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Tesla 'Smoke' while Supercharging - an explainer for Emergency Responders

When is a Tesla with 'smoke' showing not actually a fire?


We recently heard from fire agency colleagues in the United Kingdom regarding a call for fire crews to a Tesla Supercharge site where the driver had reported 'smoke' showing while charging.


Enroute, crews were informed that the site manager had isolated power to the Superchargers, shutting down power to all units. The driver of the EV had been located & informed the site manager the Tesla belonged to his son.


On arrival, crews witnessed the same 'smoking' effect & used a thermal imaging camera to monitor the high voltage battery, located underneath the vehicle (between chassis rails & the four wheels).


The TIC showed a reading of 30°C on the 'right hand side of the inside of the grill' & 'smoke' was still showing, however it was reported as 'minimal' & slowed over time following the power isolation.


The driver called his son, who spoke to fire crews & informed them the 'smoke' effect was due to water on the batteries when they were at full charge.


Tesla electric vehicle with fog cloud
Image supplied by Tesla - not an image of the incident being discussed in this post

So, what's actually happening here?


While the Tesla owner's explanation isn't quite right, it's pretty close.


We had a hunch as to what was happening, but reached out to our friends at Tesla to give us a full technical explanation:


"When the HVAC system is ambient sourcing, louvers are open, refrigerant suction temperature and coolant temperature are below 0 degC. Humidity (of the ambient air) condensates and freezes up on the radiator surface. During super charging the refrigerant suction temperature (and the coolant temperature as well) increase above 0 degC, ice melts and evaporates. This evaporation process is very visible to the eye when the radiator fan stops running (happens when the HVAC goes out of ambient sourcing mode or if HV battery heating has ended). Moisture evaporating is often reported as "steam" or "smoke" by customers."

In short, condensation is being produced, which appears from the front of the vehicle as a 'fog-like' cloud.


We've broken it down into this short video:



A quick look at Tesla Supercharging


A Tesla's high voltage traction battery charges up most effectively when at a higher temperature, so most drivers 'precondition' their battery while enroute to a Supercharger.


Pre-conditioning the battery warms it up to between 32 & 42°C (90 to 107°F). Once it's connected to the Supercharger, the rapid transfer of energy also causes heat. To thermally manage the battery, the vehicle's Heating Ventilation Air Conditioning (HVAC) system progressively cools the battery to maintain an optimum charging temperature.


This transfers heat from the battery to a condenser, similar to a radiator in an internal combustion engine vehicle. In cold weather, ice that has formed on the condenser during pre-conditioning evaporates, producing the condensation 'fog-cloud'; the average January temperature in the UK is between 1 & 7°C (34 to 44°F).


And because the HVAC system is located at the front of the vehicle, under the frunk, the fog effect is seen in that area.




How can emergency responders tell the difference?


The site manager & fire crews in the UK call out did exactly the right thing; shut off the power to charging & monitor the Tesla battery with a thermal imaging camera.


A TIC is one of the primary methods emergency responders can use to determine whether an EV battery is going into thermal runaway. When using a TIC with an EV, particularly aTesla, these are some considerations that could be used to determine responders are NOT dealing with thermal runaway:

  • The battery pack is at ambient temperature that is not rising

  • A battery pack over ambient temperature can be due to preconditioning of the battery or while fast or super charging

  • A Tesla HV battery can accept a fast charge most efficiently at approximately 32 to 42°C (90 to 107°F)

  • The vehicle has not been previously involved in a collision, or submerged in water, located near a fire & is not showing fault codes on the dashboard


Additionally, it's highly unlikely to be thermal runaway if:

  • The weather is cold - around freezing - & the EV is fast or super charging

  • TIC is showing a temperature in the normal charging range of 32 to 42°C (90 to 107°F)

  • The 'smoke' or fog cloud is showing at the front of the EV where the HVAC is located

  • Gas detection systems do not show carbon monoxide in the cloud


How do responders know if it is thermal runaway?


Of course, you should always be alert to the early signs of thermal runaway in lithium-ion batteries, whether in a road-registered vehicle, an e-bike or a mobile phone.


These include loud popping noises of battery cells bursting, whistling or hissing of venting gases & projectile cell debris from underneath the EV.


A dark vapour cloud, followed by a lighter vapour cloud will appear from underneath the vehicle; these are typically quite dense & turbulent.


Follow your agency SOPs & if any of the early warning signs are observed, take agency approved action.


EV on fire
Thermal runaway of a lithium-ion battery is different to a condensation fog cloud

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, thanks for supporting our website & research.




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