LiFePO4 Storage and charging options

FilterGuy

What, me worry?
Nice Find Will. I really appreciate all the research you do and share!!! I was hoping it would cover low-temp charging but it doesn't.

I had always heard "Never charge below 0 deg C (32 Deg F)" and in your videos you talk about keeping the temp a few degrees above freezing for charging. However...
  • When I look at the BattelBorn Manual for their 24V LiFePO I see that it states:
    "Low temperature: < 25F The BMS will not allow a charging current"
  • When I look at the spec sheet for the LiFePO Daily BMS that you recommend on the web site, it appears to say it cuts out charging at -5deg C (It is partially in Chinese so I can't tell for sure)
Both items surprised me. It seems to indicate LiFePO can charge at a few degrees below zero!?!? Can you shed any light on this? I am planning on building a system that must work in cold weather and want to know the limits I need to design to. (particularly if I build my own battery out of individual cells)

Thanks!!
 
D

Deleted member 783

Guest
Nice Find Will. I really appreciate all the research you do and share!!! I was hoping it would cover low-temp charging but it doesn't.

I had always heard "Never charge below 0 deg C (32 Deg F)" and in your videos you talk about keeping the temp a few degrees above freezing for charging. However...
  • When I look at the BattelBorn Manual for their 24V LiFePO I see that it states:
    "Low temperature: < 25F The BMS will not allow a charging current"
  • When I look at the spec sheet for the LiFePO Daily BMS that you recommend on the web site, it appears to say it cuts out charging at -5deg C (It is partially in Chinese so I can't tell for sure)
Both items surprised me. It seems to indicate LiFePO can charge at a few degrees below zero!?!? Can you shed any light on this? I am planning on building a system that must work in cold weather and want to know the limits I need to design to. (particularly if I build my own battery out of individual cells)

Thanks!!

This is my biggest concern about going LiFePO4. If the BMS tanks on a cold night, I lose my entire battery investment? That's hard to swallow.
 

Supervstech

Administrator
Staff member
Moderator
Nice Find Will. I really appreciate all the research you do and share!!! I was hoping it would cover low-temp charging but it doesn't.

I had always heard "Never charge below 0 deg C (32 Deg F)" and in your videos you talk about keeping the temp a few degrees above freezing for charging. However...
  • When I look at the BattelBorn Manual for their 24V LiFePO I see that it states:
    "Low temperature: < 25F The BMS will not allow a charging current"
  • When I look at the spec sheet for the LiFePO Daily BMS that you recommend on the web site, it appears to say it cuts out charging at -5deg C (It is partially in Chinese so I can't tell for sure)
Both items surprised me. It seems to indicate LiFePO can charge at a few degrees below zero!?!? Can you shed any light on this? I am planning on building a system that must work in cold weather and want to know the limits I need to design to. (particularly if I build my own battery out of individual cells)

Thanks!!
Most battery specs I see state charging from 25F or -5C... to 60C allowed... I think degradation starts at 32F/0C, so I’m setting my heating to kick in at 35F/2C... I’m not taking chances...
 

FilterGuy

What, me worry?
Most battery specs I see state charging from 25F or -5C... to 60C allowed... I think degradation starts at 32F/0C, so I’m setting my heating to kick in at 35F/2C... I’m not taking chances...

I will probably do the same, but it would be good to know the exact details. As an example, if degradation does not start till -5C, then charging it down to 0C or even -2C would be OK (and still leave some margin for error).

Why would I want to push it? The act of charging will also warm the battery. If you have the batteries in an insulated box, you might be able to ride out the ambient lows.

Note: I am dealing with a micro-hydro system so I have a small wattage input (only 120 watts). However, if I can keep it running 24x7 it gives me 2880 Wh per day. Enough for the small cabin I am powering. That means I want to charge over-night while the temps are the coldest.
I will probably put in a small heat-pad and thermostat to increase the ability to ride out the cold, but I don't want to 'use up' 20 or 30 watts if I don't have to.
 

Supervstech

Administrator
Staff member
Moderator
I'm a diyer...

I'm thinking of lining the bottom of my battery area with a coil of water filled pex... then plug in a thermostat controlled pipe heater to keep the pipe warm, thus keeping the batteries warm...
 

FilterGuy

What, me worry?
SuperVstech Wrote:
"I'm thinking of lining the bottom of my battery area with a coil of water filled pex"


If it froze, will the pex burst?

One similar idea I had was using freezer packs with a heat mat inside the insulated box. Freezer packs are sealed plastic bottles filled with a liquid and are designed to be frozen and then put in an ice chest. If I put them in my battery box (Unfrozen) they would provide thermal mass but if they do freeze they won't burst. (I would mount them under the batteries so if they do burst they don't drain over the batteries.)

