Last fire.. :-(

Surferdudemi

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Have not watched this thread for awhile, but reading through the original post, I'm wondering if using concrete board might be a problem. The bottoms of the (Lishen) cells I have are not covered with the blue insulation. Concrete attracts moisture at high RH, so is it possible that the cells are shorting through the bottoms (which are connected to the positive pole) when the concrete board gets moist or if water drips onto it? The negative pole of the cell is connected to the positive pole of the next cell, so if you short the bottoms of the cells, you're going to discharge that cell.
 

bgflyguy

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Has anyone shown that shorting the cases can cause fire?

"Seems like" "it would make sense" etc aren't proof.

These cells have been tested with dead shorts, over charging, and high temp charging, but they only seem to catch fire on the most agressive puncture tests or when they are literally in a fire.

It unfortunate this happened, but these cells are still a very safe chemistry. For every 100 cells we buy, there are a 1000 going into buses etc.
 

100kwh-hunter

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I read some very interesting arguments against what ever problem a cell can have...
Yes we can put pressure sensors heating sensors bms ect to protect a string and what ever system that we can think of...
But arent we forgetting the most important thing?
Cell level fuse?
The fuse will blow when the cell is asking to much aka a self discharger, short circuit or a heater.
This will stop other cells to put more energy into that cell so a heater is out of the question.
The bms will register a faulty string and shut that string down, if the bms does not do this other fuses will blow.
Oke a lot of extra work those fuses, but less work than rebuilding your home.

Or am i missing the point?
Cell level fusing, string level fusing and pack fusing.
Over v cut off, under v protection and one temp sensor to cut off charging below 0C and cutt off charge/discharge above 45C.

With best regards Igor
 

MattiFin

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I read some very interesting arguments against what ever problem a cell can have...
Yes we can put pressure sensors heating sensors bms ect to protect a string and what ever system that we can think of...
But arent we forgetting the most important thing?
Cell level fuse?
The fuse will blow when the cell is asking to much aka a self discharger, short circuit or a heater.
This will stop other cells to put more energy into that cell so a heater is out of the question.
The bms will register a faulty string and shut that string down, if the bms does not do this other fuses will blow.
Oke a lot of extra work those fuses, but less work than rebuilding your home.

Or am i missing the point?
Cell level fusing, string level fusing and pack fusing.
Over v cut off, under v protection and one temp sensor to cut off charging below 0C and cutt off charge/discharge above 45C.

With best regards Igor
This might work well with small cells when you have 100pcs 2.8ah cells in parallel. Not so helpfull when you have 280ah cell with no access to internal parallei cells.
Invidual bms for each battery string works somewhat similar and that’s what OP also recommended with hindsight.
And large fuses are also cost-prohibitive to be used at cell level.
 

fafrd

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I read some very interesting arguments against what ever problem a cell can have...
Yes we can put pressure sensors heating sensors bms ect to protect a string and what ever system that we can think of...
But arent we forgetting the most important thing?
Cell level fuse?
The fuse will blow when the cell is asking to much aka a self discharger, short circuit or a heater.
This will stop other cells to put more energy into that cell so a heater is out of the question.
The bms will register a faulty string and shut that string down, if the bms does not do this other fuses will blow.
Oke a lot of extra work those fuses, but less work than rebuilding your home.

Or am i missing the point?
Cell level fusing, string level fusing and pack fusing.
Over v cut off, under v protection and one temp sensor to cut off charging below 0C and cutt off charge/discharge above 45C.

With best regards Igor
I believe so, yes.

When a cell is shorted, either internally or externally, it is not the ‘other cells’ that are providing the current that heats the shorted cell up - it is the cell itself.

In fact, no current will flow from other cells as the shorted cell discharges - they will all maintain full voltage and overall battery will decline just because the voltage if the shorted cell will drop as it discharges.

There is no way to stop a shorted cell from discharging using fuses. The best you can hope for is an alarm causing you to check the battery before the situation has become catastrophic.

If a wire has come loose and caused the cell to short externally, you may be able to break the short and stop cell discharge.

If terminal damage or whatever has caused a cell to short internally, there is nothing you can do to stop it. If it has not gotten too hot yet, you may be able to shut the battery down, remove connections to that cell and remove that cell from the battery (watch it discharge in the middle of the yard ;), but standing by with a fire extinguisher ;and possibly an infrared thermometer may be the best you can do.

The inter-cell fuses your are proposing might held avoid problems if a loose wire causes a short between cells, but that’s really not the area of concern.

Several members are suggesting that any cell failure / degradation that can cause internal shorting and increased self-discharge will be gradual and cannot result in sudden high-current discharge sufficient generate enough heat to cause a fire.

That’s comforting and goes a long way towards addressing my concern, since with proper design, it should be straightforward to design a battery such that shorting caused externally can be pretty much impossible.

Even though I suspect these aftermarket cheapo cells are sub-prime (not suited for use in EVs), I’m not terribly concerned about them being truly defective (ala defective LG Lithium cells in some Chevy Bolts with folded tabs).

