Battery Fire (and probable cause)


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elkooo

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Joined
Aug 9, 2019
Messages
32
So this is something that generally is not admitted with pleasure:

I had a battery fire. It was probably caused by a spot welded nickel strip, that worked its way through the shrink wrap on the positive end of a cell (where the edge is already negative...). It was a 10s5p arrangement, housed in an aluminum case.


  • The fire burnt so hot, that it went through the aluminum pack enclosure(~1.5mm wall thickness)in less than 2mins.
  • From the first visible smoke to first flames was around 40secs (reaction time)
  • All cells burnt out through the tight arrangement in the aluminum casing.
  • The flames shot out though the end caps on both sides of the caseand reached around 1.2m in flame lenght (!).
  • The fire burnt full powerfor around 2mins and calmed down then.
  • By this time, the remnants of the cells werered glowing inside the damaged aluminum enclosure.


Luckily I was able to bring the battery outside at first signs of smoke. If the fire happened inside, I would probbably not have a home anymore.

The cause was most probbably due to the nickel strip that cut its way through the shrink wrap. On this battery, the cells were stapled on top of each other and the nickel was bent 180 from one 5p pack to the next one. The bend was sitting in the area, where the outer rim of the cell is already negative right under the shrink wrap(on the + side)... and due to mechanical movement / load it probbably worked its way through the plastic and caused a short. Because the heat was caught in the thight aluminum case, something caught fire (either the plastic or any gases exiting the cells) and the whole pack burnt out in a raging fire.

I post thisa warning for everyone having cells at home. Shorted cells can easily cause harm and fire. I expirienced it myself now and think it was a fair warning (no other damage than the lost pack was caused...). So please be careful!
 

jdeadman

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Joined
Mar 28, 2017
Messages
967
which is the biggest reason to fuse your cells. And for those packs that are high draw that I use (lawnmower) they do not come inside the house
 

Chablism

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Joined
Jan 27, 2018
Messages
177
Can you clarify staples on top of one another.

The molded trays used to hold power tool packs protect this edge

For laptop cells, the industry is now at a point where they double protect this edge with tape.

Tks for sharing
 

rev0

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Joined
Oct 3, 2017
Messages
372
Thanks for sharing. Definitely second the ask for pictures, and more detail (what kind of cells did you use, are they laptop cells or high drain ones?). I wonder if using paper washers on top of the cells like ebike and laptop packs commonly use would have prevented this from happening. I feel like paper has higher wear resistance to plastic. At least it would be an additional layer over the plastic on the cell.
 

cowpen

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Joined
Sep 6, 2018
Messages
47
Scary stuff. Glad you were there to react with enough time to get it outside without getting burned.
 

garolittle

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Joined
Oct 27, 2017
Messages
100
Really glad you were not hurt. Thank you for posting this. Would definitely like to see pictures. Gary.
 

elkooo

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Joined
Aug 9, 2019
Messages
32
A few more Details:

On the plus-side of the cells were no paper insulatorsnor other protectorsmounted... which probably made it possible for the nickel to cut through the shrink wrap and cause theshort.

The cells were conected with folded nickel strips as seen in the sketch:

image_tsibbk.jpg


I think the folds cut through the insulation due to mechanical impact to the pack.
The cells were Sanyo MJ1 Cells (3.5Ah;10A continuous drain).
I reckon 100amps could probably haveeasily been flowing(since the cells can deliver probbably muchmore than the rated 10A... no idea how much... 5cells in parallel, each one delivering estimated20A or more...).
The pack was for high drain application (~35A max drain out of 5p) and therefore the cells were not fused. The main fuse (40A)did not help, because the short happened on the cell level.
Pack was fully charged (4.15v/cell) when the incident happened.

Lession learned:
Never let shortage happen!! NEVER!
Always protect the positive side good as you can!
Whenever possible, fuse cells!
LiIon Fires burn only fora short periodbut savagely, especially when in a tight arrangement /tube-like enclosure!
Cells close together cause next cells to also burn! Distance means more safety!
Aluminum is not fireproof! (Even the cell containers popped open!

