How can the batteries for e-cars burn under water?

Factual Questions
41

I haven’t seen this mentioned here so far but one feature of a charged battery failure is that it starts with a runaway internal arc-welding or plasma-torch effect, vaporising anything in its path. On its way out to the atmosphere, this high-velocity jet of plasma cuts catastrophically through any adjacent cells in a chain reaction, each with their own plasma jet.

In seconds, the entire car battery assembly turns into a bulging roaring ragbag of molten and vaporised battery materials, as all the stored energy is turned into heat in a billion short-circuits.

The effect of any surrounding water is minimal until it can get into contact with the bulk of the hot vapor, as the outer casing disintegrates or blows apart through intentional safety seams.

I have seen a laptop emit a blue plasma pencil-jet that, once the thick white smoke had cleared, had cut through thick cables several inches away. Do not go near lithium cells when they blow!

42

Good power management systems monitor blocks of cells and balance external loads so that you don’t have large voltage differences that put excessive load on those blocks; and note when there are out of spec current fluctuations or voltage spiking that indicates internal faults so they can shunt around them. Cells built with good quality control will degrade in a very predictable way so until near end of life the performance degradation should be very consistent. However, that quality control includes the entire chain of manufacture from the processing of basic constituents to how they are laid up in the cell in nanoscale layers. It is extremely expensive to build and maintain this kind of manufacturing infrastructure, which is why lower grade Li-Ion/LiPo batteries have such a propensity to catch fire or explode.

Stranger

43

There’s some oxygen (not a lot) dissolved in most ordinary sources of water, so I suppose the heat from the Lithium reaction will be causing that to come out of solution as free oxygen, but it seems like maybe that wouldn’t be enough to support combustion.

44

I doubt it.

45

But hang on… Lithium is reacting with oxygen that it steals by decomposing water, and the reaction is highly exothermic… how isn’t that ‘burning’?

46

It is.

What I doubt is that the hydrogen and heat liberated by the action is combining with significant amounts of free oxygen in more traditional “burning”.

47

I always thought the violent sodium-in-water explosions were due to the hydrogen, but these guys seem to have shown otherwise:

https://www.nature.com/articles/nature.2015.16771

Excerpt:

It’s the classic piece of chemical tomfoolery: take a lump of sodium or potassium metal, toss it into water and watch the explosion. Although this piece of pyrotechnics has amazed generations of chemistry students, until now it has been misunderstood, a paper in Nature Chemistry reveals.

The explosion, say Pavel Jungwirth and his collaborators at the Czech Academy of Sciences in Prague, is not merely a consequence of the ignition of the hydrogen gas that the alkali metals release from water. That may happen eventually, but it begins as something far stranger: a rapid exodus of electrons followed by explosion of the metal driven by electrical repulsion.

Given the chemical similarities between sodium and lithium, you may be right: violent explosions during lithium/water contact may not be due to hydrogen ignition afterall.

48

Interesting article (I have to gret a full copy to read – I’m prevented here from seeing the full thing), but even they admit that the burning of hydrogen gas in air eventually happens. They’re concerned about what stRTS THE REACTIUON IN THE FIRST PLACE, ABND THAT’S WHERE THE INTERESTING STUFF IN THE ARTICLE COMES FROM.

Nevertheles, I disagree with their saying that the reaction would evenbtually peter out as the alkali metal gets covered with reacted products. If you’ve ever thrown alkali metal into water, yoiu can see it skipping across the surface – there isn’t time for reactants to cover the surface, because it’s in comnstant motion.

The alkali metal skips because it’s lighter than water (at least for lithium), but also because of a sort of “Leidenfrost effect”, where the metal is buoyed up by , not water turning into gas, but water reacting with alkali metal to form hydrogen gas (undoubtedly aided by the reactions the paper talks about). The damned bit of metal skips and skims all over the surface, partially melting, heating, and reacting. This is why you need to be very careful if you try this out at home – you could get a blob of semimolten alkali metal hurling itself at you and lodging in your skin, where it will violently react with your moist skin, and it will hurt a lot. Best to do things like this while wearing heavy clothing, gloves, and a face shield.

Robert W. Wood, and brilliant physicist and renowned kook, back around 1900 reportedly used to dress up in black robes and wander the streets of Baltimore with sodium pellets wrapped up in twists of absorbent paper. If he had an audience while passing a puddle, he’d toss the paper into the puddle and watch the reactions of the unsuspecting marks as it burst into flame.

