Last fire.. :-(

bdserv

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I started a thread about the conductivity of the cell case. Within that thread I linked to another forum members post whos opinion I value.

It's true the amperage between the cells negative terminal and the cells case is tiny. But what would happen if two or more of the cells cases touch each other? Installing an insulator between the cells is a good preventative measure IMO and it's cheap. Lishen specifically states in their spec sheet to not allow the "cans" to touch each other. I wouldn't rely solely on the thin PVC wrap especially in a mobile environment.

To each their own but if providing additional insulation between the cells can possibly prevent a catastrophic event then I will continue to recommend doing so.

During initial cell testing and balancing I was measuring a cell terminal voltage with my DVM. I made an assumption that my cell cases were negative and wedged one of the DVM probes under a bus bar on the negative terminal - not a problem since there is an insulating plastic piece on top of the case and even so, the case is negative anyway. The probe pivoted slightly and made contact with a small unprotected portion of the top of the case and to my surprise, melted the end off of my DVM probe. I was wrong - on these particular cells [Coslight 150 AH] the case is positive and can definitely supply significant current.

I was actually fortunate that this happened because it completely changed my initial plans for my pack design and I made provisions to eliminate any possibility of cell-to-cell case contact.
 

ereams65

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I started a thread about the conductivity of the cell case. Within that thread I linked to another forum members post whos opinion I value.

It's true the amperage between the cells negative terminal and the cells case is tiny. But what would happen if two or more of the cells cases touch each other? Installing an insulator between the cells is a good preventative measure IMO and it's cheap. Lishen specifically states in their spec sheet to not allow the "cans" to touch each other. I wouldn't rely solely on the thin PVC wrap especially in a mobile environment.

To each their own but if providing additional insulation between the cells can possibly prevent a catastrophic event then I will continue to recommend doing so.
+1 on this thought.....since I build mainly for golf carts, I've chosen to follow the factory's build design for EV packs and that includes a separator between each cell. I also have noticed that of the ones I've seen thus far, more than one EV, those separator's are designed to flex for when cells expand/contract. Just something I've put in the back of my mind because they're smarter than I am.
 

Surferdudemi

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+1 on this thought.....since I build mainly for golf carts, I've chosen to follow the factory's build design for EV packs and that includes a separator between each cell. I also have noticed that of the ones I've seen thus far, more than one EV, those separator's are designed to flex for when cells expand/contract. Just something I've put in the back of my mind because they're smarter than I am.
Do you know what the material is?
 

100kwh-hunter

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What I would like to know is, if you have an internal short (due to dendrite formation), how much current can these really carry
Dendrite is occurring/building up with every cycle.
The IR is also building up.
You will notice this in time.
If your string is original a 100Ah, most of us with take it out before it is getting dangerous, meaning:
I don't think someone wants to prolong the "suffering" of the string when it is not able anymore to hold more than 25Ah aka 25% SOH.
Personally i would replace them at 50% SOH.
 

upnorthandpersonal

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Dendrite is occurring/building up with every cycle.
The IR is also building up.
You will notice this in time.
If your string is original a 100Ah, most of us with take it out before it is getting dangerous, meaning:
I don't think someone wants to prolong the "suffering" of the string when it is not able anymore to hold more than 25Ah aka 25% SOH.
Personally i would replace them at 50% SOH.

Yes, exactly, so I do not think that dendrite formation can lead to high current internal shorts that have the potential of causing a fire/venting.
 

100kwh-hunter

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Yes, exactly, so I do not think that dendrite formation can lead to high current internal shorts.
My opinion about a "runaway":
There are other chemestries that get some unbalance and will cause a thermal runaway(famous red sanyo's 18650)
Short circuit due to other causes:
a pack that is NOT properly balanced.
a not matching string, 15s 420Ah cells and one 105Ah cell.
short circuit outside of the cell but in the string.
self discharger
All those causes will let the current go up beyond the cell's limits and will start heating up.
With cell level fuse protection you will and can not have a current bigger than the c rate of the cell.
I like 25% to max 50% of the c rate of the cell on a fuse.
One cell is 200Ah with a c rate of 3! we get 600Ah max, so my fuse will be 150A, max.

When the IR is rising because the forming of dendrites, and you will keep pushing 200A thru a cell that can handle not more than 50A...
It will heat up, the other cells are still 80%SOH and one cell is 25%SOH, so all the healty cells will give there power into that one cell with charging without a fuse, with fuse it will self disconnect and safe.
 
