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Things that make an EV go...POP

Updated: Apr 24, 2023

We’ve spent a lot of time telling emergency responders (& others) that loud popping noises coming from an electric vehicle means lithium-ion battery cells are bursting & thermal runaway has started.


Well…while that’s still technically correct, we have to amend that statement a little.

Following a Tesla Model X fire on 30th March 2023 in Auckland, New Zealand, bystanders reportedly heard ‘loud popping noises’ & the media ran with the idea that the vehicle’s high voltage battery had gone into thermal runaway (in other words, had caught fire).


A clip from the NZ Herald video showing the Tesla Model X on fire. Image: NZ Herald

Following our own investigation, we believe this isn’t the case, but have no doubt loud popping noises were heard in this incident.


Let’s have a look at why we’re placing our money on this being a non-battery fire.


Popping is an early warning sign


If you’ve been following our work you’ll know one of the early warning signs of a lithium-ion battery fire in an electric vehicle is loud popping noises, often describe as gunshots by some eye witneses.


This is a result of individual lithium-ion batteries (LiB’s) going into thermal runaway, an unstable & self-sustaining chemical chain reaction that produces a rapid build up of heat.

Thermal runaway is caused by LiB cell abuse; in EVs that usually due to the vehicle being involved in a collision, submerged in water or due to a fault in the cell during manufacture. It can also be caused by impurities in the battery chemistry, called dendritic growth…but, that’s an article for another time.


LiB cells are designed to contain a large amount of energy in a small space, so they are commonly encased in steel or aluminium for protection.


But if the cell fails, that energy can be released with somewhat explosive results.

If thermal runaway occurs, heat build up inside the cell causes a specially designed pressure release disc to rupture, producing the popping or ‘gunshot’ sound.


But not all battery cells make a popping noise


While we’re on the topic, a good-to-know is that not all lithium-ion battery cells will ‘pop’.

There are three form types; cylindrical, prismatic & pouch.


Both cylindrical & prismatic lithium-ion battery cells have a pressure release disc designed to rupture if heat builds up.


But pouch cells, which are kind of like the squishie yoghurt pouches you buy for kids, will typically just split at the sides & expand without making a distinct popping noise. Having said that, & just to slightly confuse things, they may sometimes make a popping noise depending on how battery cell abuse has occurred.

Cylindrical & prismatic lithium-ion battery cells typically make a loud popping noise when the pressure release disc bursts during thermal runaway

Bangs, pops & gunshots…what else could this be down to?


Modern vehicles are designed with many creature comforts & technologies to make us safer and enhance our driving experience.


As any firefighter will know, these can make some alarming sounds when things go wrong. This is not just in electric vehicles. Here’s a quick list to highlight that point:


Tyres:

Tyres are an essential part of vehicles and provide both traction to start, stop and steer, and provide cushioning to dampen bumps. But when the pressure within a tyre is released suddenly because the tyre has been damaged by a fire, this will definitely be heard as a bang or pop. Many modern passenger cars don’t carry spare tyres, so these pops should be limited to 4.


Gas struts:

The gas struts or cylinders that are designed to hold up bonnets (hood or frunk) and boots (trunk), are also a source of a bang or pop when the vehicle they are fitted to is burnt. These are filled with pressurised fluid and can release this energy with a loud & sudden noise when damaged. This can be the same with suspension components such as air suspension systems like those found in the Model S and X by Tesla.

Tesla Model X Emergency Response Guide & components key (below). Image: Tesla

Airbags:

SRS airbags are designed to use a controlled explosive charge to inflate the bag instantly if we find ourselves in a crash. They are known to make a loud bang in this situation, but this is the same when the material is burnt in a vehicle fire. The airbag can still go bang, inflating the bag if it hasn’t already been burnt away.


Seatbelt pretensioners:

An additional safety system used in partnership with vehicle airbags are seatbelt pretensioners. These devices are installed as a part of your modern seatbelt and are designed to pull the seat belt tight if your airbag is about to be triggered. This should eliminate any slack from the seatbelt and stop you from travelling forward excessively. These pre-tensioners may also make a bang or pop when they are ignited by a fire or excessive heat.


