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Tesla fog mystery solved

A sharp increase in sales of electric vehicles has uncovered a foggy phenomenon during the Northern Hemisphere winter that caused enough ‘no smoke without fire’ concern to result in reports of fires to emergency services.


However, the clouds emitted by Teslas while rapid charging have been identified as condensation from the thermal management system rather than a dangerous lithium-ion battery fire caused by thermal runaway.

Research organisation managed to get the following explanation direct from Tesla:

“When the HVAC system is ambient sourcing, louvres are open, refrigerant suction temperature and coolant temperature are below 0ºC. Humidity (of the ambient air) condensates and freezes up on the radiator surface.

“During Supercharging, the refrigerant suction temperature (and the coolant temperature as well) increase above 0ºC, 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.”


Because electric vehicle batteries charge up most effectively at higher temperatures, increasing numbers of EVs – including Teslas – use their thermal management systems to pre-heat the batteries to 32-42°C prior to rapid charging.

The DC rapid charging process itself also generates heat in the battery, which is thermally managed by, the vehicle’s HVAC system to maintain an optimum charging temperature.

Heat transferred from the battery to the condenser can melt and evaporate ice that has formed on the condenser during pre-conditioning in cold weather, causing a fog-like cloud.

However, having experienced the Taycan, the quick blast of cool or warm air before entering the cabin proves effective and the way the air sweeps around the cabin automatically due to the presence of automated vent actuators enhances the effect.

On the upside for VASA members, the added complexity of this system, and others like it, is likely to provide a steady source of customers in years to come.

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