Charging LFP below 32F - 0C freezing temperature - possible - with newer chemistry?

[SeaGal]

I'd think a few inches of foam would make heat loss negligibly low. For summer, need a manual or automatic vent opened.

very low. It's funny but last summer, the 2" of PIR around the batteries stopped them heating up as much as the garage air temp got to in the sunny afternoons!

I don't think many, if any, of you have charge current adjustable based on temperature.

I do I have temperature and SOC mappings... see...

Science based discussion on charging LiFePO4 below 32°F

It wouldn't be hard for an engineer designing a BMS to scale charge current with temperature and state of charge. Seems like it could be done in firmware. For example: 10A@-10°C, 15A@-5°C, 20A@0°C, 30A@5°C, 40A@10°C, 100A@20°C. This would allow them to better protect the battery from damage at...

diysolarforum.com

 

[beckkl]

Or to lower it at night.

I'm not sure that maintaining battery temperature at night consumes much more than warming it back up in the morning. It is just a matter of that extra 5C delta T, and thermal resistance.

I'd think a few inches of foam would make heat loss negligibly low. For summer, need a manual or automatic vent opened.

While you're at it, raise not just 5 degrees but enough that the battery can comfortably accept the power you will have available. The curves for cold temperature charging dropped quite low, like 0.2C. I don't think many, if any, of you have charge current adjustable based on temperature.

I have solar assistant configured + home assistant, so can adjust the max charge amperage based on temp. But it is sort of irrelevant since I can't get above .1C with my current production.

 

[Lt.Dan]

I'm thinking in this early portion of the morning, here is a screen shot of mine this morning:


So if heating begins at 6:45am, or so, right when solar just starts to come in (ignoring my home load, just focus on PV Charging the battery), the battery will begin heating because it detects charge current, then by 7:30 (maybe much sooner, depending on Cell temp, and insulation), the battery is warm, and ready to accept the 1kw of solar that is coming in. In that 45 minutes, you've lost maybe 4-500wh of possible solar, because it was only letting the heaters run, and not allowing any other charge current.

And by that time, if it starts charging the battery with a full 1kw, at 7:30am, and say you have a 100ah battery bank (abnormally small, but exaggerated for the example), then you are still only charging at .2C rate. If you have a 280ah bank, which I would consider minimum for a 48v bank, then its only a 0.07C rate. Should be no problem for a LiFePO4 battery at 2-5*C.

Another thing that popped into my head, is what if you don't have good sun in the morning/next day? You heated that battery all night to keep it ready for the next day, and wasted that energy because there is no charging the next day. PV might not even cover your loads, so its definitely not charging the battery. Now you lost that energy you could have used during the day because it was wasted in the night to heat.

 

[beckkl]

I'm starting to question if you are reading my replies carefully. The example you provide is not reproducible with a Seplos BMS. You cannot "let the heaters run, and not allow any other charge current". The BMS does not allow for this to happen. There is simply no settings in the BMS that will allow you to have the heaters on and the cells not accepting a charge with a Seplos BMS.

If you don't keep the batteries above 0 deg C you have no choice but to charge the cells below 0deg C, at least until the heaters get them over 0deg C which can be several hours based on others who have tested this exact scenario. Which is why I originally asked what the impact of charging at an extremely low C rate might be.

 

[Lt.Dan]

I'm starting to question if you are reading my replies carefully. The example you provide is not reproducible with a Seplos BMS. You cannot "let the heaters run, and not allow any other charge current". The BMS does not allow for this to happen. There is simply no settings in the BMS that will allow you to have the heaters on and the cells not accepting a charge with a Seplos BMS.

If you don't keep the batteries above 0 deg C you have no choice but to charge the cells below 0deg C, at least until the heaters get them over 0deg C which can be several hours based on others who have tested this exact scenario. Which is why I originally asked what the impact of charging at an extremely low C rate might be.

My apologies, I assumed the Seplos activated the heaters in the manner that @SeaGal mentioned on post #7. If the Seplos BMS does not switch off the charge mosfets, and allow the heaters, then it is pretty worthless if you ask me. Maybe reach out to Seplos and ask for a firmware update? It should be capable of such with the current hardware if I remember correclty.

 

[beckkl]

My apologies, I assumed the Seplos activated the heaters in the manner that @SeaGal mentioned on post #7. If the Seplos BMS does not switch off the charge mosfets, and allow the heaters, then it is pretty worthless if you ask me. Maybe reach out to Seplos and ask for a firmware update? It should be capable of such with the current hardware if I remember correclty.

No worries. I'm hoping that is what they eventually do. It seems like they didn't think beyond adding the heaters to help keep battery warm. Which if you had grid access is fine I guess (although likely a waste of power as you mentioned). I don't have grid access, so I think I will likely have to try and keep them from freezing at night.

 

[HighDesertOffgrid]

No worries. I'm hoping that is what they eventually do. It seems like they didn't think beyond adding the heaters to help keep battery warm. Which if you had grid access is fine I guess (although likely a waste of power as you mentioned). I don't have grid access, so I think I will likely have to try and keep them from freezing at night.

Hopefully the heater circuit isnt physically downstream of the charge circuit. In that case no amount of firmware updates may fix it.

 

[beckkl]

Well, well well. After doing some youtube searching, it looks like someone recently discovered that the BMS CAN heat with the charge mosfets off. It turns out that the BMS waits 30 minutes before turning on the heating fet when under temp protection is triggered. Kind of strange.

 

[Partimewages]

Couldn't you just put an in line (circuit) stat that closes at 35F or 2C? It would keep the heater from running until it's cold.

 

[beckkl]

Couldn't you just put an in line (circuit) stat that closes at 35F or 2C? It would keep the heater from running until it's cold.