So if substandard cells that begin to fail through accelerated growth of dendrites and increased self-discharge is a gradual process that can be noticed and reacted to by just keeping an eye on battery performance (monthly?), I’m starting to come around to the prevailing view that DIY risk may be low and acceptable (with monitoring).

I’m still extremely uncomfortable with the idea of putting a DIY LiFePO4 battery in someone else’s house were no one will be keeping an eye on it (‘set it and forget it’).
 

upnorthandpersonal

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Several members are suggesting that any cell failure / degradation that can cause internal shorting and increased self-discharge will be gradual and cannot result in sudden high-current discharge sufficient generate enough heat to cause a fire.

What I would like to know is, if you have an internal short (due to dendrite formation), how much current can these really carry? Also, how much heat does it really take to ignite the boiling electrolyte. For example, this video has been around for a while:


Personally, I've also done tests like this in my lab. I've never had a LiFePO4 cell catch fire (I know, anecdotal) unless there was an active source of sparking/ignition near venting electrolyte. Burning electrolyte is a lot easier to suppress than a thermal runaway lithium fire.
 

fafrd

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This might work well with small cells when you have 100pcs 2.8ah cells in parallel. Not so helpfull when you have 280ah cell with no access to internal parallei cells.
Invidual bms for each battery string works somewhat similar and that’s what OP also recommended with hindsight.

And large fuses are also cost-prohibitive to be used at cell level.
I’ll have to go back and read that post, but I’m not getting the concern with parallel cells (and would appreciate an explanation if you understand it).

I’ve built a 560Ah 24V 8S pack where each cell is a 2P pair of 280Ah cells (can never remember whether that is 2P8S or 8S2P).

In the case that sudden high-current shorting is possible, my 2P ‘cells’ will result in double the total discharge energy but BMSs are going to be useless to prevent the cell discharging and heating up, whether 1 BMS on a 2P battery or 2 BMSs on two 1P battery.

And I the case that and internal short will be gradual and not suddenly so sufficient to generate high heat, having a second cell discharging through the shorted cell is not going to really change the situation (all currents and resulting heat generated will be doubled, worst-case).

The paralleled cell will get discharged below LVD and may get damaged as a result (so that is an argument in favor of two 1P strings each with its own BMS versus a single 2P string with a single BMS, but avoiding loss of a second cell when one fails is not a concern for me.

In terms of the (very low) risk of a fire being starters by a failing cell, I’m not seeing how a pair of 1P batteries is any safer than a single 2P battery…
 

MattiFin

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I don't think there is such a thing? A 280Ah LiFePO4 cell is just that: a single cell, just a large one (volume wise) - or am I missing something?
Internally there is several smaller cells connected together.
In this example 105 Ah battery has two cells
(look after 20 minutes)
 

MattiFin

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In terms of the (very low) risk of a fire being starters by a failing cell, I’m not seeing how a pair of 1P batteries is any safer than a single 2P battery…
in 1P confiquration the energy supplied to internal short comes only from one cell. in 2P you have twice as much energy dumped to one failing cell and cooking it potentially lot hotter.
Tesla folks were smart enough to limit the enegy dumped to one cell with fuse wires. dumping all of the energy from 100 parallel batteries to one failing cell would make it reaaaally hot.
 

upnorthandpersonal

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Internally there is several smaller cells connected together.
In this example 105 Ah battery has two cells
(look after 20 minutes)

Right - I don't really consider those individual cells in the typical understanding of cells in the context of the discussion, unless I missed something. I would just consider one whole 105Ah 'cell' in that video as the cell, not like a battery of individual 18650s.
 

fafrd

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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? Also, how much heat does it really take to ignite the boiling electrolyte. For example, this video has been around for a while:


Personally, I've also done tests like this in my lab. I've never had a LiFePO4 cell catch fire (I know, anecdotal) unless there was an active source of sparking/ignition near venting electrolyte. Burning electrolyte is a lot easier to suppress than a thermal runaway lithium fire.
That’s a great video, isn’t it ;).

And I agree, increasing self-discharge current profile from dendrite formation would be would be the most helpful data to have.

If it’s a gradual process, we can all eat pretty easy on this concern. If it can suddenly jump from nothing to hundreds of amps, we need to protect against it.

In my 2P8S battery, I’ve got a pair of cells behaving very differently than the other 7 pairs (after top-balancing and capacity testing all 16 cells).

That pair mysteriously and suddenly is delivering ~20% lower capacity than the other 7 and exhibiting a noticeably different charge/discharge curve.

I suspect one or both of the cells in that pair may be failing for the reasons we’ve been discussing or that that cell suffers from some manufacturing defect.

My capacity testing was all done at 10A and that one pair of cells behaves noticeably different than the others when discharged at 80A and charged at 40A.

The battery functions and so I’m just observing and characterizing for now and my immediate objective is to determine whether performance of the problem pair is steady state or continues to degrade.

