Earlier this week I wrote about how Florida firefighters were having to deal with something new and troubling: electric vehicles, flooded with seawater from Hurricane Ian, have been bursting into flames, and the resulting fires have been extremely difficult to extinguish, leading to unusual methods like shoving cars into water-filled ditches, using about ten times the amount of water to put them out compared to a normal fire, and then parking the charred carcasses very far apart because the damn things will re-ignite, like demons. In that story, I speculated a bit why this is happening, but to really understand would take an actual engineer. Thankfully, one has reached out.
This engineer works at a major OEM, working with batteries and EVs. They requested anonymity, because OEMs can get weird about things their engineers reveal to the public, so we’re honoring that request. We think this knowledge is important enough for the general public that we want it out there, so here we go.
Take it away, anonymous engineer!
Why Salt Water Flooding Makes EV Batteries Burn
Part 1: Salt And Shorting
As the Fire Marshal and other commenters pointed out, the problem begins with salt bridges short-circuiting bus-bars and other high-current components in the pack – essentially exceeding the creepage and clearance distances because not only is the salt water itself more conductive than air or air-filled potting, but the salt residue left behind can be much more conductive, forming a path for current to flow.
This part can happen in anything with a battery or voltage potential – it’s why ICE cars that have been flooded and resold often have “quirks,” as some components in various ECUs have shorted and subsequently burned up or changed their behavior because the circuits around them have changed (via the residual salt shorting circuit traces). Its just that your typical lead-acid battery at 12V doesn’t have enough voltage to drive sufficient current through a salt bridge short to heat up enough to explode. Drop a wrench across the terminals and sure, it’ll blow up, but the salt isn’t conductive enough for a typical 12V battery to flow enough current to heat up enough/off-gas enough to explode.
The salt bridges are specific to the issue with salt-water flooding. Other sources of electrical shorts resulting in thermal runaway not specific to flooding could be manufacturing errors/damage in the cells (the primary cause of the Chevy Bolt fires, and probably the other LG cell fires as well), deposition dendrites that grow from the cathode or anode (depending on the cell temperature and voltage history) to pierce the separator, physical abrasion of separators or other insulating materials from the cells swelling and shrinking with each heat and charging cycle (secondary Bolt fire cause), or, of course, physical damage.
Part 2: Hard And Soft Shorts
BEV propulsion batteries on the other hand have ~300V or more (Hyundai’s newest EVs are at 800V), which is a lot more electrical “pressure” (to use a hydraulic analogy) to drive current through an electrical short, resulting in much more current (flow) and thus more heat generation. This heat will be generated in the cells themselves, and in all the electrical bussing that is part of the circuit, as well as in the salt bridge.
In the case of a “soft,” relatively high resistance short, this may take a while before the temperature gets critical, but since the battery has a lot of capacity and is pretty well sealed off with little heat transfer to the environment with the car off (as in no active cooling systems operating), the heat has nowhere to go, and things still heat up.
A “hard”, low resistance short makes this much worse since most Li+ chemistries can output much more current than is safe for the cell (and the restrictions to prevent this are in the battery control module, motor controllers, inverters etc, all of which are off), heating the cell up.
If the pack is filled with water, it is harder to build up enough heat since water is very good at absorbing heat, warming up and then turning to steam before venting from the pack, but eventually it boils off (possibly leaving behind more salt bridges). Either way, the cells get too hot through their own heating and being heated by the electrical bussing carrying the current until…
Part 3: Something Gives
Typically in Li+ cells, this something is that the graphite* in the anode (even in so-called “silicon anode” cells, graphite still makes up 80-90% of the volume) breaks down, then separator sheets inside the cell deform or melt, causing a small short between a single cathode and anode layer (monocell) within a cell. This short begins generating heat, melting more of the separator and breaking down the electrolyte, making the short worse and the cell internal heating worse.
*It’s not the graphite anode itself but the binders and the electrolyte interface over the graphite that begins decomposing first. The result is the same – you get a collapse of the anode, and this paves the way for a short when the separator begins melting, but technically the graphite decomposing isn’t really accurate. This process also off-gasses, adding to the flammable mixture waiting for a good O2 source to combust.
Part 4: Chemistry
Now we get into the cell chemistry effects a bit more. For example, LFP (Lithium Iron Phosphate, LiFePO4 etc) cells are considered safer because the chemistry contains less energy (thus less energy can go into either current to further heat the cell or be chemically converted into heat once the cell is on fire, heating other cells), and the LFP cathode chemistry seems to retain bonds to oxygen for longer at higher temperatures. Therefore, the LFP chemistry resists thermal runaway until higher temperatures, and has more trouble achieving those temperatures due to its lower energy density, so more mass to heat up with less energy than other chemistries.
In contrast, NMC (Nickel-Manganese-Cobalt oxide cathode) or NCA (Nickel-Cobalt-Aluminum oxide cathode) are more energy dense, which means more range for a given weight or volume, but more severe thermal runaway behavior. Variants of these two chemistries are also more commonly used at least in the Americas & Europe because they allow for more range, more power etc – China has a lot of LFP production and use in vehicles including their own LFP prismatic-can version of the Tesla Model 3. Anyway, any of these chemistries can break down at high enough temperatures when the anode, separator, cathode, and electrolyte all degrade, these things just happen at different relative times and temperatures with different chemistries.
