Quick n Dirty (cheap) Off Grid Freezer Set-up

Hi Mega Brain,

I'm hitting the woods in a few weeks and I want to bring my wee freezer. Its a lil danby I got second hand for 60$ a few years ago when I started buying meat farm direct. I keep it in my back shed, winter and summer. Screw the fridge, but I can't live without a freezer...especially since I will be in quarantine for 2 weeks after arriving at said woods. Won't be popping out to the farmers market on the weekend...gotta come prepared.

We will have a generator for tools (...mostly skillsaw, grinder, heavy duty drill...this is a building trip) but  I hate the idea of running it constantly.

I have a blue top 55Ah 12v battery from work..it was bought for a project, didn't get used, wasn't being maintained so I went on mission and took it to maintain (and use) until we need it at work again. Its been on a trickle charger the past 8 months and occasionally used with a cheap small inverter to run my internet stuff when the powers been out.


My idea was to upgrade the inverter, which I just did with another cheapish 1500w (3000w peak) modified sine wave inverter (just a motomaster...was on sale) and charge the battery off the generator whenever we're using tools using the same lil charger.


I've been testing it with the freezer the past couple days...here is what i've found.

-the inverter shuts down when the battery voltage drops below 12v, so no worry about over discharging.

-the inverter draw an unspecified amount somewhere under 2amps which is a lot of loss ! That's from the documentation which does not specify at what voltage that current is drawn and I can't easily measure it right now my contactless meter is at work, where covid prohibits me to go !

-the readout on the inverter reads 80watts when the freezer is actually running.

-I can run the freezer, with it doing its thing for about 6 hours on a full battery before the battery drop to just under 12v under the load

-It takes at least 7 hours on the wee charger to pump the battery back up to full.

At this rate, I will have to run the generator more than more than 1/2 of the time and won't be able to get through a full night without it IF i want this freezer to be a freezer.

Then I thought...

What if the inverter and freezer are ONLY on during when the freezer pump has to kick in? I estimate the duty cycle of the freezer to something like 10% actually running, 90% just chilling ( ) ...  The inverter came with a contact closure remote on/off switch on a phone cord. I was thinking I could wire a phone cord to a passive thermostat, inside the freezer somehow...but in reverse...so that everytime it goes OVER x degrees the thing would turn on as opposed to off (I assume the switch is a closed = ON open = OFF...need to look closer)

I'm betting that given the mad waste of the inverter itself, as well as whatever is consumed by the freezer in standby (not pumping) i could likely save a lot of energy this way, and gains would total least 30% of the battery charge which would get me through 8 ish hours, or a full night sans generator. I would hope that I can actually take it to 200% for 12 hours, and not have to run the battery down that low ever.

I'm also thinking that I could up the ante on the insulation. I could add 2" of foamboard inside...it would reduce capacity but there would definitely be gains in terms of performance.

What if i take that a step further and throw a bunch of hay bales around and on top the thing ? I know that these things should produce heat when they do their thing, but this one never seems to be warm on the outside.

If a combo of these ideas doesn't give me a 200% improvement in efficiency my next best option is probably to just buy a better battery but as per the title of the thread, quick n dirty's the game here, so that would not ne ideal right now. I also realize a few hundred watts of panels would go a long way to keeping the battery topped off as well, but that's more expense not just for panels, but charge controller etc. Maybe money is better spent on some kind of quicker charger for the battery, though it seems debatable whether a quicker charge is good for a lead acid battery (10A for instance - they exist though, and compared to another battery, they're pretty affordable).

any thoughts on this ?

anybody every do the thermostat in the freezer move before ?

thanks!

My understanding might be incomplete , but I don't think you will gain anything much by thermally switching the inverter.
I suppose the inverter/charger/ freezer might have some phantom load , but I don't eliminating that will add even an hour of run time.

From what I understand, chest freezers have their condensing coils under the exterior metal skin ,  and their evaporator coils under their interior plastic skin with a layer of insulation in between.
If so, adding insulation to the inside or outside of the fridge would be counterproductive.
Adding more insulation on the freezer  lid could  work, as  could  keeping the whole  apparatus cool via shade and/or evaporative cooling.

