Why you cannot charge LiFePO4 below 0 degrees Celsius

SherylinRM

GO GO JUICE BOOMER ZOOMER MAKER
Here is a screenshot from a Ganfeng Datasheet (the most detailed data on temperature and charging by a cell manufacturer I've found) that shows the relationship between temperature, c-rate, and soc:
View attachment 28852

Even with the above, they still consider 0*C the hard lower limit that should not be exceeded.
So I am going to assume that the numbers IN this are the C rate at which they recommend charging? Is this correct?
thank you DZL for this. :)
 

Dzl

Unofficial Forum Librarian
Staff member
Moderator
...I glazed over...Soooo far past me...... but, does this mean that the claims of the -20F chargeable LiFePO4 cells could be true?
Based on what i’ve read in this forum, not for any practical use. I’m no expert on the matter, though.
I am not an expert either but my understanding is:

The simple answer is just don't charge under 32*F

The complicated answer is... its complicated, but still no.. ;)
There are a number of variables to consider with low temperature charging (1) temperature (2) C-rate (3) SOC (4) degree of acceptable damage, (probably more). But @DerpsyDoodler is correct, practically speaking there is really no situation where you should be charging LFP at anywhere near -20*F / -30*C outside of some sort of emergency maybe.
 

fafrd

Solar Addict
From what I've seen (from spending too much time looking at datasheets of cells I don't even own...) cell manufacturers usually consider 0*C a hard limit, below which charging should not happen (probably a reasonable but somewhat arbitrary number, its conveniently round and easy to remember).

But they also usually identify soft limits at higher temperatures. With reduced maximum current up to 15*C in many cases. As you noted, its not a black and white limit, Its a gradient, and its a relationship between a few factors (Temperature, C-rate, SOC, duration of charge).

Here is a screenshot from a Ganfeng Datasheet (the most detailed data on temperature and charging by a cell manufacturer I've found) that shows the relationship between temperature, c-rate, and soc:
View attachment 28852

Even with the above, they still consider 0*C the hard lower limit that should not be exceeded.
I’m not understanding the meaning of that table.

At 10degC and starting with a cell at 0% SOC, what is it saying, that charging rate should be limited to 0.5C to achieve 100% SOC or that SOC at that temperature can never exceed 50% SOC when starting from 0% SOC?
 

DerpsyDoodler

Photon Sorcerer
I’m not understanding the meaning of that table.

At 10degC and starting with a cell at 0% SOC, what is it saying, that charging rate should be limited to 0.5C to achieve 100% SOC or that SOC at that temperature can never exceed 50% SOC when starting from 0% SOC?
I read it as “at x temperature, and for y as the existing state of charge, only push z amount of charging current.”
 

fafrd

Solar Addict
I am not an expert either but my understanding is:

The simple answer is just don't charge under 32*F

The complicated answer is... its complicated, but still no.. ;)
There are a number of variables to consider with low temperature charging (1) temperature (2) C-rate (3) SOC (4) degree of acceptable damage, (probably more). But @DerpsyDoodler is correct, practically speaking there is really no situation where you should be charging LFP at anywhere near -20*F / -30*C outside of some sort of emergency maybe.
I’m interested in what that table implies for charging at 10C (50F). If you can explain, I’m all ears.

My motivation is test I charged a cell at 50F and got a much worse result than when I charged the same cell at 77F (25C). I have not yet repeated the experiment to understand whether it was a specific issue with that cell or a common behavior for all of my 16 280Ah cells...

I understand that charging below 0C is a no-no but I am interested to understand how cell capacity is affected when charging in the 50s or even the mid-40s...
 

Dzl

Unofficial Forum Librarian
Staff member
Moderator
So I am going to assume that the numbers IN this are the C rate...
Correct, the numbers refer to C-Rate at some combination of Temperature * SOC
....at which they recommend charging? Is this correct?
That's a little less clear, here is a larger uncropped screenshot:

Screenshot_2021-01-26 GFB_100Ah_PS_EN_11FA_20191104_ pdf(2).png
Or view the full datasheet here

They give seemingly somewhat contradictory guidance.
2.2.6 states 0-60 are the absolute limits
2.2.7 shows very low C-rates under 0

Also bear in mind, while many datasheets have a table similar to this, this one is specific to Ganfeng Lithium 100Ah cell. My purpose of posting was to illustrate the general broad trend and relationship between SOC + Temperature + C-rate, it should not be treated as a reference table for anything beyond that specific cell from that specific manufacturer. From what I've seen there can be meaningful differences between manufacturers

I’m not understanding the meaning of that table.
I read it as “at x temperature, and for y as the existing state of charge, only push z amount of charging current.”
^ This is how I Interpret it too ("when temperature is X and SOC is Y don't exceed a C-rate of Z")

(@fafrd I responded to your later comments in a PM since its another subject, but short answer, the table above is probably not related to your issue).
 

