Another Tesla Megapack fire

[bdunham7]

https://www.vice.com/en/article/n7zd5z/california-county-tells-people-to-shelter-in-place-because-of-tesla-battery-fire

[rooppoorali]

The CEO is silent about it on his Twitter account. Disappointing.

[Marco]

I don't understand how these things could have a significant fire with the door closed, turn on fire hose when fire is detected and they should be full of water in no time.

[BradC]

turn on fire hose when fire is detected and they should be full of water in no time.

They are, and they keep burning. There are three parts to the fire triangle. Fuel, Oxidiser and Heat. When your "widget on fire" supplies both its own fuel and oxidiser then the only thing you can remove is heat, and when you have no way of removing the heat fast enough then it keeps burning.

All the water does is keep the surrounding stuff from going up while it burns out the entirety of the fuel source.

[Marco]

AFAIK the small amount of metallic lithium and oxygen from electrolysis is almost irrelevant, it's the electrolyte which is burning with nearly all oxygen coming from the air.

Even if electrolysis could supply sufficient oxygen, few reactions can sustain high enough temperatures under water to keep burning.

[sandalcandal]

AFAIK the small amount of metallic lithium and oxygen from electrolysis is almost irrelevant, it's the electrolyte which is burning with nearly all oxygen coming from the air.

Even if electrolysis could supply sufficient oxygen, few reactions can sustain high enough temperatures under water to keep burning.

The oxygen comes from the decomposition of metal oxides in the electrodes/SEI and works similarly to a thermite reaction. Flammable electrolytes still contribute to the overall heat released though.

Recent-ish overview article here: https://onlinelibrary.wiley.com/doi/full/10.1002/bte2.20210011

[Marco]

I don't see anything in there of the Joules available, which is the crux of the matter. If most of the available chemical energy is in the electrolyte, liberated when burning in free air, it won't do much under water.

Given they are dumping EVs into submersion tanks I think it's less thermite and more a flash in the pan after large batteries go under water, but I can't quickly find experiments.

[sandalcandal]

I don't see anything in there of the Joules available, which is the crux of the matter. If most of the available chemical energy is in the electrolyte, liberated when burning in free air, it won't do much under water.

Given they are dumping EVs into submersion tanks I think it's less thermite and more a flash in the pan after large batteries go under water, but I can't quickly find experiments.

The only people I've seen suggesting/trying submersion as a solution to controlling lithium ion battery fires are municipal services. I wouldn't take those attempts as indication that aerobic combustion is the main source of energy in lithium ion battery fires. I haven't seen much in the academic literature I have read that considers submersion as a solution.

Submersion however does seem to be like a decent idea for helping ensure re-ignition is not a concern as is often the case after EV battery fires have initially extinguished (e.g. as was an issue with the Chevy Bolt https://www.nhtsa.gov/staticfiles/nvs/pdf/Final_Reports.pdf)

But back to the main topic:
"Thermal runaway mechanism of lithium-ion battery with LiNi0.8Mn0.1Co0.1O2 cathode materials" July 2021, Nano Energy
https://www.sciencedirect.com/science/article/abs/pii/S2211285521001361
https://www.sciencedirect.com/science/article/am/pii/S2211285521001361 [publicly accessible version]
"The cathode liberated oxygen species (O2, O2-, O-, etc.) is considered to be blamed for the thermal runaway."
"The redox reaction between cathode and anode is the main heat source during thermal runaway."

I recall seeing similar findings in other papers I've read but please feel free to provide any contrary research.

However, they do attribute the released oxygen combusting with the electrolyte as a "triggering factor" albeit again, the redox of the electrode material is the "main heat source".
"The reaction between oxygen species and electrolyte is the triggering factor of thermal runaway. "

Regardless of that note however, I think BradC is correct in saying

They are, and they keep burning. There are three parts to the fire triangle. Fuel, Oxidiser and Heat. When your "widget on fire" supplies both its own fuel and oxidiser then the only thing you can remove is heat, and when you have no way of removing the heat fast enough then it keeps burning.

All the water does is keep the surrounding stuff from going up while it burns out the entirety of the fuel source.

You can't "put out" a lithium ion battery fire simply by flooding the battery systems with water because there is an internal source of oxidiser [and the heat from thermal runaway cannot be quenched sufficiently with such a method].

[bdunham7]

You can't "put out" a lithium ion battery fire simply by flooding the battery systems with water because there is an internal source of oxidiser [and the heat from thermal runaway cannot be quenched sufficiently with such a method].