49

Ah, gotcha. Yeah - there’s also the fact that the hydrogen is pretty swiftly whisked away by both its own buoyancy and the convection current that emerges from the process.

50

Some would say that that is a redundant description.

Stranger

51

Lithium ion batteries have their own oxygen from the cathode to burn with, not external sources like water .
The cathode such as LiMnO2 contains oxygen and the cathodes are susceptible to thermal breakdown, so when they get hot they release oxygen , quite a lot of oxygen, above 150C ( 302F).

The batter also contains an electrolyte which is typically Lithium salt in an organic solvent. The organic solvents will break down above certain temperatures and that breakdown is an exothermic reaction that generates more heat , so things can get bad quickly , hence the ‘ thermal runaway’ phrase. As the electrolyte breaks down it produces ethane and other combustible gasses so you have a mixture of oxygen, flammable gasses , self generating heat and lithium all held together in a little can.

Things can start with something causing a temperature rise in the battery, poor cooling, attempts to over charge, charging of a more depleted cell form adjacent cells , mechanical damage. That’s what a lot of thought goes into the design of battery circuits for both rechargeable and non rechargeable packs. Lots of fuses , diodes, balancing circuits etc. The packs are not just a bunch of cells connected together as you we see in products where you pop in a few AAA cells.

Once the cell for whatever reason gets above 90C the electrolytes are starting to generating their own heat, the solid interface arround the anode will breakdown, also an exothermic reaction raising the temperature further, now the electrolytes and the anode can come in contact making more combustible gasses, and the battery internally shorts out, so yet more heat and the we get the release of the oxygen from the cathode and that reacted with the gasses for even more heat , or a klabloooie moment.

No water needed for the lithium to react with. The electrolyte is plenty of fuel all by itself.
A cell could burn in water , but generally the water around would probably cool things down.

For an EV car fire, you have a whole lot of energetic fuel , with its own oxygen source , and that will burn pretty fiercely , and set fire to all the other stuff in the car that burns like the tyres, upholstery, all the plastic ( which is the same for an ICE vehicle)

52

Here is a video of a recent EV fire in Centennial, Colorado (South Denver). The fire department used EV blankets to cover the car to smother or slow down the fire, and transported it to a safe place to cool down. They repeatedly say it deprives the fire of oxygen.

They mention that all of their hazardous material teams carry EV blankets, and they can be used for lithium battery fires from things other than EVs, such as mobility devices.

BTW, the notes on the video mention the car was a 2019 Jaguar I-Pace, and it was charging. It also says this model has been recalled for possibly overheating while charging, but they don’t say whether this particular car had received the repair.

53

If not, it’s a good bet it doesn’t need the work anymore.

54

The thing about ice fires in a garage is it’s easy to detect a gasoline leak. A few drops are easy to smell. And once the catalytic converter cools down there isn’t an ignition source. It’s also an easy fire to fight with an extinguisher if caught early. An EV fire gives no warning and cannot be extinguished.

Both situations are going to diminish over time with the introduction of solid state batteries and the natural replacement of ICE cars with EV’s.

IMO the EV hybrids with smaller solid state batteries will be the interim battery because we don’t need 400 mile range cars. a 100 mile range car would fit the bill and reduce the environmental impact of large battery packs. There are a lot of interesting small ice engines in development that would serve nicely as generators for longer distances.

55

Actually, they say it restricts the oxygen. Which is true - it reduces/eliminates the atmospheric oxygen feeding the fire. It still doesn’t deal with the oxygen coming from the electrolyte and interactions with the lithium. They’re essentially letting it burn out in a safe manner.

I think range is an area where YMMV definitely applies. Also, you have to account for diminished range in cold temperatures which can significantly impact that range. My sister has had a hybrid for a very long time and while the vast majority of the time it just runs on electricity/battery power for her daily commute in the dead of winter the gas generator starts to kick on daily on the way home.

“This car has a 100 mile range” really means “this car has a 100 mile range under specified conditions and conditions outside that range may mean a significantly shorter range”.

56

Don’t let them get hot. Don’t let them get cold. Make up your mind!

/s

57

Jimmy Pop nailed it!