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Wolf

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Sep 25, 2018
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@daromer
Fortunately low voltage in a cell does not cause dentrits to grow it is the charging and cycling process esp with high current that does.
This is attributed to the evolution of Li metal morphology during cycling, which leads to dendrite growth and surface pitting. See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5126712/

My personal thoughts and beliefs:
Assuming that dendrites offer a resistance factor till they pierce the membrane and short the cell I believe that IR is affected negatively. I.e. Higher.
Additionally the surface area of the Anode is compromised resulting in poor performance hence capacity loss which we measure with our testers.
As with you I have no documentation for that belief.

Wolf
 

Dr. Dickie

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Sep 23, 2020
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Problem with low voltage is that the start to form dendrites. They are not visible but they are there and they cause fire in the end....
And what i know they are not visible to IR either. I cant find any datasheet that states that atleast.
Yeah, and that is what makes me concerned. But, I can't go back do it over. If I could I would.
Thanks
 

Dr. Dickie

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Sep 23, 2020
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344
Fortunately low voltage in a cell does not cause dentrits to grow it is the charging and cycling process esp with high current that does.
This is attributed to the evolution of Li metal morphology during cycling, which leads to dendrite growth and surface pitting. See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5126712/

My personal thoughts and beliefs:
Assuming that dendrites offer a resistance factor till they pierce the membrane and short the cell I believe that IR is affected negatively. I.e. Higher.
Additionally the surface area of the Anode is compromised resulting in poor performance hence capacity loss which we measure with our testers.
As with you I have no documentation for that belief.

Wolf
Well, the good news is, I did all of my charging at 500 mAmps, so maybe I ameliorated some possible damage.
Thanks for the paper, that is good stuff.
 

floydR

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Aug 23, 2017
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1,480
What would be considered high current? max charge listed on datasheet? for LG M26 cells 2500 mA?
I know I have charged a few 39 mOhm cells that my Opus BT-C3400 drops the Amps down to below 100 mA building the ma slowly till approximately 2.8v.
They charge normally after that @ 1000mA and have a Ir of 34 mOhm.
1000mA is 2/5th the max rate
Later floyd
 

daromer

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Oct 8, 2016
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Interesting about the dendrites. I will see if i find the sheets showing low voltage and dendrites. Above only seem to look at those factors.
Could be i misread it.
Dendrites and charging during low temperatures is a big factor atleast.


Wolf: I guess dendrites can form without piercing until it reaches a certain point when it bursts? => no significant resistance change. Though just an idea i have not checked :)
 

Wolf

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Dendrites and charging during low temperatures is a big factor atleast.
That is a subject that I have painfully come to grips with as the temperatures plummeted in the NE part of the US. My batteries are located in a shed that is not heated and lets say somewhat protected but certainly not tightly sealed and insulated. Hence I built my battery box with heating elements. Initially I had 200 Watts of heat which proved to be quite inadequate. I have doubled that to 400 Watts and it seems to "maintain" ~5°C.
I am incorporating new code into my ESP32 for an "emergency" 400W heating element to activate at 1°C if the present 400W still doesn't keep up.
I may also use the batrium expansion board relay for that. I will probably use a Server PS unit for that as they are rated at ~700W and should feed the 400W heating element quite nicely. Since most of the cold snaps happen at night and no charging takes place I am not to worried about cell temp till in the AM when and lately if the sun decides to grace my solar panels.

So what happens when a LI-ion cell gets charged at 0°C?

"When you charge a lithium ion cell in below freezing temperatures, most of the lithium ions fail to intercalate into the graphite anode. Instead, they plate the anode with metallic lithium, just like electroplating an anode coin with a cathode precious metal. So charging will electroplate the anode with lithium rather than, well, recharging it. Some of the ions to intercalate into the anode, and some of the atoms in the metal plating will intercalate later over 20+ hours if the cell is allowed to rest, but most will not. That is the source of the capacity reduction, increased internal resistance, and also the danger."
"This lithium plating of the anode isn't nice and smooth and even - it forms in dendrites, little sharp tendrils of lithium metal growing on the anode."
Ref: https://electronics.stackexchange.c...ge lithium ion,battery that is below freezing.
Additional Ref: https://www.sciencedaily.com/releases/2014/09/140903105638.htm
So you see my concern about charging my batteries at 0°C.
As you can see I am keeping my Battery box as warm as possible. Ignore the lack of temp sensors data on 2,3,4 as I have not installed them yet ran out of warm time.:p
Wolf

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Korishan

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Jan 7, 2017
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I think this thread has gone off topic 🤔
 

enki

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Nov 9, 2019
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Would you think it would be a good idea to implement per-cell MOSFET switches in order to cut off an overheating cell (or cells) if the temperature sensors detect an anomaly?
 

Oleksii

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Mar 18, 2020
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I was thinking about per-4-cells (one sensor in the middle between 4 cells) NTC sensor with an easy placement such sensors on a long and thin "pcb" into already made pack with holders. Them monitor such sensors with a smart algorithm.
But mosfet-per-cell that's unreal, IMO.
 

OffGridInTheCity

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Dec 15, 2018
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I was thinking about per-4-cells (one sensor in the middle between 4 cells) NTC sensor with an easy placement such sensors on a long and thin "pcb" into already made pack with holders. Them monitor such sensors with a smart algorithm.
But mosfet-per-cell that's unreal, IMO.
I have over 8,000 cells in my powerwall. 8000/4 = 2000 sensors. The price of such a scheme would really add up.
 

italianuser

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Feb 25, 2020
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I have over 8,000 cells in my powerwall. 8000/4 = 2000 sensors. The price of such a scheme would really add up.
Yes, too many for me, too.

I was thinking of something, maybe it's exaggerated but I liked the idea...

A thermal sensor moving on x;y axis... They don't cost too much. Yes I would like that. My battery is on one flat shelve... Even on a x;y;z axis would be possible...
 

OffGridInTheCity

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Dec 15, 2018
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Yes, too many for me, too.

I was thinking of something, maybe it's exaggerated but I liked the idea...

A thermal sensor moving on x;y axis... They don't cost too much. Yes I would like that. My battery is on one flat shelve... Even on a x;y;z axis would be possible...
Interesting thought. A Batrium longmon is a smallish 'unit' with temp sensors. Maybe one could have the longmon move over the pack with some kind of mechanism... If the movement wasn't too extreme the +/- power wires and network wires could be made to 'flex' to accommodate the movement. You can even have 2 or 3 longmons per pack if you could afford it - the Batrium config params would allow this.
 

daromer

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A simple approach as i have mentioned since many years and is also used on some places is to have it sealed and meassure temperature on the inlet and the outlet and meassure differences made. You wont find the area but you will find things not working :)
 
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Wolf

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Sep 25, 2018
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is to have it sealed and measure temperature on the inlet and the outlet
Exactly!
Besides in my case having longmon temp sensors on each pack, I have 4 DS18B20 on each corner of my battery box. Including the main BME680 sensor in the middle that controls the heating and cooling. Any odd readings will trigger an alarm to investigate a possible issue.
Wolf
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