If the freezer packs do get to the freezing point, they will hold it at that temperature till they are fully frozen (Phase change thermal buffering). However, there are two problems with counting on the phase change thermal buffering:
1) When they thaw they will absorb a lot of energy and hold the temp at the freezing point till they are fully thawed. (This is what they were designed for)
2) I do not know the freezing point of the mixture in the freezer packs I have. If it is lower than the min battery charge temp the buffering would occur too late. (I need to break one open and do some experiments to determine the freezing point)
This Link has some freezer packs that say they feez at 32deg F..... But I would still want to verify that.
https://www.uline.com/Product/Detail/S-18256/Insulated-Shippers-and-Supplies/Ice-Packs-8-oz

Note: In my placement of the freezer packs, I will need to account for the swelling that happens if they do freeze.

No matter what I do to try to keep the batteries warm, I will need a low-temp cut-off on the charge because I can not guarantee they will stay warm enough.

Related topic: What are people using for minimum non-charging storage temperature of LiFePO4? I have heard ranges from -10C to -20C.

There will be times my batteries will be totally disconnected and not in use. I will have a heat mat on a small 20 Watt solar panel but that will only heat during the day and when no snow is on the panel. Consequently, my system needs enough insulation and thermal mass to 'ride out' the low temps till the solar panel comes back on line. The freezer packs will provide a large "effective" thermal mass even if they do freeze below the min battery charge temp. As a last ditch effort to keep them warm I could use the batteries to drive a heat mat (with a thermostat set to -9C). However that only works till they are discharged. Then I am storing discharged batteries....not a good thing.

Note: I edited this post to correct the cold storage range.
Note: I edited this post to change the name from Freezer Block to Freezer Pack and added a link to some for sale.
 
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erik.calco

Solar Badger
I'm a diyer...

I'm thinking of lining the bottom of my battery area with a coil of water filled pex... then plug in a thermostat controlled pipe heater to keep the pipe warm, thus keeping the batteries warm...
We branched this thread into a battery temp thread? SWEET!

Fun fact... if you let your fallen leaves sit, they create a warm layer under the snow. Why? Because of biodecay. Don't believe me? Leave a pile on concrete, then shovel them when covered in over a foot of snow. Be amazed. Not ice.

That raises the question. Can you use biodecay to keep your batteries warm, or at least reduce the cost of a heating element?

How do I know? I hate raking leaves, and learned to leave some intentionally in hard to shovel areas.
 
D

Deleted member 783

Guest
If it froze, will the pex burst?

Nope, my RV is all pex and its frozen a number of times without issue.
 

FilterGuy

What, me worry?
Hmmmm.... I am not sure how I would use biodecay in a practical way, but I *really* like the way you think. Coming up with a simple passive heat source is exactly what I am trying to do.
 

erik.calco

Solar Badger
Hmmmm.... I am not sure how I would use biodecay in a practical way, but I *really* like the way you think. Coming up with a simple passive heat source is exactly what I am trying to do.
Well, can't rely on it without data. But this is totally worth collecting data on because I'm amazed every year how rare it is for the buried leaves to freeze. Downside... in spring you have mess of black gunk to clean up. You do need moisture. Dunno about air. So, first experiment might be in a bag so you can contain it. I'd prolly do after they are wet from rain. Create various tests to monitor temps over winter. Can use an empty plastic box as a fake battery for each test with a temp probe inside. Need ambient temp collection as well.

I tried to Gaggle online, but couldn't find anything quickly.

If you are already going to leave your batteries out regardless, it certainly couldn't hurt to create an insulative layer out of the leaves.

EDIT: Now that I think about it, I've also done unintentional experiments where apples, carrots and other produce turned 100% into a brown liquid in a zip lock bag over 2 years in my garage inside a cooler. Truly amazing because there is not a single solid left in the bag. What this says is that the biodecay process is possible in a sealed bag, and that lots of moisture in the beginning is a good thing, as these things begin with a high water content. I can show you a pict of black goo stains on my garage floor where carrots were not contained, and the liquid dried once exposed to lots of warm air again. Potatoes work too. High sugar content could be a common denominator impacting the speed of the process.
 
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erik.calco

Solar Badger
FYI: I posted a description of my existing system over in "show-n-tell": https://diysolarforum.com/threads/my-micro-hydro-system.1957/

As mentioned in that thread the FLA batteries died in just a couple of years so I am looking at going to LiFePO4 if I can convince myself I have a solution for the cold storage.
I'll check it out later. I am fascinated by that. I watched some videos of it. Looks like a lot of work, but very cool concept.