LPG:

A Liquified Petroleum Gas (LPG) cylinder is also a cause for an extremely loud bang and will usually be much more than a pop. We’ve had LPG in cars for a long time now, and cases like this are extremely rare. We are now seeing Compressed Natural Gas (CNG) and Hydrogen being used in high pressure cylinders also. Significant design and testing have gone into making these cylinders as safe as possible.


Fuel tank:

Unlike your favourite Hollywood movie, when a petrol or diesel fuel tank catches fire, these DON’T go BANG! and send the car rocketing through the air. The liquid fuel is able to be burnt, but usually only outside of the fuel tank. Inside the fuel tank is too much fuel and not enough air (too rich) and rather than burning or exploding, the fuel needs to leave the tank and mix with air to burn or catch fire. However, an empty or near-empty fuel tank could, in the right (wrong?) conditions, explode & cause a loud noise.


So how do we know what’s causing the popping?


It can be tricky!


If you’re called to a non-fire incident involving an EV & loud popping noises are heard, it’s a pretty safe bet that’s an early warning sign that lithium-ion battery cells are bursting due to rapid heat build up & thermal runaway is occurring.


If fire is involved & the popping noises sound similar & are repeated – you hear multiple over a period of minutes – then we’d say this is also likely to be thermal runaway.


Any loud popping that occurs while the vehicle is on fire &/or can only be heard once or twice, is likely to be one of the other vehicle components listed.


Back to the ‘chisel jawed’ heroes of Auckland


We’re still waiting on the final investigation report, but we do have some details of this incident that make us pretty confident it didn’t involve the high voltage battery pack.


The driver of the Model X hit a roadside barrier on the left side of the vehicle (passenger side in the right-hand drive New Zealand market) causing the two tyres on that side to burst (causing popping noises!).


She continued to drive the vehicle, eventually stopping when warning lights came up on the large control screen on the dashboard. Following a quick inspection of the vehicle, & for reasons unknown, she then re-entered the vehicle & accelerated away at up to 100 kph.


By this point the vehicle was driving at speed on two rims, which is what we suspect caused a fire. On Reddit, we found a description from a passing driver who saw a vehicle with the ‘left front wheel on fire’ & ‘burning rubber strewn across the road’.

The driver of the Model X eventually pulled over, but remained in her vehicle until a (& we quote) ‘handsome a*s, turbo chad, chisel jawed hero’ pulled her out of the burning vehicle. The car ’infernoe’d for another 10 minutes with frequent very loud bangs & batteries fizzing out with jets of flame before fire & police arrived’.


Another report describes ‘large flames coming out of the hood’ & that the ‘driver was still in the car when it caught fire’.


When police arrived the driver was upset, shouting ‘I don’t know why my car is burnt’.


Why are we questioning if this was a battery fire?


While the description sounds like it could be, the circumstances of the fire, reported battery state of charge at the time (yet to be confirmed) & the relatively short length of time it took to bring the incident under control, means it hasn’t made our list of ‘Verified’ passenger EV battery fires. Yet.


It’s worth noting here that even if an electric vehicle is on fire, it commonly doesn’t involve the high voltage lithium-ion battery pack. EV battery fires are currently very rare, so we treat all incidents with the same level of investigation as this one, to ensure we’re tracking data that’s as complete & accurate as possible.


But…there’s always a chance we’re wrong & we’ll let you know if the Fire Emergency New Zealand investigation comes back saying anything different.


Downloadable graphic


Want to share this information with the firefighters you know? We've compiled it into an easy graphic, which you can download & share here:

More information


If you haven't already done so, visit the excellent range of resources for emergency responders on the Tesla website www.tesla.com/firstresponders


And, of course, there's a heap of free information here at evfiresafe.com


We'll be working with fire agencies to present a new series of responder-focused webinars throughout 2023, so please join our mailing list to keep up to date with those. You can also join us on Facebook, LinkedIn & YouTube.


As always, we hope this has been helpful & thanks for supporting our website & research.


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