The recommended firefighting procedure is lots of water, followed by an extended dunking in some places.  The theory is that while perhaps you can't extinguish a burning cell, the cooling can prevent the fire from spreading to as-of-yet unignited cells.  Whether quenching can halt the fire in a burning cell depends on other factors, it isn't impossible.  In theory you could put out a thermite fire with gasoline, although I wouldn't recommend trying that.

[Marco]

Even if the energies are comparable under water, above water it's not a very high energy/power density fire relative to thermite to begin with. It can't touch steel or aluminium in an immersion cooled situation. It would belch some smoke bubbles at most.

[Siwastaja]

AFAIK the small amount of metallic lithium and oxygen from electrolysis is almost irrelevant, it's the electrolyte which is burning with nearly all oxygen coming from the air.

Even if electrolysis could supply sufficient oxygen, few reactions can sustain high enough temperatures under water to keep burning.

It's not metallic lithium, it's not electrolysis, and it's not the electrolyte, the oxidizer is in the CATHODE material itself. It might not be obvious because the modern naming scheme leaves the oxide out, but it definitely is there - originally lithium cobalt oxide (LCO), later lithium manganese oxide (LMO) or, what Tesla uses in its vehicles (don't know about megapacks), lithium nickel-cobalt-aluminimum oxide (NCA).

Burning electrolyte of course makes things extra fun because it can shoot out, but electrolyte indeed does not have oxidizer in it, it requires external air to burn, you are right about that. You are just missing the cathode completely.

During thermal runaway, collapsing cathode provides so much thermal energy that significant cooling efforts are needed to stop the reaction. The problem is, water can't get everywhere. Good mechanical design would allow water surrounding the whole battery pack AND circulate efficiently, but think about it, if you still have a localized hotspot, it can easily burn through the envelope which tries to keep water in, and if that happens, all the water in your fancy cooling system leaks out. The solution is to just pump ridiculous amounts of water through the battery pack, or at very least, try to cool everything else around and let it burn.

[Marco]

I'm not talking about a circulating cooling system, just plain pool boiling. Make the metal enclosure relatively water tight (can still have some drain, just needs to be small capacity) and have the sprinkler fill it up, assuming no one left the door open. It's not going to burn through a couple mm of steel in a pool of water, it's not thermite.

[Siwastaja]

The problem is the super low thermal runaway onset temperature of around 160degC or so in LCO/NCA chemistries. Stopping on-going thermal runaway might be even more difficult, but at very least it means cooling every internal spot of every cell cell below 160degC. Given high heat output, and limited thermal conductivity of cell materials, this is impossible by regulating cell envelope temperature by boiling - especially since boiling causes voids (gaseous water conducts heat more poorly than liquid water).

This is why it's almost impossible to stop thermal runaway once it's started. If the cells are small, with cooling channels between each cell, and you pump 20degC water at massive flow rate evenly through the whole thing, it is remotely possible. If you expect the evaporating water to do the trick - no dice. So basically with the pool boiling solution, you let it burn, and it will produce smoke, and then you just hope the pool prevents it from propagating within pack.

Sure, given enough material thickness and enough water, then you can just the thermal runaway go to completion, but the fact they don't do this suggests it requires too much materials and weight.

The total energy released during thermal runaway of a fully charged cell is approximately twice the stored electrical energy (e.g., see https://www.fire.tc.faa.gov/pdf/TC-TN16-22.pdf ). Thus a 50-gram 15860 cell, storing ~30Wh = 108 000J of failure energy, would require 108 000J/ (2257 J/g) = 47 grams of water.

This is an interesting finding, so if you have 1 ton of batteries, another ton of water would be required to fully evaporate during thermal runaway. In reality, you would need significantly more, say double that, so that only excess is evaporated, and all cells keep submerged. While at it, you would want thick enough metal walls so that blowtorch effect in localized hotspots (remember, the cells have internal oxidizer, so this can and will happen underwater) can't make a hole causing water to drain.

The net effect would be destruction of the gravimetric and volumetric energy density. Battery storage is already expensive and large; engineering it completely fire safe would likely kill the whole business model. So instead we seem to accept the risk that these things sometimes burn, producing a lot of toxic smoke, and fire brigade trying to control it by pumping a lot of water through the failed unit and protecting the neighbor units.

[james_s]

I wonder why these are catching fire in the first place. Properly used good quality lithium ion cells should be pretty robust and unlikely to catch fire. Has there been any results to the investigation of the cause?