The fact that folks like you have performed your own safety tests is comforting and the more convinced I become that cell failure from dendrite formation is a gradual process, the less concerned I’ll be in this whole subject.

Venting electrolyte seems like something it would be pretty easy to get an early warning on (and almost certainly something that could be detected early with a pack-level pressure sensor). Since I’ve got no potential sources of sparks / ignition anywhere near my battery, I’m not overly concerned about venting electrolyte bursting into flame.

(Though there is a gas hot-water heater as well as a gas furnace in the next room with airflow between them, so if you tell me an explosion from vented electrolyte dispersed over that kind of distance / volume is something I should be concerned about, that is an input I would take seriously).
 

fafrd

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Internally there is several smaller cells connected together.
In this example 105 Ah battery has two cells
(look after 20 minutes)
Whether it is a single roll or two individual rolls, these cells are essentially a long continuous ‘sheet’ / cell with a large number of small parallel connections.

There is really no difference if the cell is composed of a single jelly roll or two - it’s shorted self-discharge current across the membrane that is the concern (where the dendrites form).

Even in the (avoidable) case of mechanical damage to the terminals (sitting on a terminal), it’s unlike that there is any difference between a cell composed of a pair or rolls versus just one.
 

MattiFin

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Right - I don't really consider those individual cells in the typical understanding of cells in the context of the discussion, unless I missed something. I would just consider one whole 105Ah 'cell' in that video as the cell, not like a battery of individual 18650s.
Thats part of the point, you don’t have access or means to separate those internal connections in case of catastrophic failure. Yet you have huge amount of energy available to the fault.
”Tesla style” 100x 3Ah parallel pack with invidual fuses sounds better to me than large capacity 1x300Ah ”all the eggs in same basket” cell.
3x100Ah pack with invidual fuses might be marginally better but probably impractical.
 

upnorthandpersonal

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Thats part of the point, you don’t have access or means to separate those internal connections in case of catastrophic failure. Yet you have huge amount of energy available to the fault.
”Tesla style” 100x 3Ah parallel pack with invidual fuses sounds better to me than large capacity 1x300Ah ”all the eggs in same basket” cell.
3x100Ah pack with invidual fuses might be marginally better but probably impractical.

Yep, I see your point. However considering that the market seems to be going towards higher capacity cells (Tesla 9Ah 4680 as well), plus the impracticality you mention, this might be a purely theoretical exercise.
 

Posplayr

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Agreed (in my case, add making flexible bus bars). However using something like MG Chemicals 847 makes for a good and stable connection compared to dielectric grease, and helps even out the contact with the terminals.

I would use DeOxit (or something similar) to chemically strip any microscopic corrosion as opposed to sanding. I even spray this into the terminal ends before crimping (which supposedly produced gas-tight seals). Glue-filled heat shrink completes the weatherization of the crimp, but you need some type of Noalox for the open terminals.

[Tom posted this so I will correct my statement above]

That is the wrong application for dielectric grease, you should use a product such as Noalox that improves conductivity and prevents corrosion.

Dielectric grease is for applications like inside a terminal block - to repel water and prevent fretting corrosion.


 
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Gazoo

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Whether it is a single roll or two individual rolls, these cells are essentially a long continuous ‘sheet’ / cell with a large number of small parallel connections.

There is really no difference if the cell is composed of a single jelly roll or two - it’s shorted self-discharge current across the membrane that is the concern (where the dendrites form).

Even in the (avoidable) case of mechanical damage to the terminals (sitting on a terminal), it’s unlike that there is any difference between a cell composed of a pair or rolls versus just one.
This is one of the other fires I was thinking of:

 

Surferdudemi

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I tested some concrete board. Got it wet and measured the resistance. It's about 1Mohm, so that's not likely the culprit here. Also saw on another thread that although the case is positive, there's about 1500 ohms between that and the positive terminal, so conduction through the case also doesn't seem like an issue (6 mW of energy through that 1500 ohms also doesn't seem like it would heat up much over time).

Has anyone inspected the surfaces where the cells are stacked together in any of these incidents to see if there is any evidence of scorching or conduction? I know that many of them are pretty well wrecked from the photos, so maybe can't be seen.
 

Gazoo

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I tested some concrete board. Got it wet and measured the resistance. It's about 1Mohm, so that's not likely the culprit here. Also saw on another thread that although the case is positive, there's about 1500 ohms between that and the positive terminal, so conduction through the case also doesn't seem like an issue (6 mW of energy through that 1500 ohms also doesn't seem like it would heat up much over time).

Has anyone inspected the surfaces where the cells are stacked together in any of these incidents to see if there is any evidence of scorching or conduction? I know that many of them are pretty well wrecked from the photos, so maybe can't be seen.
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.
 

Pyrofx

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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.
But Gaz , it cost me 9 bucks to insulate my 16 cell bricks. And it made the pack 3/32" thicker when done. 😜.
I used the cutting board / chop sheets. Tough thin plastic and never to worry about contact.
 
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