Lithium battery storage temperatures

Estragon · June 22

It's time to replace my 48v flooded bank, and I'm strongly considering something like Midnite Solar Powerflo. One thing giving me pause is the cold winters at the cabin.

I know charging is a Bad Thing below freezing, so I'd build an insulated box with some sort of heating if/when required. The PF16 has heating built in, but it's pretty heavy to get to the cabin and installed (and out of stock at NAZ). The 5kw rackmount isn't heated, but easier to move. I'd likely use a Midnite AIO for closed-loop charging.

The cabin isn't used much in the winter, so I'd turn the system (including the batteries) off when not there. Midnite's specs for both batteries is:
Charging Temperature   32°F to 122°F(0°C to 50°C)
Operating Temperature -4°F to 122°F (-20°C to 50°C)
Storage Temperature 14°F to 113°F (-10°C to 45°C)

It seems odd that the storage temp window is narrower than the operating. Anyway, it gets to -40F at times in winter and even in an insulated box the batteries will likely end up cold-soaked to something approaching that while everything is turned off.

Does anyone know if this is going to be a deal-breaker?

BB. · June 23

I would certainly contact the battery supplier, if not the mfg., regarding minimum storage temperatures and warranty support...

If you can dig (or have) a small cellar, something like 4 feet or more below ground level, there is a good chance that (in most areas of the world), the ground temperature will not drop much below freezing.

Some charts for Toronto Canada areas that suggest the frost line:

Soil seasonal variations vs soil depth:

https://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm

Construction "Frost Line" example for Toronto:

https://www.thepostholediggers.ca/frost-line-facts-post-hole-digging-and-frost-lines-in-toronto/

You can probably check with your local building department for more information on Frost Line levels for your area...

Of course, other issues may arise--Can you did a cellar (stone/rocks?), will it remain dry during winter?

Design of cellar? Need an insulated lid/door/walls to prevent heat flow from "over cooling" the cellar air (hear rising, cold settling).

Some examples of electric battery heating pads (if you will be going in winter and have a genset to "preheat" the batteries for winter stays (as well as insulated boxes/storage area):

https://www.batteryskills.com/how-to-keep-lithium-batteries-warm-in-cold-weather/

Or use RV tank heating pads (12 volt example):

https://www.amazon.com/Heater-Fresh-Water-Holding-Gallons/dp/B07ZJMW636/ref=sr_1_5?th=1

-Bill

BB. · June 23

I was just thinking... "Heat Pipes" have been used to take heat from the ground and bring it "up" (for example, used to keep ground frozen around Alaska oil pipeline supports).

Another would be using ground heat with small fan/or elevation change to bring "relatively warm/non-freezing" air to the battery storage area using plastic pipe of some sort:

https://milligansganderhillfarm.wordpress.com/2013/12/01/earth-tubes-how-to-build-a-low-cost-systemto-passivly-heat-and-cool-your-home/

-Bill

Dave Angelini · June 23

I would spec compare the midnite bat with the Discover rack mount. They both will close loop but the Discover rack may be easier to move. I think the powerflow also is available in a rack? Look at Rosie also!

Lot's of things changing out there! Good Luck! Stay Safe !

JRHill · June 23

I have pondered this dilemma as I have the same issue but we are here year around. And it sure doesn't hit -40f. An insulated pit with a heavily insulated lid and a cheap wireless thermometer is about the only thing I can see to do. At least you don't have to do any maintenance to the batteries. Hurray!

With self heating batteries I can't get by the problem of the blocks becoming discharged because the panels are under snow - if we had to leave. But the bigger problem is what to do with three Catahoula farm dogs here by themselves.... They and livestock are the reason we can never both be gone at the same time.

Dave Angelini · June 23

Drop the dogs at the vet and try and hire someone to go out and take care of livestock.
That is what a few I know do. If the reason to leave is compelling, you might have to bite the big one.

You can buy a whole battery/inverter with solar inputs for chicken feed theses days.

Horsefly · June 24

I built two insulated & heated battery boxes for my two DIY 24V LiFePO4 batteries. They are in use in our cabin in Colorado at around 9,000 ft elevation, so the temps get pretty cold in the winter. One of the two has been there for 3 winters now, and the other has been there 2 winters. They stay on and connected to the SCC all winter.

The heating elements in both batteries are (if I recall correctly) 24W output, so they heat very slowly. My experiments before the build convinced me that low and slow heating is critical, so that the cells don't get too hot near the heating elements before the other side even gets warm. My math (mostly confirmed by my testing) indicated that even with no solar for over two weeks, the batteries would be able to continue keeping the cells warm. The heat is set to turn on when the battery gets below 40°F, and turns off when it gets above 50°F.

I think I probably posted about the system here somewhere, but I'm not sure.

Having said all this, there are lots evidence that if the battery is truly in storage (no power in or out) they can do fine completely frozen. At least this appears to be true for LiFePO4, but I'm not sure about other chemistries. Fact is, calendar aging of the cells - which is much more likely to kill them before you have used them up - essentially stops when they are kept cold.