You can do a lot of things. Just was trying to figure it out using the BMS as-is, as it is supposed to be able to handle it. Turns out it was just some poor documentation on seplos's part.

 

[grizzzman]

Couldn't you just put an in line (circuit) stat that closes at 35F or 2C? It would keep the heater from running until it's cold.

Perhaps to avoid an on edge trigger?

 

[Daddy Tanuki]

Yttrium doping of the cathode can permit safe charging down to -20°C.

the fact I have to quote Sunshine tells you something.

the only cells that are safe to charge below freezing are the ones where the maker says you can. so in the case of Winstons pay more, get Yttruium doping of the cathode and charge to way way below zero... other wise stop dancing and figure out a way out of the hole you dug.

 

[sunshine_eggo]

the fact I have to quote Sunshine tells you something.

the only cells that are safe to charge below freezing are the ones where the maker says you can. so in the case of Winstons pay more, get Yttruium doping of the cathode and charge to way way below zero... other wise stop dancing and figure out a way out of the hole you dug.

OH THE HUMANITY!!!!!!

 

[Pranegus]

We are getting drilled in by Will and other people knowledgeable about the subject - do not charge below freezing.

Like every other knowledge - that piece of advice has an expiration date - since technology moves on.

There are more and more manufacturers are coming out - saying - we don't do low temperature -cut off, but low temp charge limitation.
"When charging lithium iron phosphate batteries below 0°C (32°F), the charge current must be reduced to 0.1C and below -10°C (14°F) it must be reduced to 0.05C. Failure to reduce the current below freezing temperatures can cause irreversible damage to your battery. "

How do LiFePO4 batteries perform in cold temperatures? | RELiON

How do LiFePO4 batteries perform in cold temperatures?

www.relionbattery.com

Can be charged at temperature down to -20°C for cold weather use (sub Zero version)

Charging Your Lithium Battery Below Zero - DC Battery Technologies

How to correctly charge your Lithium Battery is a frequently asked question. By following this simple guide, you will have all the knowledge to look after your battery efficiently!

dcbattery.tech

My first chemistry question was when looking deeper into the subject - why is everyone focused on the freezing point of water? There is no H2O in common LFP chemistries. Every solution has a different freezing point - it would be quite a coincident that all batteries are freezing a 32F 0C.

My potential unpopular opinion on the subject:
All that public bashing on missing low temp CUTOFF is leading manufacturers to not develop low temperature chemistries.

There is still research going on low temp charging , but not as much as there could be, with the single minded focus of the DIY video publishing community - in ripping batteries apart and putting sensors ins freezing WATER and waiting for the current to drop to zero.

Study on the Capacity Fading Effect of Low-Rate Charging on Lithium-Ion Batteries in Low-Temperature Environment

By taking a cylindrical LiFePO4 power battery as the research object, the cycle performance test was conducted under different charging current aging paths in a preset low-temperature environment and combined with EIS results to analyze the dynamic characteristics of the battery during the aging...

www.mdpi.com

When you look at various scientific studies - you can see that the effect on LFP is gradual.

View attachment 182635

It essentially means - +1C is not massively safer then -1C Zero is just random value chosen for familiarity.

Impact of Test Conditions While Screening Lithium-Ion Batteries for Capacity Degradation in Low Earth Orbit CubeSat Space Applications

A wide variety of commercial cylindrical lithium-ion batteries are available for use in nanosatellites (CubeSats) that cycle in low Earth orbit (LEO). This space application differs greatly from the conditions used to create the manufacturer datasheets that CubeSat teams rely on to screen cell...

www.mdpi.com

Fluorinated Solvent Molecule Tuning Enables Fast-Charging and Low-Temperature Lithium-Ion Batteries​

Yanbing Mo, Gaopan Liu, Yue Yin, Mingming Tao, Jiawei Chen, Yu Peng, Yonggang Wang, Yong Yang, Congxiao Wang, Xiaoli Dong, Yongyao Xia"

https://onlinelibrary.wiley.com/doi/10.1002/aenm.202301285

-20C is bad for Lithium - do not get me wrong - that will kill current chemistries pretty fast. But 0C is a rather arbitrary cutoff in my opinion.

Lets get the debate up and point out the flaws in my Thesis.

I think they chose 0C arbitrarily as a value familiar to many, even though +1C and -1C aren't very different. It's kind of like the qwerty keyboard layout, which was designed for mechanical typewriters to prevent keys from sticking. And although there are already more ergonomic ones, out of habit this is still used.
Still, I wouldn’t charge at low temperatures unless absolutely necessary.

 

[Daddy Tanuki]

I think they chose 0C arbitrarily as a value familiar to many, even though +1C and -1C aren't very different. It's kind of like the qwerty keyboard layout, which was designed for mechanical typewriters to prevent keys from sticking. And although there are already more ergonomic ones, out of habit this is still used.
Still, I wouldn’t charge at low temperatures unless absolutely necessary.

It takes so little power to keep batteries warm through the night that it is really a moot point I think.

I am heating three packs of cells 2 x 16s winston 400 ah and one pack of calb 3p16s 500 ah. The most expensive part is the 6mm aluminum plates for the heating pads to attach to and spread out the heat.

The heating pads draw 50 watts to heat all of the cells per hour, and keep them between 15-25 °c. Thats less than the parasitic load for many of the Chinese AIO inverter units that a lot of members use here on the forum.

So figure 50 watts from say 1600 to 0900 or 17-18 hours. a little a little under 1kW from a 70kw bank. and I can charge immediately at the full out put of my PV array which in the morning tapers up of course but I have seen upwards of a 120 amps at 53 volts before.

YMMV