Newbie from Central PA requesting help

[Zeito]

Hello everyone - I am hoping to get some help or information so I can make a decision in regards to my off-grid hunting cabin.

There is a ton of great information on these forums, but unfortunately for me I don't understand the vast majority of it! I am a complete dolt when it comes to anything electrical/solar/invert/convert/etc. When I read things on there about VMS, and MPPT, and 12v/24/v/48v, etc. I might as well be reading about how Dr. Brown needs the lightning bolt to create 1.21 gigawatts of electricity so it can fuel the Flux Capacitor in the Delorean time-machine.

I apologize in advance as this will probably end up long-winded, but here's what I I'm trying to come up with. Currently I have an off-grid hunting cabin in Central Pennsylvania. I do have a simple solar setup now - an el-cheapo (bought on e-bay) 100w single solar panel feeding into a 10a charge controller hooked to one deep cycle marine battery. From that battery I have a 5 bank 12v fuse panel that feeds 8 or 9 LED (camper) lights spread thruout the camp. We currently have a small (I think it's a 7.6 cubic foot) PROPANE refrigerator that does the job but is a bit small for what we want. We like to keep the fridge stocked with cold "beverages" and food so we have things available when we visit. The cabin is fairly close to my house so we are there quite frequently. Anyway, we started kicking around the idea of getting rid of the propane fridge for several reasons; one being that propane is insanely high priced at the moment; we would like a bigger fridge/freezer; and quite frankly having to remove everything from the fridge/freezer to defrost it one or two times a year and waiting for a couple of DAYS for it to cool back down just plain sucks).

I started to look into a 12v fridge/freezer, but honestly paying out around $1,000 for a fridge (about the same size I have now) just doesn't seem too cost effective - in addition to knowing that I will require either an upgrade or a totally new (bigger) solar system than I have now. Laying out (basically) a couple grand and not even getting a larger fridge is hard to justify - even at the high propane prices it would take YEARS to recoup that cost. If I'm going to lay out a bunch of $$$, I at least want a bigger and better fridge - otherwise what would be the point? So, at that point, I started to kick around the idea of running a full-sized house fridge (120v) with an inverter because you can get those refrigerators for practically nothing - heck I have a spare one in the basement of my house that I could use... for FREE.

So in summation... is it worth it to pursue? And which way - 120v with inverter or 12v with a much higher cost? And how much "juice" would it require (solar panel "watts", charge controller "amps", battery bank "amp-hours")? I know just about enough to be able to look at the KW usage and calculate how many watts/day or amp-hours/day and that stuff, but when it comes to trying to figure out how much battery will be used, and how much incoming charge I need to recharge, etc. I am lost.

Thank you all in advance!

 

[sunshine_eggo]

Short version:

You want to power a residential fridge with solar/battery?

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~10 cu-ft fridges (about the same overall physical size as your RV fridge, but with more usable interior space) can be had for about $400. They use about 0.9kWh/day. You would have to provide 0.9kWh/day and at least that much in usable battery capacity.

Assuming great solar conditions (great weather, optimized tilt, full sun exposure from sunrise to sunset) you'd need 300W of solar - maybe 500W in winter.

If you get lead-acid, you'd need 150Ah of 12V battery (50% usable capacity) to power the fridge for 24 hours without charging. If you invest in LFP, you'd need 100Ah. Note that LFP may not perform well in freezing conditions as they can't be charged below freezing. They typically protect themselves; however, if they don't heat up above freezing on their own, they may not get charged. Some utilize self-heaters that heat the battery with incoming charge current to allow safe charging.

A 1000W pure sine wave inverter might cut it, but you'd need to confirm it can handle any surge associated with the compressor starting up.

 

[Zeito]

Awesome info, eggo! I was looking at a 18 cu-ft fridge that uses around 1.1kWh/day and came up with 400w-600w in solar panels, and about 200Ah in batteries (lead-acid) so maybe I wasn't that far off? Another question would be where "amps" comes in to play with the charge controller. Do I need, or would it be better to have a 10amp controller, 20amp, 40amp? Does it matter? And why? Thanks again!