Estragon · June 28

Thanks for the comments & advice.

I've asked for and received advice from NAWS (NAZ?) and Midnite Solar, and unfortunately neither can tell me one way or another.

The buried vault idea appeals on a couple of levels. The temp is moderated a lot even with less than frost level depth. Frost level is ~7ft here, but that can be improved with EPS insulation extending around the excavation. If I could dig a vault once, and avoid the 11 years of SG checks and watering etc I did on the FLAs, I'd take the win. Water is a problem though. Spots not liable to snowmelt flooding nearby likely have bedrock 2-3 feet down. Some exploratory shovel work may be in my near future.

I like the heat pipes idea, just because.

Powerflo is available as a 5kw rackmount, and that's what I'm looking at now. I've also been looking at Discover (which does have an internally heated option) and others. Interesting that Discover has a lower allowable storage temperature, but also says "Storing the battery module outside specified temperatures will result in irreversible damage and void the warranty". It doesn't say whether it's the hot end or the cold end of the range that causes irreversible damage.

I did set up small separate solar systems with vertical panels to keep the FLA banks charged when the main arrays get snow covered. Maybe I could rig up some heating from them.

In checking various forums, opinions vary widely. I've run across a few who store in extreme cold and say it's fine. I've yet to find anything like real research where someone at a university stuck a battery in a freezer for a few months and then did real testing. I'll keep looking.

I might have to put the project off for a bit if the US and Canadian governments can't stop changing the trade rules anyway.

Thanks again for the input.

Dave Angelini · June 29

Unattended offgrid is the benchmark ! If it is too high of a risk, Use what you have been !
Good Luck !

JRHill · June 29

I was thinking that but didn't want to be negative. Off grid is easy. Off grid is hard. But with respect to Dave I have to agree that lead batteries (flooded+) might be the better choice. He didn't say that, exactly.

"I've asked for and received advice from NAWS (NAZ?) and Midnite Solar, and unfortunately neither can tell me one way or another."

Nobody is gonna want to answer that question. Until that place is inhabited full time lead is the best choice. For us I did the unforgivable - I put my LFP04 blocks in the crawl space 4' away from everything but the panels. I have heard all the 'You are crazy" comments and more. "I'm gonna lose everything." But until God sends rain on a more regular basis and the non-resident land owners and their guests quit blowing tannerite, shooting tracers and burning campfires, etc IN THE FOREST during burn ban there are greater risks than my batteries in the basement where we live full time and I watch everything everyday.

In any situation it's not known the full scope of the decision to be made. But from what is known, lead batteries would be a good path for the next change out.

"I did set up small separate solar systems with vertical panels to keep the FLA banks charged when the main arrays get snow covered. Maybe I could rig up some heating from them."

That was a really good move.

Estragon · July 15

After a lot more reading, without anything conclusive, I'm thinking that storing (but not charging) the LiFePo batteries should be okay down to about -30C.

One paper I ran across tested cells down to extreme (i.e. ~liquid nitrogen temp) cold. The purpose of the paper was more for safe transport, what happens if a nail punctures a cold cell, etc., but it did note no damage to the cell after the extreme cold soak.

Potential issues noted in some papers included mechanical damage from freezing of electrolyte (depends on exact formula, but lower than -30, and microscopic mechanical damage to electrodes.

If I do go ahead with the lithiums, I'm now planning to put them in an insulated crawlspace and in a well insulated battery box. This should keep them at something close to the mean winter temperature, which will probably not be lower than around -18C.

I've run into another issue with the Midnite gear though, which I'll put in a separate thread.

Dave Angelini · July 15

Any problems in the past with temps around 0F?

The LFP's that have caused my folks issues have been depending on the bms electronics to do what they are suppose to. Did they test them or just find out that the contactor/bms did not do what it was suppose to. Expensive mistakes.

The good news is LFP is getting close to the cost of a set of AGM's

Estragon · July 15

Good point about the BMSs. From what I've read so far, many/most are a black box with limited user control. I can see why, but it does limit those of us who are more hands-on.

JRHill · July 15

"After a lot more reading, without anything conclusive, I'm thinking that storing (but not charging) the LiFePo batteries should be okay down to about -30C."

"Storing but not charging..." just let them go. Maybe in future millennia they will find them encased in something like ash and determine where our primitive society was at the moment. Storing them frozen at partial to less than full charge - that's a bad decision for expensive high quality batteries, in my opinion. If the temp only got to -10 but they are guaranteed to -30, as an example, how do you argue that when they fail? And there you are with ruined batteries when you need them. That's just crazy when the conditions are that severe.

To me, at this point in time, I'd be suffering lead batteries in temps like that because I KNOW how they hold up. And I've not been shy at saying that to others. OK, now beat me up for being so conservative for the things as they are at this moment in time.

Lithium battery storage temperatures

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