For my powerwall project, I am thinking about an arrangement, were on top of the cell compartment (made of concrete sheet material) sits a supply of fine vermiculite. It is only shielded from the below cell compartmentwith amesh & thin paper sheet to pervent it from falling into the battery compartment. Upon a fire, the plastic mesh &paper sheet is burnt immediately, giving way to the vermiculite which can extinguish flames/ insulate the cells at least until further measures are taken.

image_utzmry.jpg

image_lqezqz.jpg
 

OffGridInTheCity

Active member
Joined
Dec 15, 2018
Messages
2,125
@elkooo - do you think that cell fusing would have stopped the fire in this case?

It seems to me that the keys are:
1) NEVER USE DAMAGED CELLS - dents, holes etc.
2) BMS - NEVER OVERCHARGE - saw a great video on this. If overcharge (above 4.2v or max for your Lithium Ion chemistry) current continues for very long (minutes even) a fire can start.
3) BMS - Do over discharge - not quite as obviously catastrophic but when in series, one of the cells/packs can actually go < 0v as the overall battery continues to deliver voltage and I'm not sure but I think this can cause fire? It totally destroys the chemistry of the cells for sure.
4) Short protection - battery design/arrangement, fusing (cell and battery level), physical protection from things falling on to the battery and any wear/tear over time.
5) Avoid heating cells - going beyond specs or testing on charge/discharge amps.
6) Physical protection - ambient temps and some physical protection so you don't hit the battery with tools or jab it with piece of pipe or wire while working, or drop anything on it.

I'm sure there are others.... but these come to mind and but by taking care of all the above they seem safe enough to me so far :)

However, I do wonder about life-span. My bank is perfectly well behaved right now, but what will happen 5 or 10 or 15yrs from now when individual or collective cells fail from too many cycles. Will cells embedded in the packs become dangerous in some way as their capacities go lower and lower in uneven amounts? My plan is to use the packs all the way down to 40-50% of their original (today's) capacity.
 

rev0

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Oct 3, 2017
Messages
372
Elkooo, where did you buy your cells? Just want to ensure they are real LG MJ1 cells, they're one of the most faked cells along with HG2, 30Q, and a few other "high performance" cells. I assume you capacity tested these prior to assembly anyways though. Fake cells definitely forgo the common safety features like PTC/CID.
 

Geek

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Aug 15, 2017
Messages
920
rev0 said:
Elkooo, where did you buy your cells? Just want to ensure they are real LG MJ1 cells, they're one of the most faked cells along with HG2, 30Q, and a few other "high performance" cells. I assume you capacity tested these prior to assembly anyways though. Fake cells definitely forgo the common safety features like PTC/CID.

This is an interesting possibility. Given that the cell vented from the side and not from the top. Even the fact that it caught fire in the first place. It really should not be possible if the cell is fitted with proper protection.
 

elkooo

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Aug 9, 2019
Messages
32
The cells were genuine (they came from a 100% thrusty source). I capacity testet them all and made matching packs (divergence was only 1mAh from one 5p block to all others). The BMS was also OK (since the same pack was running well with old cells...) and is able to handle the new cells.

I think the cell vent mightonly work for a limited amount of gas. As soon as the cells are too hot (fire around the cell), there may be too much pressure and also particles that block the vent. Possibly the venting gas ignited on a glowing hot piece of metal (all was in a tight package).
This firewith severalhundreds of C burning around thecellsmight have caused the vents of the cells tosimply not keep up with the amount of pressure building up. And the containments of these cells are also quite fragile (I did pierce cells by accident before, either mechanically or when I didn't behave propperly during spot-welding).

I think it was just a chain of unlucky events that caused the fire:
1: Short on a + side of one of the packs
2: Ignition of either venting gas or some of the plastic around the cells / in the pack
3: Heat buildup inside the casing and subsequent total loss to fire

The fire would most porobablyhave been prevented by just one or multiple of the following precautions:
fusing on cell level (not really possible due to high drain application)
having extra insulation on + side of cells(paper rings / plastic rings...)
being more careful assembling the pack overall
other form-factor of the pack (powerwall-style...)
no mechanical impact on the pack
 

ChrisD5710

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Sep 13, 2017
Messages
161
This is why I do not use nickel strips. I fuse each cell and use straps to raise the plus busbar away from the cells.