[Marco]

you would want thick enough metal walls so that blowtorch effect in localized hotspots

It will not have the speed and focusing to put any real sustained heat on any spot. It won't even touch the walls under water. Need just enough steel to withstand water pressure with some ribbing and way it's going to burn through, or do much burning for that matter. It will trigger the sprinkler long before the entire pack is on fire.

[NiHaoMike]

Have they thought about putting the batteries on boats? If one catches on fire, sink it.

[Marco]

Open water is not available everywhere. A couple IBCs full of demineralized water and a 300 gpm pump can be put any where.

The existing enclosures could probably already withstand the pressure of 2 meter of water, it's just the access doors which can't.

[Siwastaja]

I wonder why these are catching fire in the first place. Properly used good quality lithium ion cells should be pretty robust and unlikely to catch fire.

The problem is in massive quantity. Tesla surely uses good quality lithium ion cells, but if you have metric gazillion of them, "unlikely" becomes reality every now and then.

Li-ion is fundamentally volatile and manufacturers have done great job managing the risks, but it is impossible to do perfect job. Small manufacturing defects are enough, and some go unnoticed no matter how good quality management system you have.

Now the interesting question is, should they concentrate into making the cells more safe, or to put the same resources into managing the risk outside the cells as discussed in this topic. Easy answer is "do both", but this all needs to be engineered for a certain price point.

[james_s]

The thing that worries me is these same sells are as far as I know getting used in the cars too, also in large numbers. So far the cars don't seem to be catching fire very often, but once there are a lot more of them I wonder if that will change. There must be a lot more batteries in cars overall at this point than in megapacks.

[Marco]

If the edge cases become too common, they might start needing special tow trucks which can keep cooling down battery packs during transport, using those under the car sprayers which are being brought out left and right. So they don't run the risk of suddenly towing a flaming wreck.

Maybe even mandate an external connection to the cooling system which can force through water through the battery packs if enough of the structure survived (one way, with the pressure just breaking a seal). I think that makes more sense than ramming an injection spike into the battery pack.

[Siwastaja]

Or, just accept what it is. Gasoline cars burn left and right all the time, no one blinks an eye. Buildings burn left and right, and you are still allowed to wrap the building in bitumen and styrofoam so that the whole thing burns like a torch. People are only interest in improving the safety to a certain level, and li-ion safety is certainly within the general level of safety already.

[Marco]

Or, just accept what it is. Gasoline cars burn left and right all the time

Partly why they have fire fighting foam in the first place.

Changing situation, changing optimization of equipment investment. Clearing roads after accidents ASAP has large economic incentives, so special equipment will likely start making sense, assuming the dominant battery chemistry doesn't change soon.

[NiHaoMike]

Clearing roads after accidents ASAP has large economic incentives

What about minimizing the number of accidents in the first place? More mature autopilot technology would help with that, as would tightening up requirements for driver training.

[james_s]

Or, just accept what it is. Gasoline cars burn left and right all the time, no one blinks an eye. Buildings burn left and right, and you are still allowed to wrap the building in bitumen and styrofoam so that the whole thing burns like a torch. People are only interest in improving the safety to a certain level, and li-ion safety is certainly within the general level of safety already.

But do they burn as often? I've seen them burn too, but there are hundreds of millions of cars on the road just in the USA and probably billions worldwide. How many of these megapacks are there in the entire world? Already we've seen 2 or 3 of them catch fire.

[Black Phoenix]

Or, just accept what it is. Gasoline cars burn left and right all the time, no one blinks an eye. Buildings burn left and right, and you are still allowed to wrap the building in bitumen and styrofoam so that the whole thing burns like a torch. People are only interest in improving the safety to a certain level, and li-ion safety is certainly within the general level of safety already.

But do they burn as often? I've seen them burn too, but there are hundreds of millions of cars on the road just in the USA and probably billions worldwide. How many of these megapacks are there in the entire world? Already we've seen 2 or 3 of them catch fire.

The problem here is that it is news. Last time a Tesla burned, it was news on the US and Worldwide - I saw news in Portuguese and even China.

Strangely because China have a ton of problems with batteries blowing up and burning in 2 wheel vehicles, EVs and even public buses, but those are not normally wide reported as the Tesla one was (wonder why /s).

I bet with you that if tomorrow an accident with a Tesla kills someone and creates a fire that damages property, it will be news everywhere around the world. A ICE car burning and having the same damage in property is not as impactful as one of the most wealth new tech companies product having a problem, who his CEO is a constant Twitter controversial commenter.