Electronics > Projects, Designs, and Technical Stuff
Deliberate Lithium cell overcharging
Siwastaja:
--- Quote from: amyk on May 11, 2019, 06:54:21 pm ---
--- Quote from: Siwastaja on May 11, 2019, 02:21:25 pm ---Physical nail penetration might be the best, if it needs to demonstrate a typical cause.
--- End quote ---
Even then, you might not get a fire.
--- End quote ---
Indeed - often the manufacturers even document a nail penetration test, with a typical result of "no fire".
Nail penetration causes local internal short circuit, but if the shutdown separator works as intended, the ion transfer stops / is limited, limiting the temperature rise below the runaway onset temperature.
Smaller cells tend to perform better because they are more isothermal, with safety layers reacting better to the average cell events (think about the PTC cap of a 18650, and compare it to a massive prismatic cell.)
Note that it is dangerous to rely on all these cell-level protections; the risk is hugely increased when abusing cells, especially physically. However, the risk is still awkwardly small when your target is to demonstrate a battery fire.
To say it in other words with made-up but representative numbers, assume that a risk of li-ion fire, using high-quality cells, is, say, 1 failure per 10^6 packs. Now, overcharge the cells severely, and the risk might go up by 1000x, to 1 failure per 10^3 packs; a huge safety issue if you have epidemic BMS failures; but still low enough so it's almost impossible to demonstrate. I have seen so many overcharging events, and never a fire resulting. Have been lucky - or unlucky, depending on what you want to see.
Re: explosion, this is a classical case of there being two completely different meanings for the same word: the scientifically defined exact meaning, and the colloquial meaning. I wouldn't make a big fuss about it, people do and will always call almost anything an "explosion" if it feels like it to them, and this isn't wrong, just how language works.
Zero999:
--- Quote from: amyk on May 12, 2019, 12:01:44 am ---That was definitely not an explosion, there wasn't even a "big bang". If it was an explosion there would be a shockwave and shrapnel everywhere.
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Whatever, perhaps you'll feel comfortable poking a lithium ion battery with a skewer, whist holding it in your hand.
It's good they way the myth about water and lithium ion batteries is busted in that video, as it doesn't burst into flames, when wetted.
tszaboo:
--- Quote from: james_s on May 11, 2019, 08:05:22 pm ---There's no need to abuse the cells prior, lithium battery fires occur when cells are overcharged, or when they short circuit internally which is also what happens when they are overcharged. Worn out or abused packs tend to be more vulnerable because they can get out of balance but it's the overcharging that makes them burn.
That said, I don't really see what the point is here. The best way to fight a lithium battery fire is plain old fashioned water and lots of it. Take away the heat and the fire goes out.
--- End quote ---
Well, yes and no. Sand is probably the best to put out the fire on the short term, and let it burn on your term later.
"Lithium reacts intensely with water, forming lithium hydroxide and highly flammable hydrogen. The colourless solution is highly alkalic. The exothermal reactions lasts longer than the reaction of sodium and water, which is directly below lithium in the periodic chart.
2 Li(s) + 2 H2O -> 2 LiOH (aq) + H2(g)
At 750oC lithium reacts with hydrogen to lithium hydride (LiH). The white powder that forms releases hydrogen gas upon later reaction with water, in amounts of 2800 liter per kilogram hydride. As such, lithium can be applied as hydrogen storage."
So once the cell is damaged, it might catch on fire days after it has been put out, just because lithium reacting with water. Besides it emitting H2, which might explode, it could emit Hydrogen fluoride, which is poison.
Siwastaja:
--- Quote from: NANDBlog on May 12, 2019, 08:28:35 am ---"Lithium reacts intensely with water, forming lithium hydroxide and highly flammable hydrogen. The colourless solution is highly alkalic. The exothermal reactions lasts longer than the reaction of sodium and water, which is directly below lithium in the periodic chart.
2 Li(s) + 2 H2O -> 2 LiOH (aq) + H2(g)
At 750oC lithium reacts with hydrogen to lithium hydride (LiH). The white powder that forms releases hydrogen gas upon later reaction with water, in amounts of 2800 liter per kilogram hydride. As such, lithium can be applied as hydrogen storage."
So once the cell is damaged, it might catch on fire days after it has been put out, just because lithium reacting with water. Besides it emitting H2, which might explode, it could emit Hydrogen fluoride, which is poison.
--- End quote ---
Your point is quite moot, as there is no (or very little) metal lithium in a lithium ion cell, in a similar way that there is no metal sodium in your table salt: they react similarly with water: nothing happens.
Similarly, chlorine gas is extremely poisonous, yet NaCl doesn't easily decompose into it.
Chemistry can be unintuitive for a layperson: seeing the same atom as in a well-known poison or dangerous substance doesn't necessarily mean anything (think about H2O, or many CN (cyanide) containing safe substances for example).
But for the exact same reason Li or Na is so freaking reactive, it's also extremely unreactive going back to the opposite direction. In a li-ion cell, it's in the "reacted" or unreactive form, the same as Na is in your table salt. In metal lithium primary cell, it's in the reactive metal form.
Although, there might be some corner cases where a long-time abused cell has developed small amounts of lithium metal (see: lithium plating, for example, during overcharge), which, even if, due to the minuscule amount, doesn't produce much energy when reacting with water, could, in certain conditions, trigger another reaction. But I don't think this is very relevant in the big picture - the damaged cells are dangerous as long as there is significant stored chemical energy left in them, and the actual trigger can be anything. It's probably a false assumption that lithium-water reaction has anything to do with it.
Feel free to store burned cells in sand after the initial fire has been taken care of with a lot of water, and the battery has been dismantled - probably a good idea. Sand is easy to obtain in large quantities, and provides good combination of thermal insulation and thermal capacity, withstanding high temperatures. During an incident, getting sand inside and around a pack is probably much more difficult and destructive to the surroundings, compared to simply pumping some water through a hose.
beanflying:
There is one obvious suspect local (very suspect ;) ) who would love to help you build a device to overcharge some Lithium Batteries. I am sure Clive would be up for a build in some form and some video of it in use for your training.
http://www.bigclive.com/
https://www.youtube.com/user/bigclivedotcom/search?query=lithium
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