A fast charger seems pretty necessary.

A very small solar panel, the kind designed to be a trickle charger , could counter some of the daytime losses.

That is interesting... Did you allow the freezer to run for 24 hours so it stabilizes before taking your duty cycle reading? I think the phone relay idea might work not sure on that too many variables. Those cheap inverters are horribly innefficient at low draw. Does the freezer have an always on light on the switch? I find that is a huge dead draw over 24 hours. Next I would suggest you let the freezer coast by setting it to its lowest temperature setting, adding some water jugs to add mass run it off the generator all day and figure out how much it rises overnight. In that way you stay well within the freezing temperature and are in effect using the freezer as a thermal battery thus preserving your battery for other things... Add on a 125 watt panel and you could probably do just that and always have a charged battery and run it straight off the sun in daytime with no genny and no nighttime load at all. Just some ideas to play with...
Cheers,  David

Good comments above. I agree that, unfortunately, full sine inverters can be pretty wasteful in terms of standby power draw. Frustrating.

Regarding insulation, I agree with William. Adding insulation inside is counterproductive. Adding insulation outside while running (except a cap over the lid, where big losses are) will just make the fridge freezer run longer and work harder.

If there is empty space inside, though, a chunk of foam or blanket that sits on top of the food (like a second drop-in lid) will make a positive difference. Keeping it full is important, even if it's blocks of ice on top of everything else.

The thermostat idea is interesting, in theory. I don't know a source for a mechanical thermostat that clicks on at -10 C and off at -20 C. No doubt they exist, but it would take some digging. Call some heating/cooling contractors perhaps?

But here's the thing: a freezer doesn't have to be powered all the time. 30 minutes on, 90-120 minutes off would probably be quite safe. That makes me think about a 12 VDC timer wired into the inverter's on/off switch. I know those exist, though I have no source to recommend.

Reliability? Either way, you would be trusting the quality of the relay built into the inverter, which switches the main power feed on and off.

Luck!

Jay Peters wrote:Hi Mega Brain,

. Screw the fridge, but I can't live without a freezer...

Back in 2014, I spent 6mos offgrid and very broke in central Texas.

I had a 100w panel, 2 old RV batteries, a cheap,cheap,cheap 1000w MSW inverter and a $30 PWM solar controller.

After messing with different scenarios,  I put a timer on the freezer power cord and set it to turn off daily between 2200 and 0900.

It worked enough to get by.


SS

great thoughts all - thanks as ever.

The Freezer is currently on the medium setting, and has quite a bit of ballast in it right now. Its not being asked to freeze anything in these experiments, just to keep things frozen ! That would be the goal throughout I think. If I need to make Ice, that can be done on the genny while using tools.

Re: The insulation/physical design of the freezer. Thanks for the info - I always wondered with their not being any coils on the exterior like a fridge. Instead I will endeavor to just keep the thing in a cool shady spot, Maybe get some foam as an inside lid.

Re: Timer - that was my first thought, but I wasn't sure where to find a timer that would allow multiple short on/offs over the course of the day and run off 12v (easily and quickly - don't have much time to get this together). But I will investigate further.

Re: Thermostat I'll have to keep looking - maybe a there's a programmable 12v thermostat available that allows for N/C or N/O. Or a passive one that works in reverse. BRAINWAVE : I suppose I could always use a 12v Normally Closed relay to accomplish this, though that adds a small draw to the battery as well.

Something like this > Thermostat contact closes when temp drops below X (normally turning on a heating element > This will instead trigger the Relay to OPEN turning OFF the Inverter. When the temp rises above X, the opposite would be true. The trick is again finding a thermostat that goes into the negative temps


Re: Fast Charging I have an automotive charger that does up to 10 or 12A  - I will see how fast that charges the set-up and perhaps just use that instead if needed.

