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Battery Fire (and probable cause)
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 case and reached around 1.2m in flame lenght (!).
  • The fire burnt full power for around 2mins and calmed down then. 
  • By this time, the remnants of the cells were red 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 this a 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!
Fulstein, Korishan, rev0 And 5 others like this post
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
100kwh-hunter likes this post
******Hi My name is Jason and I have SOCD (Solar Obsessive Compulsive Disorder)*******
Current Powerwall - 1400 Cells 7s200p (modular 40p packs) ~ 12kWh of storage     4x 315W Canadian Solar Panels

Working on the next 7s40p packs     ~2.5kWh

Waiting on 2000 Cells of unused Sony vt4 (2000mah 30A) ~ 15kWh      hehehehehe  More Power
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
Can you upload some pictures to show what you think the problem was? Were you using traditional 4x5 holders that somewhat keep the nickel off of the batteries? Or was your nickel sitting right on top of the cells?
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.
Scary stuff. Glad you were there to react with enough time to get it outside without getting burned.
Really glad you were not hurt. Thank you for posting this. Would definitely like to see pictures. Gary.
A few more Details:

On the plus-side of the cells were no paper insulators nor other protectors mounted... which probably made it possible for the nickel to cut through the shrink wrap and cause the short.

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

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 have easily been flowing (since the cells can deliver probbably much more than the rated 10A... no idea how much... 5cells in parallel, each one delivering estimated 20A 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 for a short period but 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 compartment with a mesh & 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. 

OffGridInTheCity, hbpowerwall, bogptrsn like this post
@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 Smile

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.
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.

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