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MattiFin

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When the IR is rising because the forming of dendrites, and you will keep pushing 200A thru a cell that can handle not more than 50A...
It will heat up, the other cells are still 80%SOH and one cell is 25%SOH, so all the healty cells will give there power into that one cell with charging without a fuse, with fuse it will self disconnect and safe.
Cell-level fuses won't help anything with rising IR situation:
In parallel connection the cell with highest IR takes smallest current of them all.
 

bdserv

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Yes, exactly, so I do not think that dendrite formation can lead to high current internal shorts that have the potential of causing a fire/venting.

The following is from https://spectrum.ieee.org/less-fire-more-power-the-secret-to-safer-lithiumion-batteries#toggle-gdpr

"A Lithium-ion battery pack is invariably composed of one or more compartments, or cells, each of which has two electrodes covered by an extremely thin polymer film, called a separator, which prevents their coming into direct contact. Permeating the porous separator is the electrolyte, a material—today generally a liquid—that allows lithium ions to move back and forth during charging and discharging.

The slightest damage to the ultrathin separator can put the electrodes into direct contact and create an internal short circuit, which can generate enough heat to make the cell catch fire. The heat of the fire may then overheat adjacent cells, resulting in a chain reaction that can easily cause the whole battery pack to explode.

So it’s the integrity of the cell’s separator that matters most. Of course, every effort must be made during the manufacturing process to prevent damage to the separator, but even a perfectly fabricated separator can fail in operation if dendrites later damage it.

Dendrites are sharp bits of lithium metal that grow from the anode. These fibers can spread like kudzu vines into the electrolyte, pierce the separator and make their way to the cathode. It’s amazing how such tiny little things can cause so much destruction: They were responsible, for example, for the fires that grounded the worldwide fleet of Boeing 787s in 2013.

Dendrites tend to grow when the battery is overcharged, because that’s when the lithium ions migrating into the anode can no longer find a berth. Normally, the ions slip between the atomic layers of the anode, a process called intercalation, but when the space between the layers is all filled up—as can happen during overcharging—there’s nowhere else for the lithium to go but onto the surface. There they form the seeds of a metallic crystal, which grows with each new charge-discharge cycle."
 

upnorthandpersonal

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The slightest damage to the ultrathin separator can put the electrodes into direct contact and create an internal short circuit, which can generate enough heat to make the cell catch fire. The heat of the fire may then overheat adjacent cells, resulting in a chain reaction that can easily cause the whole battery pack to explode.

Yes, with battery chemistry that inherently exhibits thermal runaway. LiFePO4 is no such chemistry.
 

bdserv

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Yes, with battery chemistry that inherently exhibits thermal runaway. LiFePO4 is no such chemistry.

Technically, even LiFePO4 chemistry exhibits thermal runaway under the right [wrong] conditions, but at least in the studies I've read the effect is nowhere near as significant as it is in other lithium-based chemistries, and when it does occur it typically does not spread to adjacent cells. That aside, I get what you're saying.
 

100kwh-hunter

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Cell-level fuses won't help anything with rising IR situation:
In parallel connection the cell with highest IR takes smallest current of them all.
Sorry, no my friend. it is just the other way around...
They keep pumping A's into that poor cell...swallow or choke...a fuse prevents both..
They want to balance...hence a self discharger...more dendrite forming, hence higher ir henche heating up...
Now this circle you will prevent with propper fusses, and regulair testing will save you a good fuse....

The slightest damage to the ultrathin separator
Indeed...when the internal layers CAN connect, a fire WILL occour.
So it’s the integrity of the cell’s separator that matters most. Of course, every effort must be made during the manufacturing process to prevent damage to the separator, but even a perfectly fabricated separator can fail in operation if dendrites later damage it.
In the factory there are NO dendrites, you will only get dendrites when you use the cells...
But when we discard of the cells? 0.001% SOH or 25%SOH
In the factory they must watch out for MOISTER and there vacuum to be correct...

So it’s the integrity of the cell’s separator that matters most. Of course, every effort must be made during the manufacturing process to prevent damage to the separator, but even a perfectly fabricated separator can fail in operation if dendrites later damage it.
sounds like a factory i will not buy my cells from....first take a deep dive into the fabrication processes.

