambient temperature becomes a factor.

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

Wavy Gravy

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#19
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#20
Sorry - I've been away for a bit and been super busy with other areas of my build but wanted to update this post.

Over the last week I've done a ton of experiments.  Here are my current findings:

I have 3 inverters:
  1. 120 watt Samlex pure sine wave
  2. 300 watt Bestek modfied sine wave
  3. 1000 watt Bestek modified sine wave (this wil be the "main" power inverter for our build for a simple 120v system on the road
I also got a dedicated 12v power meter to run some tests.
[Image: IMG_20160621_162235_zpssbwjrbco.jpg]
The 120 watt Samlex draws 2.9 watts in standby state. Sadly, it won't even think about starting the fridge compressor.

[Image: IMG_20160621_162344_zpswwq4mfmb.jpg]
The 300 watt Bestek uses 3.5 watts in the standby state.  It came close, but can't quite start the compressor.

[Image: IMG_20160621_162751_zpsojqwcwyr.jpg]
The Bestek 1000 watt uses 8.2 watts in its standby state.  It very easily starts the compressor.


From the above, it's easy to see that the larger the inverter, the more power it will take to even have powered on.  There are also two facts that are readily apparent from my tests:
  1. It takes a large inverter to start the compressor
  2. Once the compresor is running, it takes very little power to sustain running of the motor (about 60 wats, or about 5 amps on a 12 volt system)
So here is the plan moving forward.  My next test will be using the 1000 watt inverter to start the compressor, and then switching over to a small, more efficient inverter to sustain runing fo the compresor.  Since my build is already going to use the 1000 watt inverter, this isn't realy an added component.  I will just have a remote way to trigger the inverter to come on just long enough to start the compresor, then switch to a small inverter to continue runing the compressor once it's started to keep power loss at a minimum.

Of course there is a balancing act here:
- Having both inverters live to the same line at the same time would be bad - at least one would most definitely blow, especially if the phase angles of each were 180 degrees apart.
- Too long of a pause during switching will allow the compresor motor to stop, rendering the entire technique useless
- Time will tell how much the compressor motor will like being switched live between two sources that will likely be at different phase angles.  In the wind industry, we do this all the time to coax an induction turbine in sync with the grid with no issues - but it's hard to say how such a small motor will react.  That surely won't stop me from experimenting to find out though!

This week I will build the switching mechanism and give it a try.  I'll report back with findings, good or bad.
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Oberneldon (06-23-2016)


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