Dzl

Unofficial Forum Librarian
Staff member
Moderator
Since this topic is gaining interest again, here are a few more tables showing the same trend as the Ganfeng Datasheet for other manufacturers

CALB SE series datasheet is the clearest but least detailed (and doesn't acknowledge SOC):
Screenshot_2021-01-26 CALB_SE pdf(1).png


ETC 228 is quite similar in presentation to Ganfeng, but min of 0*C and SOC doesn't appear too important:
Screenshot_2021-01-26 Microsoft Word - 228Ah 产品规格书-20190510 V3 docx - ETC-228Ah-20190510-V3 pdf.png

Here is the Ganfeng table again for comparison (the only one shows that shows temperatures below 0)
screenshot_2020-11-28-gfb_100ah_ps_en_11fa_20191104_-pdf-png.28852


CATL only differentiates <80% and >80% SOC in their chart for the 202Ah cell:
Screenshot_2021-01-26 CATL202 pdf.png

Unfortunately, the two most popular cells here at the moment EVE 280, Lishen 272 do not post tables like this. However Lishen gets the award for the simplest and easiest to understand advice:
Screenshot_2021-01-26 Lishen 272ah battery date sheet pdf(1).png



Its easy to get overwhelmed looking at these tables (information overload), but ultimately most people don't need to spend time on them beyond maybe understanding the general idea, or using as a reference when you have a question. If you don't want to obsess over small details you don't need to if you follow some general, fairly simple, best practices. The ones (off the top of my head) that pertain to charging:
  • LFP cells are happy at roughly the same temperature range that is comfortable for humans (cool to warm). 15-30ish (60 to 85 F) is ideal.
  • Lower C-rate's are less stressful for your cells, particularly at extreme temperatures
  • Don't float or hold cells at 100% SOC or above ~3.4ish volts for extended periods of time
  • Its not necessary to charge to 100%

*Note All of these charts come from datasheets I've uploaded to the resources section. For more context, I encourage anyone interested to review one or two.
 

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SherylinRM

GO GO JUICE BOOMER ZOOMER MAKER
Correct, the numbers refer to C-Rate at some combination of Temperature * SOC

That's a little less clear, here is a larger uncropped screenshot:

View attachment 34656
Or view the full datasheet here

They give seemingly somewhat contradictory guidance.
2.2.6 states 0-60 are the absolute limits
2.2.7 shows very low C-rates under 0

Also bear in mind, while many datasheets have a table similar to this, this one is specific to Ganfeng Lithium 100Ah cell. My purpose of posting was to illustrate the general broad trend and relationship between SOC + Temperature + C-rate, it should not be treated as a reference table for anything beyond that specific cell from that specific manufacturer. From what I've seen there can be meaningful differences between manufacturers



^ This is how I Interpret it too ("when temperature is X and SOC is Y don't exceed a C-rate of Z")

(@fafrd I responded to your later comments in a PM since its another subject, but short answer, the table above is probably not related to your issue).
I think that the bottom line here is that we should cautious when charging at cold temperatures.
I for one have always heard it is better to warm the cells before doing a high C charge.
As I can no charge higher than 0.05 C [10 amps for 280 Ah cells] I do not worry much about it.
Even so, I would never charge at a low temp regardless.


Thanks again DZL for this info. :)
 
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Surferdudemi

Solar Enthusiast
Do BMSs implement any of the temperature modelling and current limitations? The paper that Will posted at the beginning was a great explanation, though it was based on Li-ion, not LiFePO4. Would be great to have a BMS that implemented the correct charge/discharge limits and rates based upon what it measures in the cells and the recent history.
Also, what's the implication for clamped vs unclamped cells? Clamped cells cannot dissipate heat to the outside - at least not as well as cells surrounded by air that can circulate. I think my Volt uses liquid heating / cooling to maintain cell temps. The engine runs at low temps to heat up the battery coolant.
 

