How low can you go

billybob9 · May 2019

My Schumacher 6V load tester finally came in and I tested all my batteries. They were all in the Week area so I guess I'll have to spring for new ones. There' enough juice in them to cycle my small fridge one time . So for my emergency Corona cooling I rigged up a 1 panel 12 volt system for now. I drew it out with as much INFO as possible. It kept the fridge cool for 12 hours but crashed out quick when the sun went down..

Image: https://us.v-cdn.net/6024911/uploads/editor/ap/u97ykta6hako.jpg

Estragon · May 2019

Is there room in the fridge to add thermal mass (freezer packs, frozen water bottles, etc)? When starting out with the cabin project, I used a cooler with such items frozen during the day and put in the cooler overnight, and it works to keep stuff cold pretty well.

I also threw a couple of winter jackets over the cooler overnight, which also helped.

I have a small portable DC fridge which doesn't take a ton of running watts, but seems to have a nearly constant duty cycle, so overnight kwh are actually quite high. Okay to run during the day with solar, but prohibitive to run overnight on battery power.

billybob9 · May 2019

I just checked the temperature gauge in the fridge at (7:30 AM ) . It read 43 degrees F but this temp is just air temp and not items that have mass. The Liquid items are colder and the Ice trays in the small freezer compartment that were ice have melted but not by much. So I would say the liquid temp is about 40 degrees and still in the safe zone. Yes, I do have blue ice packs and lots of liquid in containers. The ambient temp last night was 60 degrees F so that helped. I'm going to get a thermometer to put in a glass of water to get an actual reading. I'd be very interested in an actual test from someone else with real numbers.

Estragon · May 2019

Not sure if it's what you're looking for, but I have a ~4cu.ft. site built, heavily (4-6" rigid foam) insulated chest box with danfoss-based DC compressor and evaporator. It can be set to fridge or freezer temp, but I generally use it as a freezer (~ -18°C). I can turn it off, and it will take 3+ days to get to >0°C. A cement board and tile interior gives a fair bit of thermal mass even with little/no food or freezer packs.

A similar sized fridge on my boat, but with much less (maybe 1") insulation and acrylic interior will warm to near room temp in less than a day.

Both use the same type compressor. The cabin box uses ~200-250wh/day at freezer temp. The boat has a small freezer shoebox shaped evaporator cooling the main fridge box, and takes 1200wh/day at mostly fridge temp.

BB. · May 2019

I fear that your single 100 Watt solar panel is not enough to run the fridge and properly run the battery bank... If the high voltage on the charge controller (battery bus voltage) is 12.9 volts during the middle of the day, those batteries are not nearly fully charged.

Just to give you an idea of what the system power would need to be (pure guesstimates):

48 watts fridge * 1/0.85 AC inverter eff = 56 Watts DC input
guessing a 1,000 Watt inverter takes 15 watts just "powered on" = 15 Watts
56 W + 15 W = 71 Watt load 24 hours a day

Sizing the battery bank:

71 Watts * 24 hours per day * 2 days of storage * 1/12 volts * 1/0.50 max discharge = 568 AH @ 12 volt battery bank

Sizing the solar array. First based on 10% rate of charge (5% to 13% typical, 10% nominal full time off grid):

568 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,070 Watt array

If this is an "emergency power system", you could cut the battery bank AH by 1/2, and that would cut the array by 1/2 too... Or even cut the array by another 1/2 if you are only running the fridge a couple days a week).

Then there is sizing the array based on your average hours of sun per day... Fixed array in San Diego:
http://www.solarelectricityhandbook.com/solar-irradiance.html

San Diego
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical :
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
5.00	5.49	6.06	6.50	5.91	5.42
Jul	Aug	Sep	Oct	Nov	Dec
5.66	5.96	5.79	5.39	5.32	4.89

Say worst case sun is December (and you may need a genset during bad weather):

71 Watts * 24 hours per day * 1/0.52 AC system end to end eff * 1/4.89 hours of sun = 670 Watt array minimum

Anyway, lots of guesses here--But should give you some idea of what is important and what changes you can make to better meet your needs.