I do think it will be useful to have the Inverter shut down automatically (with an overide for lights or whatever in the evening - this would probably just take the form of me unplugging the phone cord *remote*) as it is certainly still drawing some current. Unfortunately I can't say how much right now without my meter. (Also, its a modified Sine Wave, so stepped, not a Pure Sine wave unit, so its a bit crappier, but also in theory a bit more efficient)

After writing this last night I had a look at some cheap solar stuff. Given that the load is light, maybe it would be worth it for me to buy a "cheap" package. If I'm going to spend more money I would rather do so on a panel/charge controller than a battery. When its time to buy batteries I will probably try to buy a a bank (where the on I have has already been used a bit it would risk unbalancing a bank and always draining first).

Comme ca :

https://addison-electronique.com/en/products/battery-s-chargers/ono-solar-module/

or piece something together ...

https://addison-electronique.com/en/products/battery-s-chargers/solar-charge-controller-solum-15-15e/

https://addison-electronique.com/en/products/battery-s-chargers/130-watts-polycrystalline-solar-panel-for-12v-system/


Next steps :
1) Try turning the thing off for a period of time and monitoring the Temp...I wish I had a temp stick for that ! (they're cool - I use them at work...thermometer/humidity reading that connects to a network and shows you analytics via web app)
2) See what the inverter is actually drawing with "no" load.
3) Test a "quick charge" set-up
4) Research Timers and Thermostats and relays

Would this be the kind of temp controller you needn

https://www.amazon.com/gp/aw/d/B019I3YCFS/ref=psdcmw_306533011_t1_B011VGAPOC

I have used a similar controller on my fridge to freezer conversion, which worked great,  except for the condensation.

That looks right, though the wiring diagram is a tad confusing looking like its connecting 12VDC power supply input to 110v supply for Heat/cooling - i'm assuming though  the outputs are simply relays and are rated for AC/DC up to 110v /10A.

If that's what it does, it would work fine. I wonder what IT draws ? Have to look more after work

First off, I'm the growie, Hubby's the electronics/electricity guy so much of what's above went in one eye and out the other.

But...Hubby has a little meat chicken business to get us farm tax status and due to all the regulations, we have to have pros inspect and process them to be legal to sell, so we have to do about 120 ~5lb chickens all at once and any that don't sell fresh in two days have to all go in the freezer at once.

So...he calculated the percentage of salt in water that would absorb the heat from the "cooled chickens" and thaw the salt water while freezing the chickens.

Why is this important - because most meats will thaw at a lower temperature than water. So putting extra "ice" in your freezer won't protect the meat, but putting old pop bottles full of brine will thaw before the meat protecting the meat from thawing. We figure if we have a power failure, our huge freezer is good for two days in moderate temperatures (it's in the shade).

The issue of "not putting insulation on the outside of a freezer body" is accurate *when* the freezer is running. But if you know it isn't going to run - no power - there's no reason not to have some insulating blankets of some sort ready to go on when you unplug the freezer from the generator. If you get one of those remote temperature sensors - either on a wire or electronic, we've used both - so you know what the temp is near the top inside of the freezer, you could start the generator, bring it up to temp, stop it, and cover the freezer with the insulated blanket and see how long it goes without needing to be run again. Once you've got some idea of the timing, you set up a schedule to deal with it. I wouldn't necessarily want to do this for my entire life, but if it's short term, it might be the cheapest short term solution.

If you're interested in trying some brine bottles, I'll ask Hubby what concentration he decided to use. This isn't going to change the total amount of energy required. It will just keep the meat safely cold with less frequent use of power to keep it cold.

Thanks Jay - that seems like a great strategy to extend the OFF portion of the duty cycle.

I wonder if where ocean water sits in terms of density as compared to the brine your using. I will have access to lots of that ! Either way, it sounds like water of any density greater than unsalted water would be a benefit, whether it crosses the threshold your speaking of or not (as compared to regular ice as ballast I mean)

Due to covid reality, mill breakdowns and fire warnings my trip is being postponed...

however I will still be trying to get this system together in preparation.

I've ordered one of these to do the thermostat control. https://www.amazon.ca/gp/product/B00G9HUKDA/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

i did a bit of research and it seems like the temp ranger is 0-47 f which should also make is suitable for running the freezer as a fridge if I wish.

also probably gonna add a solar panel and charge controller.