Your only hope of finding more data on biodecay short of someone else jumping on this thread is to search online. I told you all I know about it. It definitely produces heat. Enough to amaze me. But enough to keep your battery going through winter? IDK. I'd still have BMS protection, and considered a heating pad that kicked in under certain temps regardless. The leaves would simply reduce the need for the heating element to kick in. You can always have a record cold freeze. The leaves and produce waste look very promising for at least expanding the time your battery can be operational and charged. But, probably should not be your only line of defense.

You may be able to find more info on gardening or horticulture, as I believe this is how some plants survive, IIRC. It might explain how some animals survive. E.g., a possum built a nest in my boat, giving it protection from elements and wind. However, the nest was all built from sticks and leaves which did show some decay when I found it in the nest. Perhaps it also produced heat? Doesn't seem like a lot compared to the leaves under the snow. But, I don't know. The look on his face when I found him, though, was priceless.
 

erik.calco

Solar Badger
OH, one more thing. This ironically works because the snow is on top of it. Not that you want a cold layer above. It's more like an igloo concept. It just allows the heat to get trapped underneath the snow. So, you want some form of insulation around the biomass and batteries. A potential setup would be to dig a hole in the ground. Put in a cooler. Put a bagged layer of biomass in the bottom. Put batteries in. Souround sides with bagged biomass. Be sure nothing can leak on top of batteries and short posts, of course. Assume if any leak in biomass bags occur, you will have some buildup of liquid at the bottom. Cover with lid and insulation. A setup like that would trap the heat generated.

Would have to additional protect from water coming in, and seal where cables go in and out.

If you can wait until next year, do this year with a simulated battery or cheap LA and put in a probe to collect temp data to contrast with outside temps.
 

FilterGuy

What, me worry?
Erik, You got close to another idea I have been thinking about but is currently on the 'reject' list. If I can dig a hole deeper than the frost line, I can use the earth to keep it above freezing. However, digging up my batteries is not the first thing I want to do when I arrive at my mountain retreat. (Particularly since the soil is more rock than dirt).

I have thought about doing a thermal well with a heat exchanger deep in the ground. Pumping anti-freeze fluid through it and into another heat exchanger in the battery box might work.....but then I have to figure out how to power the pump. Then my thought went to convection flows and/or heat pipes..but that would be hard to guarantee. The idea got impractical fast.

The only reason I even mention this idea is that it might trigger a good idea in someone else mind.
 

Ron

Solar Enthusiast
I figured using the beer cooler method of an insulated box. I thought of using a heat lamp and a heater pad also. The electronics will be in a separate box just above the battery box. Now for another strange thought, I have (so many!) To help the controllers last I wonder about using an air filter on a large air supply intake. I once bought one that could be cleaned and reused indefinitely.
I am trying to have a controlled environment so the CC lasts longer.
 

FilterGuy

What, me worry?
> I figured using the beer cooler method of an insulated box.
Yup, I am leaning that way as well. I just wish I knew the R value of the beer cooler, then I could run some scenarios with outside temps to calculate the result. Either way, I am already convinced I need to add a small solar panel (30 W?) driving a heat pad on a thermostat inside the box. I am playing with some heat loss calculations for a wintering box with R30 insulation all around. It is not looking too bad. I'll post what I have once I have worked through them and feel confident I have them right. (My college thermal dynamics was a looonnnggg time ago) The big problem is predicting what the worst case temp profile might be for where my cabin is. The historic low is -40F but that happened only once.
 

Ron

Solar Enthusiast
> I figured using the beer cooler method of an insulated box.
Yup, I am leaning that way as well. I just wish I knew the R value of the beer cooler, then I could run some scenarios with outside temps to calculate the result. Either way, I am already convinced I need to add a small solar panel (30 W?) driving a heat pad on a thermostat inside the box. I am playing with some heat loss calculations for a wintering box with R30 insulation all around. It is not looking too bad. I'll post what I have once I have worked through them and feel confident I have them right. (My college thermal dynamics was a looonnnggg time ago) The big problem is predicting what the worst case temp profile might be for where my cabin is. The historic low is -40F but that happened only once.
I considered a broken freezer or fridge. The price might be right. I will probably buy some thick hard foam and insulate a wooden box with it. I also considered bags of cinders made into a large box. I will put the box in the greenhouse that will be attached to the house so it won't get too cold. I think some sort of thermostat to turn on the heater pad will be used. I could make my large heater bulb(infrared?) into the first thing my diversion box will power up. I have two I used for keeping a water bucket from freezing. the chickens seemed to like it. On a side note, I woke up with no electricity and pointed out that this could be the future for many and that's one of the reasons for dad buying all this solar stuff. Plus the property is off-grid.
 
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