-Bill

billybob9 · May 2019

I could try to get my fridge closer to 32 F ( 0 C ) but would not want to crystalize at that temp so 34 F would be max. If you could do it with a freezer without freezing that might work. Once you freeze and thaw then freeze and thaw again it's a goner. I don't have any produce in my small fridge but if an emergency came up where I had no choice I would like to know what I could get away with and for how long.

billybob9 · May 2019

Bill

Yes. you are so right and guessed my local pretty good Thanks. This was just a test and since my batteries were shot I was just running the fridge without charging the battery much. The cycle for the fridge is 15 minutes on and 45 minutes off. So this was like having a totally dead battery in my car and giving it a push, Then popping the clutch to let the generator get me to where I wanted to go. Thanks and if anybody wanted to write the book " How low can you go " it would be you and an expert on food spoilage.

BB. · May 2019

Just one other rule of thumb... For a 12 volt FLA battery bank, we would be suggesting +400 AH @ 12 volts for a 1,000 Watt inverter running a standard compressor refrigerator... (and 200 AH @ 24 volts, or 100 AH @ 48 volt battery bank per 1,000 Watts of AC inverter or even 1,000 Watts of solar panels--Minimum for starting loads, and typical cost effective maximum for solar panels).

And, generally, we suggest a 1,200 to 1,500 Watt minimum AC inverter to run a standard refrigerator + a few other things around the house (LED lights, cell phone and laptop charger, etc.).

-Bill

billybob9 · May 2019

Bill

Thanks for the "We Suggest" INFO I really do appreciate it and I have that system already. A 24V system , Batteries 940 AH, 1500W Inverter, MPPT 40 amp controller ( new ) and 640W panels. I do run a full size fridge with this system and use a transfer switch to AC at night. This is a system that you would see on a boat. Even the VHF. Bottom shelf batteries, Next shelf Inverter and wiring lugs, Next shelf 12 volt stuff and on

Image: https://us.v-cdn.net/6024911/uploads/editor/nj/0122x7sz7bmy.jpg

top controller. BG

paulcheung · July 2019

What batteries make up that battery bank? Just curious.

billybob9 · July 2019

Those are Interstate golf cart batteries 224 AH that I got at Costco for $75 ea. They are now 5 years old and maybe I can get another year out of them. A good deal but now these are $93 and the AH is 210.

BB. · July 2019

My concern... Large/heavy electrical power sources (aka batteries and inverter and charger) in a plastic structure/enclosure.

I would suggest you look at "something else". From metal cabinets to wood+sheetrock/cement board for fireproofing.

One of the big issues with plastics (and to a large degrees even metal--i.e., steel loses most of its strength >~450F) structures is that they get "warm" and fail if you have problems... I fear that collapse of your battery and wiring support fails and causes more issues like short circuits. And with plastics, you got a good amount of fuel that is not too difficult to light off.

I want to keep you and your family safe.

-Bill

paulcheung · July 2019

billybob9 said:

Those are Interstate golf cart batteries 224 AH that I got at Costco for $75 ea. They are now 5 years old and maybe I can get another year out of them. A good deal but now these are $93 and the AH is 210.

When I read your post say 940 AH battery bank. I am wondering how you charge them with the panels you have. since they are only 224 AH. that is fine.
Cheers

stmoloud · July 2019

billybob9 said:

I could try to get my fridge closer to 32 F ( 0 C ) but would not want to crystalize at that temp so 34 F would be max. If you could do it with a freezer without freezing that might work. Once you freeze and thaw then freeze and thaw again it's a goner. I don't have any produce in my small fridge but if an emergency came up where I had no choice I would like to know what I could get away with and for how long.

I'm not absolutely sure about the freeze-thaw-freeze-goner trope. I need a couple more batteries and probably another 600 watt to run my fridge freezer 24/7 if I get a string of 2 or more really cloudy days. So yes I've been down that freeze thaw route a number of times which means thawed out burgers and dairy at room temp sometimes for a couple of days. Frozen veges are acceptable, although not as good as they were intended. And I've never been sick as a result. Mind you our winters here will only get to a max of 63 F, and that will only be for maybe max 2 hours a day

How low can you go

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San DiegoAverage Solar Insolation figures

San Diego
Average Solar Insolation figures