Sam Stephens wrote:

Jay Peters wrote:Hi Mega Brain,

. Screw the fridge, but I can't live without a freezer...

Back in 2014, I spent 6mos offgrid and very broke in central Texas.

I had a 100w panel, 2 old RV batteries, a cheap,cheap,cheap 1000w MSW inverter and a $30 PWM solar controller.

After messing with different scenarios,  I put a timer on the freezer power cord and set it to turn off daily between 2200 and 0900.

It worked enough to get by.


SS

Hi Sam,

That's a very good solution!
I run my apple fridge the same way in the summer-autumn.

It's a small fridge with a little freezer compartiment
I have the freezer compartiment filled with frozen water bottles. During the night everything is kept cool from the melting water bottles.
During daytime, the fridge runs on the excess solar electricity. (and freezes the water again)

For a pure freezer you could wire the 'superfrost' button in (or something like that) in Aduino/PIC microcontroller, or just into a timer.
So that you make an extended cooling run of the freezer (cold reserve), during the peak-hour of the solarday (when energy is excess/free/wasted anyway)
The excess cooling is then used to bridge the gap between the night and the next solar day.

What do you think of this idea?

Winter is a different problem however

Greetings,
Steven.

Hi Jay Peters;

Jay Angler's hubby here.  You have an interesting problem that has a lot of facets to it.  I have read the whole thread (to 22:00 PDT June 28 / 2020) and you have received some very valid advice and also a bit of advice that I am not so comfortable with.

If I was confronted with your problem I would try as much as possible to use thermal storage in some medium (in this case I think brine is the medium of choice) rather than electrical storage in a battery.  A brine thermal storage medium is inexpensive, probably the majority of the mass is readily available on site, and has an infinite cycle life.  A battery is expensive, is heavy and the full mass has to be transported to your site, and has a significantly limited cycle life.

My problem was to freeze 500 pounds of fresh chickens in a short period of time using a domestic type 25 cubic foot chest freezer.  The answer was to fill a large number of 710 mL. pop bottles (we don’t drink much pop, but these things aren't hard to find...) with an 18% brine solution and put them in the freezer a couple of weeks ahead of time and let the freezer freeze them solid.  On chicken day we put a layer of brine bottle into the freezer, then a layer of chickens, then a layer of brine bottles and repeat until all the chickens are in the freezer and any extra space in the freezer is full of brine bottles. (Making alternate layers like this spreads out the cooling power in the freezer and by shortening the path heat has to travel between chickens and brine bottles speeds up the freezing process.) An 18% brine solution freezes at about -14C and the chickens freeze at about -3C to -4C so there is about 10 to 11 Celsius degrees of temperature difference between the brine bottles and the chickens driving the heat from the chicken into the brine bottles. Using this method the freezer can freeze 500 pounds of chickens solid in 24 hours and at the end of that time the brine bottles are completely melted.  They can be removed from the freezer at this point but we just leave them in. The freezer runs a few days to re-freeze the brine bottles and then they act as power failure protection - and are ready to go for the next round of fresh chickens.

