LiFePO4 Storage and charging options

FilterGuy

What, me worry?
Looking at the specs of LiFePO4 cells, the short term storage usually goes down to -10deg C. However, the discharge temp is often specified down to -27deg C. They may be counting on internal heating during discharge. The other possibility is that a discharge current prevents whatever chemical reaction damages the battery at cold temps. If so, how small can the current be? If a tiny discharge can prevent damage, it could offer a great way to help prevent low temp damage without a large drain on the battery. Does anyone know about this?

Either way, this made me realize that if I am using the batteries to drive a heat pad, I get two heat sources: The heat pad and the battery. I guess I can use the internal resistance of the cell times the current^2 to calculate the battery heating effect in each cell (P=I^2R).

Edited to clarify that one possibility is that the discharge current prevents whatever chemical reaction damages the battery at cold temps. (I had previously assumed the damage was due to formation of Lithium. That is true for charging at cold temps, but I don't know what the damage is for cold storage)
 
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FilterGuy

What, me worry?
I looked at a few LiFoPO4 specs and only found internal impedance.... and it was really low (.1mOhm.) If I assume that is the resistance, the resitive heating would be *really* small. I am probably missing something on this.... or the low-temp discharge is about the chemistry... not internal resistive heating.

Meanwhile I found this interesting company that claims their LiFePO4 can be discharged at -40: https://www.grepow.com/page/low-temperature-battery.html
 

FilterGuy

What, me worry?
Interesting, I'll have to look into those. With my particular set-up I will be using 24 volts so that immediately limits the 'off the shelf' resistive heat sources available. There are several 12V solutions but I would have to put two in series.

The other issue is the temp control. For storage heating using the battery itself, I do not want it kicking on till something around -5c or maybe even lower. For storage heating using solar, I want it to kick on as high as 10c (50F).
 

erik.calco

Solar Badger
I think using a cooler is virtually a no brainer both cuz of insulation and to help keep it dry, not to mention it is cheap.
The challenge with under ground, besides digging, is keeping water from flowing in. Even coolers are not completely sealed. An ideal hole would have a sealed concrete foundation with walls that rise above the surface, so flowing water goes around it, like a micro house foundation.

Note that on warm days, you'd want to pop the top and let that warm air in. The secret to my beers never freezing in the winter in my car was when I drove with the heater on, I'd pop the top to let it warm up. Then put lid back on while the car was parked idle in the freezing night winds (not garaged).

But if you are going to do this regardless, I'd definitely throw in some leaf and other biomass bags with moisture. As long as it can decay, it will produce heat. The only question is how much and will it produce enough. But, it's easy free heat.

These leaves will sit on my concrete to make shoveling this boat port area easy:

1575066528741.png

They'll get wet from rain or snow melting to kick off the decay process. In spring, I'll rake the wet black partially decayed mucky mess around the base of a tree next to it.
 

erik.calco

Solar Badger
@DThames the first guy to post a 100A version of that for $11 is king of DIY for a day!

If i get outside before the next snow fall during the day, I'm going to gather some of those leaves into a black bag for an experiment. I'll put in front of my front door, where water from ice filled gutters always comes down when it melts, then refreezes to create a skating ring for outdoor critters. I'll use bricks to keep it from blowing away. And, we'll see if I get the same ice pond I normally get there. The problem with that north facing spot is the sun never hits it.
 

FilterGuy

What, me worry?
@DThames the first guy to post a 100A version of that for $11 is king of DIY for a day!

If i get outside before the next snow fall during the day, I'm going to gather some of those leaves into a black bag for an experiment. I'll put in front of my front door, where water from ice filled gutters always comes down when it melts, then refreezes to create a skating ring for outdoor critters. I'll use bricks to keep it from blowing away. And, we'll see if I get the same ice pond I normally get there. The problem with that north facing spot is the sun never hits it.

Putting leaves around the batteries seems impractical. However, it occurs to me that if you buried a coil of 3" tube in a large pile of leaves and then routed that around your storage box with a 'smoke stack' for convection, you might be able to harvest some of the energy of decomposition. The challenge will be to make the pile large enough that you don't freeze the leaves that are adjacent to the coiled tube. (Unfortunately my cabin is in a pine forest and harvesting enough bio-mass would be a challenge. Otherwise I would be tempted to try it.)

Note: This idea came to me while I was looking at an alternative to burying the batteries below the frost line (Also impractical). You could get warmer are from the ground using earth-tubes:
https://milligansganderhillfarm.wor...st-systemto-passivly-heat-and-cool-your-home/
I am not sure if this is practical for my situation either, but it has more appeal than burying the batteries.
 

erik.calco

Solar Badger
Putting leaves around the batteries seems impractical. However, it occurs to me that if you buried a coil of 3" tube in a large pile of leaves and then routed that around your storage box with a 'smoke stack' for convection, you might be able to harvest some of the energy of decomposition. The challenge will be to make the pile large enough that you don't freeze the leaves that are adjacent to the coiled tube. (Unfortunately my cabin is in a pine forest and harvesting enough bio-mass would be a challenge. Otherwise I would be tempted to try it.)

Note: This idea came to me while I was looking at an alternative to burying the batteries below the frost line (Also impractical). You could get warmer are from the ground using earth-tubes:
https://milligansganderhillfarm.wor...st-systemto-passivly-heat-and-cool-your-home/
I am not sure if this is practical for my situation either, but it has more appeal than burying the batteries.
Yeah, the problem I have with burying batteries is keeping them dry. Water loves gravity. A ditch is a potential pond.

The leaves thing needs some testing. I have no doubt it works. I just don't know HOW WELL it works, or the best conditions to artificially induce. I keep thinking about the irony of how snow on top allows them to contain and feed off their own heat, like an igloo effect. You think about those really cold nights out of nowhere. How would all that cold quickly get through 18" of snow? It wouldn't. The leaves are protected from wind and have a thick insulation from sudden ambient changes. Yeah, 3 weeks of deep freeze would works its way through. But, those temporary drops wouldn't . You'd have to emulate that type of insulation on top of having decaying leaves. The lid of a cooler by itself is probably not enough.

What we need is real data collecting -- a science experiment with controlled measured variables. But that's a lot of time and money to setup, especially if you want to try different variations over time to find the optimal solution.

I get a ton of leaves on my property. I wish I could donate some to you.

The only thing I can say about pine needles is they make great kindle. When we went camping, we'd try to find them to get a fire going. But, I doubt they work as well for natural decomposition. But, who knows. That would require testing.

I just wish we could find more online on this. You'd think there would be a lot of data on it somewhere. Lithium batteries seem to of created the first really high demand use case. Decaying leaves is just too insignificant of a potential contributor for the needs most care about, like heating a home or cabin. Thus, no one has been collecting data on it.

Farmers with animals have to know something though. They have all kinds of ways of keeping animals alive in barns. Though, barns are also presumabley kept dry, which makes them less than ideal for decomposition.
 

Maast

Compulsive Tinkerer
Meanwhile I found this interesting company that claims their LiFePO4 can be discharged at -40: https://www.grepow.com/page/low-temperature-battery.html
Actually just about any lithium battery can be DISCHARGED below freezing, way below freezing at that. It's CHARGING below freezing that will kill a battery. It's dead easy to make a insulated box with a heating circuit though. Check out https://www.diysolarforum.com/threads/operating-lifepo4-in-low-temp-environments.1713/
 
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