ChrisD
 

Generic

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Oct 23, 2018
Messages
217
Thank you for sharing your experience with cell failure! I agree that you can't really fuse high drain cells. And most high drain cells do not have PTC protection to limit current during a short circuit, so the best thing to do is to have extra insulation on the positive cap of the cell. A thick plastic ring would be better than the paper rings. Maybe double wrap the cell body too? Couldn't hurt.
 

rev0

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Oct 3, 2017
Messages
372
I still feel that paper washers would be better for resistance to mechanical wear (e.g. nickel strip cutting into it) due to the fibers, and better against heat as well. And it must be worth something that it's how nearly every commercially built pack is done!
 

Generic

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Joined
Oct 23, 2018
Messages
217
rev0 said:
I still feel that paper washers would be better for resistance to mechanical wear (e.g. nickel strip cutting into it) due to the fibers, and better against heat as well. And it must be worth something that it's how nearly every commercially built pack is done!

Interesting. Most cells have a plastic washer under the PVC wrapper, and then when packs are built, paper washers are used. I guess you get the best of both worlds that way. Good point!
 

elkooo

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Joined
Aug 9, 2019
Messages
32
I think the washer underneath the shrink wrap is somewhat smaller than the diameter of the cell - at least in the case of those LG-cells - may it be due to smaller fabrication or due to heat used in shrinking in the cells.

Will redo the 10s5p pack but with 3D printed top covers (which get a layer of paper glued on them for the fibers - I like the idea of having two materials in combination for extra safety).
I will probably post the build in a separate thread and put a link in here...
 

Doin it

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Joined
Mar 17, 2019
Messages
331
OffGridInTheCity said:
@elkooo - do you think that cell fusing would have stopped the fire in this case?

It seems to me that the keys are:
1) NEVER USE DAMAGED CELLS - dents, holes etc.
2) BMS - NEVER OVERCHARGE - saw a great video on this. If overcharge (above 4.2v or max for your Lithium Ion chemistry) current continues for very long (minutes even) a fire can start.
3) BMS - Do over discharge - not quite as obviously catastrophic but when in series, one of the cells/packs can actually go < 0v as the overall battery continues to deliver voltage and I'm not sure but I think this can cause fire? It totally destroys the chemistry of the cells for sure.
4) Short protection - battery design/arrangement, fusing (cell and battery level), physical protection from things falling on to the battery and any wear/tear over time.
5) Avoid heating cells - going beyond specs or testing on charge/discharge amps.
6) Physical protection - ambient temps and some physical protection so you don't hit the battery with tools or jab it with piece of pipe or wire while working, or drop anything on it.

I'm sure there are others.... but these come to mind and but by taking care of all the above they seem safe enough to me so far :)

However, I do wonder about life-span. My bank is perfectly well behaved right now, but what will happen 5 or 10 or 15yrs from now when individual or collective cells fail from too many cycles. Will cells embedded in the packs become dangerous in some way as their capacities go lower and lower in uneven amounts? My plan is to use the packs all the way down to 40-50% of their original (today's) capacity.
I wonder about this myself.. I hope all cells will degrade over time equally.. but that might not be what happens. I purchased ev batteries so all cells were the same to try and avoid this.. but in that case there should be something that tells u that individual cells are losing capacity more than others..
 

elkooo

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Joined
Aug 9, 2019
Messages
32
I think single cells loosing capacity in a large bank are not that problemathic (they will be carried through by its neighbours). The larger problem would be cells that self discharge a lot. They may eventually get hot and cause a fire if not fused.
Therefore I think a thermal camera can be a good thing (draw a lot of current from your bank for a while and look for the hot spots... there may be your problems). This all applies for larger banks - smaller high drain packs are a completely different story. There every single cell matters and you better have equal cells!
 
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