HRTKD

Boondocker
Do BMSs implement any of the temperature modelling and current limitations? The paper that Will posted at the beginning was a great explanation, though it was based on Li-ion, not LiFePO4. Would be great to have a BMS that implemented the correct charge/discharge limits and rates based upon what it measures in the cells and the recent history.
Also, what's the implication for clamped vs unclamped cells? Clamped cells cannot dissipate heat to the outside - at least not as well as cells surrounded by air that can circulate. I think my Volt uses liquid heating / cooling to maintain cell temps. The engine runs at low temps to heat up the battery coolant.

My BMS understands low temperature and will cut off charging at the temperature I have it set to. I have it set to cut off charging slightly below the level that my solar charge controller will stop charging. Both settings are above 32° F. I have battery warmers that should keep the battery temperature between 35° F and 45° F. The BMS is using a single temperature probe that I have located at the top of the battery, positioned in the middle, between cells 2 and 3 of a 4 cell battery.

The consensus on heat dissipation is that it isn't a concern unless you're charging/discharging at a high C rate. Most of us are not exceeding .5C.
 

Surferdudemi

Solar Enthusiast
My BMS understands low temperature and will cut off charging at the temperature I have it set to. I have it set to cut off charging slightly below the level that my solar charge controller will stop charging. Both settings are above 32° F. I have battery warmers that should keep the battery temperature between 35° F and 45° F. The BMS is using a single temperature probe that I have located at the top of the battery, positioned in the middle, between cells 2 and 3 of a 4 cell battery.

The consensus on heat dissipation is that it isn't a concern unless you're charging/discharging at a high C rate. Most of us are not exceeding .5C.
I saw that many of them implement a low temp cut-off, and Will tests that in a couple of his videos. I was thinking more of throttling in a smoother way. Kind of like how some hybrid inverters (e.g., Sunny Island) change frequency to tell AC inverters to change output power.

Has anyone tried putting multiple temp sensors and connecting them in series + parallel to get an "average" temp? You're limited by the number of inputs to your BMS, but they are just resisters. If you put two in parallel and then two of those pairs in series, you would get some kind of average of the temp on the four thermistors for one input to a BMS. I can probably model that in Excel to see what happens in various scenarios ...
 

Hedges

I See Electromagnetic Fields!
For some of those tables Dzl posted, I saw a simple rectangular limit:
Max charge rate 0.2C, temperature range for charging 7 to 55 degrees C.
If your panels are no more than 0.2C and you can configure those temperature limits, you'll be OK.
If low temperature disconnect is 0 degrees C, can't have more PV panels than 0.1C (unless your system has programmable charge current limits.)

If your PV panels amount to more than 0.2C of your battery, you're going to have to try harder to remain within limits.
 

upnorthandpersonal

Administrator
Just to add to the discussion: I recently tried to charge a LiFePO4 battery at -17C and didn't manage purely because I could not get any current into the battery (due to increased internal resistance). I did pull full capacity after the test, but it's not conclusive no damage was done. I'll try again soon and increase the charge voltage (and/or keep the charger on for much longer) to force some stuff.
 

Bill Taylor

Solar Enthusiast
I know that LifeBlue had temp sensors inside the battery case, you can see the temperature on the Bluetooth app. It also has a temperature controlled "charge inhibit" function, so you cannot make a mistake and charge the battery at a rate that will damage it. I don't know why more companies don't have cold weather charge inhibit built into the batteries. Seems like a no-brainer to me.
 

upnorthandpersonal

Administrator
Most BMS's used here for DIY packs also have low temperature cut-off these days, so I think it is quite common. I think most of the batteries Will reviewed lately also have this feature.
 

TGPB

Solar Enthusiast
My batteries are going in an off grid cabin that is mostly unoccupied in the winter months. I have a diesel generator set up to autostart when the batteries get down to 12v. I installed 2 x 12w 12v bee hive heating pads under each battery. I have a Honeywell aqua-stat in the battery enclosure that is set at 34F with a 5F differential to turn on the heating pads as needed.
 

Just John

Photon Sorcerer
What I am seeing, is the suggestion that those with battery heaters might shoot for about 60 degrees Fahrenheit. Looks like below 50, don't try a high rate charge.
 
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NobodyOfNaught

New Member
I believe the only cells capable of below 0C charging are Winston LiFeYPO4 cells the yttrium gives them a wider range.

But they are more expensive
 
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