Running and old refrigerator off battery power, and solar charging the stuff project investigation

Soo here I am investigating for another potential future project.
The same good ol' friend which decided to let me overhaul his 24CV (chinese volts LoL!) drill hinted me at this stuff.
Lets say he has an ooold fridge in a house in the country. No AC power, had some once lent from a neighbour. He asked me to investigate about a sort of solar panel powered charging system feeding a stationary battery from which the fridge could be run at least 4 hours. Modern fridges can do with little above 1000W/day, but that fridge is old (and more than probably thirsty like an old clunker lmao) and it's gonna start from ambient temperature each time someone gets there and decides to keep the beers & lunch cool. Thus, I guesstimate the battery should at least have 500+Wh of useable power. Considering I'd program the charging system to charge the cells at a little above 3.9V/cell (≈⅔ SoC, minimum battery stress), which would also allow to have extra power for days of additional party time (raising the voltage/SoC) and/or a little use of other appliances, let's say we're aiming for a big pack, ≈900-1000Wh.
24V inverters have cutoff voltages at 20-21V; that fits well for a 7S li-ion battery. Could do with 49-56 LiitoKala 26650-50A/B cells, 98-112 Samsung ICR18650-26Fs, any combination of those two… well, I haven't investigated other possibilities much, but the aforementioned looks great and is affordable.
Optima Yellow? Comparatively expensive I believe.

Not yet evaluated how the charging would be done. Of course, through buck/boost CC/CV modules. Pretty philistine with regards to which solar panels and what wiring.
Mmmkay, that's pretty much everything for the time being.
Oh! The inverter! Briefly took a peek at it in Aliexpress a while ago, and the amount of power inverter offers is just insane. However, this stuff drew my attention:

Looks great, imho. 8^)

Suggestions and whatever useful welcome, of course.

Cheers my dears ^:)

Originally posted on Mon, 02/06/2017 - 06:03; lil fixup.

Take a look at real wattage per square meter of solar cell per day. It can be done but it’s a power hog nightmare.

I’m not a solar power expert by any means but I have tinkered with it and running a fridge off of battery power.
I would not recommend trying to run the fridge directly off of solar power. But rather by batteries that are augmented and recharged by solar power

I have a 22CuFt fridge that I ran during the last hurricane outage off of a 800Ah Marine battery and a true sine wave inverter.

Now I have a more sophisticated solar power system with 1000watts of power available, and a Prowatt inverter to convert DC to AC

Don’t forget that inverters aren’t terribly efficient so you have to accommodate for that loss.

Assuming your old fridge can run off of 150watts, that would require 51amp hours per four hours…(we’re ignoring start up for the moment)
Solar Power Calculations

Startup is the only problem. You have to have about 800watts available at startup for “modern” refrigerators but once running they only consume about 100-200watts. For an older fridge, I dunno….I used a Kill-A-Watt device and measured the max startup wattage it needs and also the running wattage. But once down to temp, they only run every few hours if the insulation and seals are good.

From experience, you need about 1000Ah of battery power available to power the fridge for 24 hours without running into any startup problems even though you will not use all that but you need it for start up surge. I highly recommend a minimum of two, 700Ah batteries for a good margin of safety during start up surge.

A good 100watt solar panel can provide about 6amps. So 300watts of solar panels should provide about 18amps for charging per hour.
That would potentially put 108Amps back into the batteries.

So for solar charging, at least 300watts of solar power for at least 6 hours to replenish the batteries. Obviously, more panels will recharge the batteries in less time but 6 hours would be what I would call the most you can safely expect on a given day with clouds passing etc. Often less.

I think the questions of when the solar panels will be engaged, and how long they can/should stay engaged, need to be answered. That will help determine how much power the solar panels need to be able to generate. If they will be engaged all the time, but power draw will be short bursts of time with long periods in between, the panels can be much smaller in order to (re)charge the batteries at a slower rate. If the solar panels will be somehow disengaged most of the time, only to be engaged when the fridge needs power, they’ll need to be able to keep up with demand. I agree with this recommendation from Zebretta:

Leaving the (much smaller) solar panels engaged all the time will be easier anyway, since dis-engaging them means they will have to be disconnected from the batteries and either covered or put into a very dark room so that they don’t produce any electricity.