Your problem is closely related to my problem, though there are some differences.  The main one is that we were looking for a really hard freeze for long term storage of the chickens.  Efficiency was secondary as we are on grid power.  Also secondary was speed of freezing the brine bottles as we could take weeks to freeze them if need be.  For this reason I chose to set the freezer thermostat to the lowest possible temperature which seems to be about -20C to -21C.  The colder you keep your freezer the larger the temperature difference between inside and outside and thus there will be more heat infiltration into the freezer resulting in higher losses.  You might choose to have your freezer during the brine melting phase of operation at a slightly higher temperature when not powered (say ~ -10C) which will reduce these losses, but also require a lower brine percentage (See the brine table). To get the large heat absorption effect of the phase change of the brine from solid to liquid the freezer has to be able to freeze the brine solid.  You would do this when running the freezer on your generator for a few hours each day and the freezer thermostat should be set as cold as possible to maximize the temperature difference between the inner walls of the freezer (where the evaporator coils are) and the brine bottles so the brine bottles freeze as quickly as possible. Then, when the freezer is not powered the brine slowly melts at -10C (or whatever temperature you select) and holds the freezer temperature (and your food) at that temperature until the brine is completely melted.  The goal would be to have enough brine in your freezer so that all of the brine has not melted before you next run your generator.  (Using gasoline to run a generator that runs a battery charger to charge a battery then then discharges to power an inverter to run a freezer has a lot of energy conversion steps to the cycle and there are losses at every step along the way!  Bad news...)  Rather than layering the brine bottles with your food as we do I would suggest building an inner wall with them inside the freezer where they are as close to the evaporator coils as possible to speed up re-freezing when the freezer is powered by your generator and where they act as a barriers to capture infiltrating heat before it gets to your food when the freezer is unpowered.

A few notes on making the brine bottles:  Sea water is about 3.5% salt by weight and you will need a much higher percentage to make this scheme work.  I will paste in a brine table below that relates salt percentage to freezing temperature.  You have a few of choices to get the correct percentage:

1/  You could start with sea water and evaporate out some of the water to get to the correct salt concentration.
2/ You could start with sea water and add more salt to it to get the correct salt concentration.
3/ You could start with fresh water and add the required amount of salt.

Even though I have sea water available I chose option 3 as it is simpler to get the percentage correct. You just weigh the water, weigh the salt, and mix accordingly.  I used pickling salt as it was the least expensive salt readily available at the grocery store.  Ice melter salt would be cheaper if you can get it - but they don’t sell much of that in the Summer...  Since the pickling salt has large crystals you have to shake it up a few times to encourage it to dissolve and the process takes a few hours.  I also add a couple of drops of food colouring to the brine bottles for identification purposes as I also use pure water ice in pop bottles for other purposes and I want to be able to tell the ice and brine bottles apart.  I mostly use 710 mL bottles as they are a reasonable compromise between volume contained (heat absorption capability) and surface area (heat transfer capability) but you can use other sizes.  In your case, 2L. bottles might work better as you want high heat absorption capability at moderate heat transfer rates.  It would also be easier to build and contain them as an inner wall in your freezer as many 710mL bottles have irregular shapes while the body of most 2L bottles is a simple cylinder.  Do leave a little bit of head space in the bottle to allow for expansion and contraction.

+++++++++++++++++++++++   look at brine chart below   +++++++++++++++++  

      I have a few comments about other aspects of this problem:

1/  Using a large inverter to power a light load is very inefficient due to the parasitic losses in the inverter.  If you absolutely MUST power the freezer from a battery and an inverter for long periods of time (overnight) you might find it wise to buy a smaller inverter just for the freezer.  Chose one that is loaded to ~70% of its capacity and the whole setup will be more efficient - but make sure the inverter can handle the start-up surge of the freezer.  Better to run the freezer off the generator though and use thermal storage as mentioned above.  Fewer energy conversion steps = higher efficiency!

2/  Comments were made about NOT insulating the inside and outside walls of the freezer and the reasons stated are valid as this is where the evaporator and condenser coils are.  The reasons stated do not apply to the lid and floor of the freezer.  Insulating the heck out of them is possibly a good idea. Do be aware that if you do a good job of insulating the floor and lid of the freezer that may bring the outside surface of the floor or lid below the dew point of the surround air which could result in condensation.  This may promote rust and eventually damage the freezer.  The soluton is to insulate in such a way as to prevent ingress of moisture containing air.  Also, make sure the rubber lid gasket is in good shape.  If not, it can be a huge heat leak.

3/  I have used the temperature sensor inside the freezer trick in the past, but the plan was to run the freezer at refrigerator temperatures which are outside the range of the regular freezer thermostat.  This worked well except that the freezer has no way of dealing with the liquid condensation that collects on the walls and in the bottom.  Regular fridges run the evaporator cold enough to freeze out the condensation so it doesn't soak your food - and then you periodically have to either manually or automatically defrost the evaporator.