Dendrites tend to grow when the battery is overcharged, because that’s when the lithium ions migrating into the anode can no longer find a berth. Normally, the ions slip between the atomic layers of the anode, a process called intercalation, but when the space between the layers is all filled up—as can happen during overcharging—there’s nowhere else for the lithium to go but onto the surface. There they form the seeds of a metallic crystal, which grows with each new charge-discharge cycle."
Dendrites will form with EVERY charge or discharge, from 0.01% higher and up.....
Even sitting for one year at full SOC or even 15% SOC, they will form.
If the SOC is optimum for the cells chemistry it will not form, every chemistry has its "dead" point.
So it is not only with a "over" charge or voltige...
Yes, with battery chemistry that inherently exhibits thermal runaway. LiFePO4 is no such chemistry.
Read my other comments again, and yes...you are right, lifepo is not such a chemistry that will turn into a heater or a self discharger...
Long before this will happen with lifepo., most power-wall and ess users will take out those cells out of service, simply due to SOH...
And the chemestry itself will not allow with normal use to self combust no matter what the cause can or would be...

Technically, even LiFePO4 chemistry exhibits thermal runaway under the right [wrong] conditions
You are right...indeed if you would "go for it" you can manage a thermal runaway, if you don't go for it you won't manage a runaway

With best regards Igor
 

fafrd

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Yes, with battery chemistry that inherently exhibits thermal runaway. LiFePO4 is no such chemistry.
I saw the statement that for Lithium batteries in general, dendrites form especially when cells are overcharged.

Do we know whether this is true for LiFePO4 as well?

Between discharging below 10% and charging above 90% (or 85%), is there any understanding as to which is ‘worse’ from the point of view of LiFePO4 cell capacity degradation in general and dendrite formation in particular?
 

fafrd

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Yes, this has been confirmed in the literature:

So does that suggest that it is better to minimize maximum charging than to minimize maximum discharging?

For various reasons, I’ve switched from a top-balanced LiFePO4 battery to a bottom-balanced battery (full discharge nightly, variable never-to-full charge depending on daylight / solar production).

I’ve chosen this because it’s a much better fit for my use case (time shift) but now I’m wondering whether it’s better for a LiFePO4 battery (or any lithium battery, for that matter) to be drained to ~10% daily and never charged close to full than to be charged to ~85% or 90% daily and never discharged close to empty…
 

upnorthandpersonal

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The thing one should take away from all this is that over-charging (and keeping a battery at high voltage) and over-discharging are both impacting cycle life and that dendrite formations lead to micro-shorting which eventually lead to cell failure. Using a cell within its normal parameters will also cause issues over time (calendar aging + cycle life) but this is of course normal. However, from the document one can infer that these micro-shorting due to dendrites won't lead to catastrophic venting or thermal runaway, but instead a steady decline in performance and increase in losses manifested through warming of the cells near end of life.
 

Bob B

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So does that suggest that it is better to minimize maximum charging than to minimize maximum discharging?

For various reasons, I’ve switched from a top-balanced LiFePO4 battery to a bottom-balanced battery (full discharge nightly, variable never-to-full charge depending on daylight / solar production).

I’ve chosen this because it’s a much better fit for my use case (time shift) but now I’m wondering whether it’s better for a LiFePO4 battery (or any lithium battery, for that matter) to be drained to ~10% daily and never charged close to full than to be charged to ~85% or 90% daily and never discharged close to empty…

I've seen posts indicating that the temperature increases in the cells significantly at the bottom end of the SOC compared to the top.
I have never verified this myself, but it might be worth your time to do some testing .... increased temps in the cells in another thing that can shorten the cycle life.
 

fafrd

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I've seen posts indicating that the temperature increases in the cells significantly at the bottom end of the SOC compared to the top.
I have never verified this myself, but it might be worth your time to do some testing .... increased temps in the cells in another thing that can shorten the cycle life.
Good suggestion - thanks.

What maximum discharge levels are recommended for being ‘gentle’ and extending cycle-life?

I’ve been discharging my 8S pack to 25.3V (or ~3.16V on my well-bottom-balanced pack) at a discharge rate of ~20A or less than 0.04C and the voltage pretty quickly snaps back to 25.65V or just over 3.2V per cell soon after low voltage disconnect…
 

Supervstech

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I've seen posts indicating that the temperature increases in the cells significantly at the bottom end of the SOC compared to the top.
I have never verified this myself, but it might be worth your time to do some testing .... increased temps in the cells in another thing that can shorten the cycle life.
This kind of makes sense from a specific load standpoint... say a 1000W load at 3.4v per cell vs 2.8V per cell... as the voltage drops, the amps go up,causing the cells to heat up.
 
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