24V Tiny Cabin

[LoganSovereign]

Hi everyone,


I am currently trying to build my off-grid system for my Hunting/GetAway Cabin up here in Northern Ontario Canada. I just bought 4 250w 37.2Voc panels for solar and I’m thinking about getting the EPEVER 40a Tracer controller with an MT50, but I'm not sure exactly what I should do to keep the batteries above freezing in the winter... I want to have a 24v LiFePO4 system with no inverter (only DC appliances), but my only source of heat will be my wood stove.

I'm afraid that when I have been away from the cabin or leave the cabin with the cells not fully charged in the winter; the batteries won't be charged when I come back because the low temp cut off will disconnect the batteries... I know the cells like to sit around 50%, I'm more worried about me not having power when I come back to the Cabin..

Do you have any suggestions?

 

[cass3825]

Some folks have considered diverting the panels output to an electric pad heater under the battery bank, but this is more of an art than a science at this point so I would not recommend it as a viable solution for your remote/unmanned scenario.

 

[upnorthandpersonal]

Take the batteries home with you when you're not there. Make it so they sit in a nice, easy to carry box with an easy way to disconnect.

 

[JoeHam]

As you likely know the only problem is with discharging below freezing. LFP batteries also have an extremely low self discharge rate while not in use. Personally I would leave the battery charged when I left. Upon return you can use it while you get a fire going before you hook up your solar panels.

True confession, I have a 60Ah LiFePO4 battery that I store at 100% SOC for over 5 years now. I think by the time it significantly degrades there will be newer battery technology I will want to replace it with. YMMV.

 

[Craig]

Consider using LTO batteries this is how I chose to solve the problem.
These can be charged and discharged at -50C they are a little more expensive than LiFePO4 but have 4 to 5 times as many cycles in them.
Honestly we do not know if they will wear out first or just get old first. I will let you know in 10 years I guess.
Also the voltages are a bit wonky but if you go inverterless it will not be a problem.



LTO spec sheet

 

[Steve_S]

LFP can discharge even at -10C BUT they cannot be charged until over 0C and even then lower temp needs slower / lower charge. They also take time to warm up / thaw out, does not happen in an hour ! I know what it's like here, I'm not far from Algonquin Park.

As you will have an SCC (solar charge controller) setup the panels so you can tilt them for winter & summer optimization, in winter likely a 30deg where in summer around 50 should be ok. 30 Degrees in winter would keep the snow off pretty well, so the SCC can maintain the battery pack.

I scooped up a used RV Furnace (Suburnan NTS-20) out of a wrecked 4 year old RV with regulators & all for $300. It only needs 12V 1A to run, I'm on the 3rd 20 Lb tank (just started it). I decided I am putting a 100Lb on there which should last 2 years. I have a separate 100W/12 Solar Panel attached to big Marine AGM and that takes care of everything. Now, I live out here off-grid my powerhouse (75' from home) is hyper insulated and only 14'x6' and also doubles as my pump house as the well head & 50 gal pressure tank is in there, so this solution works a treat and it's painless.

Look in my signature at the links, there is a link to "about my system".

Hope that helps, good luck.
Steve

PS Check this site out, fill in the info an know what your solar angles & degrees should be.

Solar Angle Calculator

The solar angle calculator tells you by location the optimum angle to get the best out of your system.

www.solarelectricityhandbook.com

 

[LoganSovereign]

Logan's situation is exactly what I'm faced with. I have a one room cabin in Northern MN. Used heavily in summer, intermittently in winter. I'm looking at 24V LiFePO4 to power a fridge and lights in the summer, just the lights in the winter. This will be my first solar setup. I don't really want to haul the batteries home if I don't have to. My current thought is disconnect the panels when we leave in the fall. Show up in January and use flashlights/propane while the wood stove cranks the temp up. Batteries will be inside the cabin. Once the batteries reach temp, reconnect solar. Does anybody see an issue with this? Trying to keep it as simple as possible so anybody using the cabin can do it.

Hi Drat, I have no updates for you as of now, other than potentially Battleborn is coming out with a battery with a heating pad? But reading your comment it seems you don't mind NOT running anything while you're not there? I would suggest you just get a battery that has Low Temp disconnect or a controller that has Low Temp disconnect, so you can leave your cabin and your low temp disconnect on the BMS or Charge Controller will not charge the battery if it is too cold. Because you are not running anything while you're gone you dont have to worry about the battery sitting at a low state of charge.

 

[SCClockDr]

My thoughts:
Disconnected on departure is disconnected.
BMS low temp disconnect depends on technology & 0% error rate. Northern MN in winter is no place to need a battery in mid winter to survive. Just sayin'.

 

[SCClockDr]

Battery & inconvenience of finding a replacement. 40 below is 40 below & unnecessary exposure has risks. Minimizing those risks is the wise approach IMHO.

 

[SCClockDr]

I'd query them about reducing the final charge % to between 65 & 50% prior to disconnect. LiFePo4 guidance I've read would suggest 100% SOC less than ideal for storage.

 

[erik.calco]

I'd query them about reducing the final charge % to between 65 & 50% prior to disconnect. LiFePo4 guidance I've read would suggest 100% SOC less than ideal for storage.

I think what they are accounting for is the normal self-discharge rate, which averages around 3% per month. He wouldn't actually be storing them for 6 months at 100% SoC.

 

[Craig]

I think what they are accounting for is the normal self-discharge rate, which averages around 3% per month. He wouldn't actually be storing them for 6 months at 100% SoC.

On top of that they said discharge for cold weather would be higher so if he started at 50% then he could lose most of his capacity by the time he returned. I think its best to do what Battle Born recommends since they are the one warranting it

 

[SCClockDr]

I'm not differing with y'all, just suggesting you ask for more data to support their recommendation. I get you are willing to accept their recommendation on face value & I'm fine with that being we've had this discussion.

 

[Delmar]

How deep is the frost line in Northern Ontario, and what construction is the floor of the cabin? Wonder if you could dig a small cellar in the center of the cabin that would maintain the batteries above freezing. Otherwise may be an application for lead AGM batteries. I'd rather destroy a cheap set first.

 

[Steve_S]

In my location in Ontario, frost depth is 4'. I have a Frost Protected Slab Foundation with Radiant Pex in it and 4" of HD XPS Foam under & surrounding it. (not 2" as required by code). Even though I am full time here, I have a separate Powerhouse/Pumphouse which is heated by a recovered RV Furnace for now, but it is also plumbed with Radiant Pex as well for future Solar Heating., It is also an FPSF Foundation with 4" of foam.

 

[Aussiecroc]

Gosh my 200 amp hour 12 volt battery weighs 16 Kg. Its going in my motorhome. Pick it up and take it home! No theft, no problems - find a use for it at home with another system. You have an asset that you can use anywhere. House boat car picnic cabin events yada yada. Solve all your problems. There are hundreds of guys in a group here that have had lifepo4 in their rvs for 7 years (winstons) They have been on some of the roughest roads in the world and are doing well. I'm sure your trip home from the cabin wont damage them if they are packed well.

 

[Craig]

I have these and a box to put them in.

DC Miniature Circuit Breaker 2 Pole 1000V 6KA 16A-63A USA Stock! | eBay

2 Pole Directional Breaker. Din rail mount. DC Mini Circuit Breaker Perfect for Solar or any other DC Application.

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Don't buy on eBay I can get you a better price pm me if interested

 

[erik.calco]

I need to split both pos. and neg. to send to DC load center as well as to the inverter.

Use a bus bars. That way you can have separate switches, cable gauge and fusing as needed for the connection between the bus bar and the bank as well as each load.