 

[sunshine_eggo]

Looks like you had a clue.

Amps is the maximum amount of current the charge controller can send to the battery:

600W/12V = 50A, i.e., to utilize 600W of solar on a 12V system, you would need a 50A controller.

 

[Zeito]

Good stuff - thanks a ton!

 

[Brett V]

I ran a refrigerator and upright freezer on a 1500 watt inverter with very good results. I'd suggest going with a 24 volt system to to keep the battery cable size requirements a little lower, and at least from my experience, the 24 volt inverter has a slight advantage with the surge when the compressor kicks in.

 

[Zeito]

What size cable did you use? I assume you are referring to the cables that go from the battery to the inverter. Thanks!

 

[Brett V]

I had about a 5 foot run from the batteries to the inverter and used 4 gauge wire. There are lots of wire size charts and calculators out there to see what will work for you.

 

[Doggydogworld]

LiFePO4 is more cost effective than lead acid if you cycle every day. It costs 3x as much per nameplate kWh but only ~2x as much per usable kWh because you don't want to discharge lead acid more than 50%. LiFePO4 should last 5x as many cycles. Maybe even 10x. They do need to be above freezing to accept charge. But if you have plumbing you probably keep the cabin above freezing anyway. And if not, your fridge doesn't really need to run when it's below freezing.

A system that will run the fridge 95-98% of the time is pretty cheap and easy. That last few percent gets expensive. You have to handle long cloudy stretches and such. 95% is good enough if the fridge holds stuff that won't spoil (e.g. Cokes) or is cheap enough to just throw out (hot dogs).

Your math looks good to me. Pay attention to inverter self-consumption. Some inverters will suck more energy than your fridge!

 

[Zeito]

I've read a couple of conflicting articles about LiFePO4 - one says NOT to attempt to charge your LiFePO4 batteries if it is below freezing (will damage the internal cells) and one says that if it is hooked up to solar that it CAN be below freezing (the charge from the solar will keep the internals of the battery above freezing). Does anyone know in real-world experience which case would be true? I do NOT keep heat in my camp when I am not there, and sometimes in the dead of winter temps can get below zero - would that damage the battery?

 

[Doggydogworld]

I've read a couple of conflicting articles about LiFePO4 - one says NOT to attempt to charge your LiFePO4 batteries if it is below freezing (will damage the internal cells) and one says that if it is hooked up to solar that it CAN be below freezing (the charge from the solar will keep the internals of the battery above freezing). Does anyone know in real-world experience which case would be true? I do NOT keep heat in my camp when I am not there, and sometimes in the dead of winter temps can get below zero - would that damage the battery?

You can use LiFePO4 down to -20C but you can't recharge them below freezing. Some vendors put heating strips in the battery pack. So you can in theory use power from the solar panels to first heat the cells above freezing then start charging them. I assume these type of packs have circuitry to figures it all out.

The BMS should prevent charging when the cells are below freezing. The battery can still run the fridge though, at least for a while. And as I said the fridge shouldn't run often in sub-freezing temps. The freezer will use some electricity, though, especially if it's frost free because the defrost cycle uses a heating coil.

Maybe insulate the battery pack and direct airflow from the fridge's external coils over it

 

[sunshine_eggo]

I've read a couple of conflicting articles about LiFePO4 - one says NOT to attempt to charge your LiFePO4 batteries if it is below freezing (will damage the internal cells)

this is correct.

and one says that if it is hooked up to solar that it CAN be below freezing (the charge from the solar will keep the internals of the battery above freezing).

This is stupid and senseless.

Does anyone know in real-world experience which case would be true? I do NOT keep heat in my camp when I am not there, and sometimes in the dead of winter temps can get below zero - would that damage the battery?

IIRC, Will has done tests demonstrating immediate and permanent capacity loss when charged below freezing. The chemical reaction gets sluggish. If you push too much current, it causes the lithium to plate the terminals. Lithium plated to the terminals is permanently out of commision.

It's technically not a binary switch. One could charge very slowly at lower temperatures, but no one publishes that info.