Additionally, if your friend is really only going to need the fridge to be cold for no more than 4 hours at a time, then the fridge doesn’t need to be active for that long either. It can run for 2 or 3 hours, then shut off. It’ll stay plenty cold inside for another hour (or two, maybe). If you hook everything up with a switch to turn it all on, the switch can be a timer, set for the appropriate length of time. Then, your friend doesn’t have to remember to turn anything off at the end of his drinking binge. :wink:

With these two considerations in mind, your system can be planned in such a way to be much smaller (cheaper) and still plenty effective for the purpose. I’d say too, if your friend is going to want lights to work off this system, I’d recommend fixing up some LED lights for him that can run from the battery side, instead of through the (lossy) inverter. That way, the inverter only needs to run while the fridge is actively running, saving probably a fair bit of residual drain. Radios can run off of batteries as well.

Zebretta, I was already stating my intention to assemble a big battery pack above in the OP.

Your battery figures, on the other hand, seem a bit overkill for our requirements. I was talking of a 7S8P LiitoKala INR26650A/Bs or 7S16P Samsung ICR18650-26FMs, this means a 1032Wh peak (25.2V × 40Ah) battery pack, from which around 670Wh of power would be available if charging at up to 3.92V/cell (23.24V × 28.8Ah). I think this is a nice enough power reserve to enjoy a fun afternoon, even with some led lights running off efficient buck converters connected to the battery pack. Even if made out of the Sammies, such a big bastard would handle in excess of 2500W surge power demand with ease, and a tad more with the INR26650-50A/Bs, so no big deal. The power inverter would trip much earlier.

There's plenty of sunpower here where we live :-) . As a kid, I was taught our area had a sort of arid subtropical microclimate. Plenty of ravines here, and hell in summer LoL.

Been reading a bit about the panels. Their open circuit voltage is around 80% higher than nominal. Had a couple of max 6A output buck/boost modules, with 10-40V input range. This means I could use 4S 5V panels. Already aware of the problems with shades over them, but since the house's roof is flat and free, 0 issues I believe.

Barkuti reporting for BudgetSolarLightForum.

Cheers ^:)

Barkuti, if those solar panels are going to be Always On™, then you’ll need some way of siphoning off the excess power that will continue to be generated once full charge is reached on the batteries.

Ok. Wasn’t trying to get too technical. Just reading the topic of running a refrigerator with batteries and solar power and I had to do that for about a month after we lost power after hurricane Wilma.

Just speaking from that experience doing so.

I don’t even know what kind of refrigerator you’re talking about (1 cu ft?) but you said “old” so I assumed it was a full size and would need even more power than mine did. You know what happens when you make assumptions haha!

Carry on

You may also need to place a larger capacitor on the motor for start up. Something the size of an air conditioners capacitor.

I think one person to either speak to or watch some of his videos is Julian Ilett on YouTube. I think if anyone can provide the answer, he is your man.
He uses solar power quite extensively in his home and uses it for testing a whole range of electrical devices for his YouTube channel

The fridge's outer volume is 0.75m3 (nearly 26.5ft3) or more, so go figure. Prolly well in excess of 14+ft3 internally; no expert in these matters, though).

Capacitor? How about a pack of 2-3mF 450V electrolytic cap? Why?

Cheers ^:)

After living off grid for over 20 years now I really like my Servel propane refrigerator. Not really enough sun to go solar
If you get that inverter please report back for a quick review, I’ve got a use in mind for a cheap inverter

Well, there actually are much (like ⅓) cheaper units, but those look a bit suspicious. You know, like buying UltraFake batteries LoL!

