04.3 Thermal runaway - hazards & behaviour
What are the hazards of thermal runaway in an EV?
Off-gassing
Ignition
Explosion
There are three main hazards when an EV battery goes into thermal runaway:
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Off-gassing - a toxic, flammable cloud of vaporised electrolyte caused by rapidly heating lithium-ion battery cells
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Ignition - when the off-gassing ignites, the pressure of the escaping vapours forms a jet-like directional flame
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Explosion - where off-gases are unable to escape, they may build up & cause a vapour cloud explosion
Understanding how thermal runaway in an EV behaves and progresses will assist in early identification and help establish appropriate emergency response.
What are the early warning signs of thermal runaway in an EV?
When arriving at a vehicle fire, look for one or more of these signs that you might be dealing with an electric vehicle lithium ion traction battery fire.
A dashboard fault code is often first indication of battery short circuit
Large cloud of dark & light vapour - highly flammable
Popping noises as cells vent & whistling/hissing noises as gas escapes
Ignition of jet-like, directional flames at up to 1000 degrees celcius
Reduction in vapour cloud as it is consumed by flames
Increased fire activity as plastics are involved & projectile debris
Now, let's look at some examples of each.
Watch: Electric car in thermal runaway:
OFF-GASSING
Video: Via Professor Paul Christensen, location and original source unknown
Watch: Electric van in thermal runaway;
IGNITION
Video credit: Unknown
An electric Nissan Shuike on charge at a DC unit ignited, detroying four other vehicles.
0.08
0.15
0.23
0.26
0.31
From 2.00
Dark cloud of heavy metal particles
Whistling noise of venting gases
Lighter vapour cloud above vehicle
Small vapour cloud explosion, vapour cloud is consumed
Jet-like, directional flames
Listen for popping noises, watch for debris projectiles in centre of the screen
Watch: Electric car in thermal runaway;
EXPLOSION
Video credit: Unknown
Please take note at:
0.20 secs
0.22
Vapour cloud of toxic gases venting from battery cells (note: it's incorrectly described as 'thick smoke' on this video)
Vapour cloud deflagrates (explodes), with possible ignition of heated battery cell
Interesting points about EV battery fires
1.
Lower state of charge, less likelihood
An electric vehicle battery under 50% state of charge (SoC) is less likely to go into thermal runaway overall. If it does, it may off-gas without ignition.
For this reason, international maritime rules state EVs should be carried at 30% SoC or less.
2.
There is no such thing as a 'fireproof' lithium-ion battery
All lithium-ion batteries used in EVs can go into thermal runaway, regardless of chemistry or form type (ie. cylindrical, prismatic, pouch or blade).
While there may be some slick marketing online, almost every EV manufacturer is represented on the EV FireSafe database.
3.
Fire behaviour may differ based on chemistry and form type
Some chemistries and form types may result in larger vapour clouds, a more active jet-like flame or 'firecracker' type behaviour. Typically, this does not change how an incident controller would manage an EV battery fire.
Research in this area is ongoing.