Part 5: Why The Fires Can Re-Ignite
The electrolyte, separator, and cathode at least all typically contain some oxygen. This means that they can ignite, even in the absence of air (made worse by any Li metal that forms within the cell, which will react with almost anything). When burning, the lack of enough oxygen for a stoichiometric reaction means that some of the gasses released can burn more if they find a new source of O2.
This causes the hottest cell to rupture (if a pouch) or vent (if a cylindrical or prismatic can), venting hot combustion products or partial products, still-burning particulates, and bits of molten aluminum and copper from the cathode and anode current collector into any air gap within the pack (hopefully there is a manifold designed to carry this stuff away from other cells & electrical connections, but not always).
All this hot, flammable debris consumes all the O2 in the pack, but it’s still not enough for stoichiometry to be happy, thus you see fires outside of the pack fueled by H2, CO, and hydrocarbon chains generated by the thermal runaway, and constantly re-ignited by the molten bits of aluminum, copper, etc. that form the current collectors and other pack components.
Douse the whole pack in water and you can cool things off enough to stop the fire, but the internal short from the salt bridge may still exist (and has probably been joined by other shorts from the molten metal and other debris from the cell or cells that ran away already), which then starts the process over, reigniting the whole mess.
All of this is basically a long-winded way of saying that salt water in high voltage packs spells a bad time, but current EV design has a long way to go to prevent thermal runaways from propagating through the pack, and the high energy density of Li+ cells (even LFP) makes this considerably harder than with NiMH or lead-acid batteries.
Wonder if we might eventually see a mandated requirement for EVs to be equipped with a standardised mechanism to discharge the battery pack(s) after a crash, fire or flood damage.
A lot of industrial equipment with significant electrical battery storage (eg. Some Active harmonic filters, Variable speed drives, etc) will discharge to the earth connection when they lose power, to limit risk of electrocution or other exciting electrical quirks that come from dumping a bunch of electrical energy into a dead system with no control.
Obviously don’t have a suitable permanent earth connection on a ground. But perhaps something emergency services could be equipped with as EV fleet grows – a load cell or portable electrical grounding system they could connect to my mythical and entirely unproven emergency discharge point to dump majority of the stored energy out, reducing risk of electricity and fire.
Trouble is, 100kWh is a LOT of energy to dump into anything in a short time, a load cell that can comfortably cope with 100kW is a small towed trailer full of heating elements and a big cooling fan, and that is going to take an hour to dissipate that 100kWh.
Yes so the fact they are exploding alot means they will get worse every year.
Yes so new EVs are exploding like fireworks 9n new cars do you think it will happen less often after aging and no maintenance?
“Ultimate Inferno pack.”
You have a bright future in marketing.
“BEV propulsion batteries on the other hand have ~300V or more (Hyundai’s newest EVs are at 800V)…”
Imagine being the poor sap tasked with breaking these cars apart for recycling.
Being an insurance adjuster, I have total lossed cars that were flooded, due to learning my lesson where I had one that I tried to fix (under about 65% of the value of the vehicle) and then going back over that car for months due to one electrical gremlin after the other. I ended approving for more than the car’s value to fix, since once you start fixing an insured’s car you can’t stop part of the way. Very expensive lesson learned.
You also get the raw sewage in the car, which has a unique smell all of its own. Once you have smelled sewage you won’t forget it. It always smell the same, so I could walk into a house and know it had a sewage overflow the minute I walked into the basement.
insurance guy are you responsible for the electrical Vehicles reigniting fires and ruining everything where they are in the everything
Has anyone noticed that Jason requested an engineers opinion but ignores the fact that David Tracy is an engineer?
Bad call Jason.
Have you ever noticed there’s more than one discipline of engineering?
No I just realized all engineers don’t drive trains. I blame Sheldon Cooper for this.
David was a cooling systems engineer, not a battery engineer. David wanted me to talk to a battery engineer!
Has anyone ever tested applying the foam used for airplane crashes for EV crashes as it smothers flames? How about storage with a system similar to restaurants with grease fires? Or how about spray foam insulation which blocks out any air , isn’t flammable, and can be peeled off when needed?
Well, yeah – it’ll keep oxygen away from the fire thereby suppressing the visible flames – but the HEAT is still there, with nowhere to go.
Okay how about the system when heat builds up system blows fire suppressed powder like restaurants kitchens?
Your info about flood water is spot-on. As a storm water professional, I’ve seen people in flood situations wading through it like it was a day at the beach. These people are crazy.
The thing is, if the ground is flooded, that means the wastewater conveyance system (sanitary sewers) is flooded too. Whatever material people flush is now in direct contact with a person wading around in shorts and flip flops. Same goes for septic systems, industrial chemicals & wastes, you name it- anything that is on the ground or underground is now suspended in that flood water. That includes bird poop, which the surface of the earth is covered with.
Even breathing it is bad, let alone wading in it.