UltraFake SZ19650 9900mAh (Yellow :-D ) ROFL!

Cheers ^:)

That old fridge is likely going to be a real power hog, depending on its age. From my experience, an ’05 18 ft3 is in the 450 kw/yr range. The old mid 90’s fridge was more than double that, as measured with a killawatt meter, forget the exact numbers now.

Cheapest off grid way is to find a used RV/Caravan propane refrigerator. On the other hand your liitoKala 26650 battery sounds pretty cool. If you built the battery series strings, so they were removable, you could balance charge them separately with a 1-8S hobby charger, if they needed that? I have a boatload of laptop packs with the Samsung 26f’s looking for a similar large battery project.

Somehow the 500W inverter seems inadequate?, but this is based off a really cheapo 400W Coleman inverter. Of course if you looked at the poor excuse of an inverter … you might only expect to kick over a small tv or laptop, which was its purpose.

nickelflipper, there are units at ⅓ of that price, who claim 500/1000W continuous/peak power. I think this unit is a good one, with battery and output voltmeters and higher quality innards.

Cheers ^:)

I’m guessing 5-7.5 mF, the capacitor helps start up the compressor motor and greatly shortens those current spikes as the motor starts. It’s good for the system on both ends; compressor motor and batteries.

I helped with the electrical on a small hunting shack in the middle of no where. Step up is 1-45 watt solar cell, 2 large AGM trolling batteries and a smart charger that keeps them topped off. Lights are LED’s of course and it has 4 12v power ports. Someone always brings an electric camping refrigerator, 4-5 cubic feet and it has a 12 volt power cord. We can run everything for the weekend then give it another month to recover.

Understood, my new fridge only takes 120W to keep it running, once it has started. Very interested if your chosen inverter unit will handle initial compressor startup, sounds like it might. Very interesting thread :slight_smile: .

all above advice seems good.

if there is a budget (always seems to be the case)

it seems to me that you can make a powerwall if somone has the excess cells already (but that is a project on its own).

1) 2 or 4 car batteries
2) a cheap 30 amp solar controller
3) (the expensive part) the right sized inverter my guess is 2kw
4) 200-400 watts of solar panels

as was mentioned a timer for the fridge and he should be good to go for a weekly beer bash.

I would put my time into building a usb/12v output box in addition the the inverter and your buddy can make use of lots of creature comforts. fyi some solar controllers come with usb outlets these days.

imho lion cells make sense when weight matters, in this case it does not.

if you do have starting issues just grab a “hard start” kit.
and any capacitor you use has to be non polarised and very low esr.
a serious can of worms was opened here too.
you WILL need a bms for li-ion packs.
and li-ion do not like float charging like lead acid.
since this appears like a getaway shack to party and get drunk golf car batts are probably best.
one could use the system heavily over the weekend and the panels have lots of time to charge it back up.


1) Car batteries? LoL! Starting batteries are a complete waste of money in this regard, just a couple of deep cycles and they take substantial damage (it's advisable to keep them above 80% SoC, the higher the better). NOT designed for this purpose! Even Optima deep cycle batteries are designed for just 50% DoD.

2) Solar controller? I'd do nicely with standard CC/CV supplies, no need for specific controllers for lead batteries.

3) 2KW inverter? That's close to half of the household AC mains power I have. Absolutely over the top bro.

4) 200-400W of solar panels? That's a helluva lot of power for our needs.

snakebite, no float charge device gaming here, fellow. I said CC/CV buck/boost supplies. And yes, a BMS is due, namely because my friend told me he was not going to power up that big ol' household fridge. We may go with a smaller battery, and no inverter. Also notice that commercial inverters designed for 24V (12S) lead batteries fits well with a 7S li-ion setup, under-discharge protection wise.

Cheers ^:)

A modern fridge will likely be cheaper than a larger battery pack, inverter and solar panels
Does not be brand new but it should be very energy efficient

A good 1kW inverter will be well enough to start the motor and run some lighning.