4/  I looked up the temperature controller suggested by William Bronson and it appears to be a nice 12 volt unit - which is a voltage you have available - but the only rating given in the specifications for the relay contacts is 10 amps at 12 volts DC.  There is no 120 volt AC rating so I would hesitate to use this controller to switch the 120 volt output of the inverter going to the freezer and I doubt that a 10 amp rating would be high enough to use the controller on the 12 volt input side of the inverter.  The internal relay may well have a rating for higher AC voltages but you would have to buy one of these controllers and take it apart and inspect the information on the relay itself to find out.  Keep in mind that AC voltage current rating on switching contacts are frequently higher than the DC voltage current ratings because the AC voltage crosses "0" twice a cycle which helps to extinguish the arc that happens with the contacts open.  With DC there is no help in extinguishing the arc so the current rating has to be lower. If you want to use this controller the safest alternative might be to use an external relay with a 12 volt coil and contacts sized appropriately for the load of the freezer.

5/  As you mentioned, one of things you would like to have this setup accomplish is to slow down the cycling of the freezer so it stays ON longer when it is running and stays OFF longer when it is not running.  There are 2 ways to accomplish this:

A/  Add a lot of thermal mass to the freezer.  The brine bottles will do this very well as they absorb and release large amounts of heat as they  melt and freeze.
B/  Increase the hysteresis of the thermostat controlling the freezer so there is a larger temperature change between when the freezer turns ON and when it turns OFF.  The internal freezer thermostat has a fixed hysteresis so if you want to change the hysteresis you will have to use an alternate thermostat or controller such as the one suggested by William.  Some external temperature controllers may have adjustable hysteresis and some may not.  It appears that the Inkbird controller may have this, though they call it "difference value".

6/  Make sure that whatever you do, that there are at least a few minutes between compressor starts to allow system pressure on both sides of the compressor to equalize.  Starting against a head of pressure is very hard on the compressor.

I hope this helps... Vince

Vince - that is a super rich response. Can I get an apple here ? Can I give an apple ? I don't know how that fully works but I know a lot more about this freezer equation now. Excellent.

Not that I will be doing this right now anyway, but given this info I think the best bet is to, as you say ditch the idea of running the freezer off the battery at all, this lightening the battery load greatly and keeping it free for other basics. Lights and devices mostly. Maybe by then Ill want a fridge haha...would be good to find a 12v unit though since they are fairly available out of old RV's and would be much more efficient.

I would also probably not bother with the smaller freezer and instead try to find a larger chest freezer nearby on craigslist, not transport my small one, and get as much mass in as possible. I would opt for the rectangular 2L milk jugs and line the whole of it. I've made brine at 5% before for fermentation purposes so that should be easy enough once I figure out the % i need considering the mass available and desired temperature.

On another, non-freezer related note. This project has evolved a bit since playing with the thermostat I bought. If/Once I get it to work I'll post about it but here's the rundown:

My generator is a firman with keyfob remote start/stop. I'm working on building a voltage sensing circuit to turn it on and off with an arduino and a 433Mhz transmitter board.

The goal would be to code it to check the voltage every minute or so. Once it drops below X (11.75v maybe) it would would make a start attempt and start rechecking the voltage more regularly to ensure the start was successful. If not, rinse and repeat a few times...not sure where to go if it doesn't work...let's get it functional first (Maybe rig it to an old cell phone to send me a text or otherwise warn me.) It would then go through an as yet undetermined duty cycle to charge up the battery. This could be time based or rely on the voltage sensor.

This may be completely unnecessary but I have the stuff, and winter is coming. I'll decode the keyfob with my SDR (software defined radio) and take it from there. Then once I have a solar panel integrated it will be an automated backup for when the sun isn't shining.

I was speaking to my buddy, a red seal electrician who specializes in solar installs in BC...there's a word for the commercial version of this unit...but I can't remember it.

Thanks again for all the great responses, and thanks Vince for clearly demonstrating how the thermal mass is really the most appropriate and likely efficient solution to the initial problem discussed.

cheers !
jay