I’m surprise that the high voltage stuff in EVs are not sealed against water intrusion in the first place.
$$$$$$$$$”
6 figures is still not enough?!?
Water has a pesky habit of getting wherever you don’t want it, so trying to completely seal all electrical components (even just the high power circuits (since not actually HV)) in a vehicle that still needs to have certain flexible/moving points and actually be used and maintained would be quite an issue.
Covered in better detail by Rootwyrm
I wonder how Tesla’s new design would hold up with the 4680 batteries. They are basically encased in a very rigid foam, but I do not know how watertight it is but it covers the bus bars and all electrical connectors on it as well.
and for anyone with a Dodge with a foam filled rear quarter panel you know the foam traps condensation so that body panel rusting from the inside out would just as easily be the anodes and cathodes on the batteries under the foam.
I did not say nonflammable, but it may help with water intrusion to the cells that would potentially reduce the risks in flooding.
The part of this that was most interesting to me was the bit at the end about why they reignite. It’s one of those things that seems obvious once it’s been pointed out: you can put the fire out, but the conditions that caused it are still present. You still have a big mass of fucked-up, shorted-out Lithium cells, and some of them still have enough energy in them to start a fire. So you can smother the fire and cool things down to the point where the burning stops, but once things dry out there’s nothing stopping another cell from kicking the whole process off all over again. Kinda neat, in a way.
I’m still not sure that this is a real problem or just hyperbole from antidotal accounts. The statistics are pretty clear that the risk of a vehicle fire is much greater for hybrid and liquid fuel vehicles than for pure electric vehicles. https://www.autoinsuranceez.com/gas-vs-electric-car-fires/ Yes, EV fires are harder to put out but the number of fires per 100,000 cars is 60 times less than a gasoline vehicle. How many gasoline powered cars burned due to shorted batteries after the hurricane?
Reports were that 4 cars caught fire.
I think that in this case, it’s less that EVs are more fire-prone overall and more that EVs present a new class of hazard in the aftermath of natural disasters. ICE cars can and do catch fire, but not typically right after they’ve been flooded and are just sitting there amongst the wreckage of a hurricane-ravaged neighborhood. Also, the fires they cause are really quite nasty—hard to extinguish, and prone to randomly reigniting. This doesn’t seem to be something that had been considered much before, but it’s going to be more and more of a problem as both EVs and flooding continue to become more common.
Some ICE cars have trouble making fire in the cylinders where you WANT it.
You certainly do know everything about everything, don’t you?
The only ICE fire I’ve ever experienced was the result of a power steering hose failing and spraying fluid on the exhaust manifold.
Try a Plymouth Valiant where the owner repaired a leak in the fuel line with gasket sealant that was susceptible to degradation when exposed to gasoline. Has anyone ever seen an engine block glowing orange right after driving it? I have.
For me it was the oil supply line to the turbo developed a hairline crack and sprayed the exhaust manifold.
EV fires aren’t news because they’re more common than other kinds of fires. They’re news because they’re different than other kinds of fires in ways that people don’t expect. We’ve got a pretty well ingrained understanding of how fire works. And when fire stops working like we expect, it’s _terrifying_!
It will be interesting to see how much of the difference in the prevalence of fires in EVs vs gasoline vehicles changes over time, and whether the difference is largely down to the fact that the fleet of EVs is significantly newer on average.
Yeah, this. “Don’t let your EV get flooded because it’ll catch on fire” is the kind of warning that most people are going to have a very puzzled reaction to.
You’re right but EVs not caught in floods are also bursting into flames in huge numbers without being close to a hurricane. Or ramming other vehicles while operating on autopilot.
“Huge numbers”? Citation needed, please.
Don’t forget to control your car fires for age of vehicle.
EVs are still new and expensive enough to be far better maintained than the average ICE that catches fire.
Yes so new EVs are exploding like fireworks 9n new cars do you think it will happen less often after aging and no maintenance?
Did you read the article?
Its just that your typical lead-acid battery at 12V doesn’t have enough voltage to drive sufficient current through a salt bridge short to heat up enough to explode. Drop a wrench across the terminals and sure, it’ll blow up, but the salt isn’t conductive enough for a typical 12V battery to flow enough current to heat up enough/off-gas enough to explode.
I did read the article. It is HARDER (not impossible) to make lower voltage batteries catch on fire due to these types of shorts. Per the article’s last paragraph: “makes this considerably harder than with NiMH or lead-acid batteries”
Yes but you neglected the 10 times harder to put out
https://www.autoblog.com/2022/10/26/kia-sportage-recall-fire-risk/
Yeah we seem to be getting alot of EV zealots from the old site since everyone else stopped reading their rants. I expect the political crap posts to start soon.
That is terrible data because it doesn’t seem to account for the fact that there are hardly any EVs older than like 5 years. New cars do not catch on fire very often.
Yeah not sure where you got your math skills from but they called and want your diploma back. You are confused on likelihood of an ICE fire vs probability of an ICE fire in comparison to an EV. Sure there are twice as many ICE fires but that is from the fact that there are 1900 times more ICE vehicles than EVs.