What happens when a Chevy Volt’s or Toyota Prius Plug-in Hybrid’s battery catches fire? A team which recently presented data pertaining to multiple types of plug-in car fires at the 2015 SAE World Congress found out first hand.
One finding was lithium-ion battery packs can reach temperatures as high as 750 degrees Fahrenheit when ignited, they need significantly more water to put out, and are capable of reigniting hours later.
More than wanton destruction, researchers Andrew Blum and Richard Thomas Long conducted full-scale fire tests to understand how fires for electrified vehicles are different than for conventional vehicles. Sponsored by Exponent Inc., an engineering consulting firm, the aim was to develop data supporting best approaches for emergency personnel to handle these fires.
“The overall goal of this research program was to develop the technical basis for best practices for emergency response procedures for EDV battery incidents, with consideration for suppression methods and agents, personal protective equipment (PPE), and clean-up/overhaul operations.”
Blum and Long noted that 188,000 highway vehicle fires overall were reported in 2013 alone. With safety protocols in place, and given EVs are a slim minority, next to none of these so far have been reported as plug-in electrified vehicle fires. When an PEV has caught fire it might become national news, but more information was sought on electric drive vehicles (EDVs) fires that do happen, and statistically could more often occur as PEVs increase in number.
“A key question for emergency responders is, ‘what is different with EDVs and what tactical adjustments are required when responding to EDV fires?'” the researchers said.
The tests included two types of li-ion battery packs found in current models from unnamed manufacturers. The veil of anonymity was thin however as there are but a handful of potential test subjects and descriptions provided all but gave the identity of the cars away.
The first, a 4.4-kilowatt-hour battery was sourced from a plug-in hybrid which has a flammable organic electrolyte in the cells. It’s secured in the rear cargo area of the vehicle with a metal casing.
Though not stated, this battery precisely fits the description of only the Toyota Prius PHEV.
The second type was a larger, T-shaped pack that’s almost as long as the car. It is used in an “extended range electric vehicle,” and the cells of this 16-kwh li-ion battery also have a flammable organic electrolyte.
This car too is given away by its description as most likely belonging to a Chevy Volt or Cadillac ELR. No other vehicles have packs fitting this battery’s description.
Though other battery chemistries are also in use by hybrids, such as nickel metal hydride, the team chose to focus solely on the up and coming li-ion chemistry.
How EV Fires Are Different
Whether a vehicle relies on an internal combustion engine (ICE) or rechargeable batteries, the study found a lot of overlap for fire suppression methods. For both situations, basic steps for responders are the same: make sure the vehicle is immobile, remove any passengers and then put out the fire.
“The key differences between ICE and EDV tactics include the need for copious amounts of water to extinguish an EDV battery fire, the high voltage electrical hazards associated with EDVs, and the recommendation to store all EDVs 50 feet from other structures or vehicles post-fire during overhaul operations and storage.”
To understand the amount of energy released by an EDV battery fire, the team took one of the 16-kwh packs and heated it with propane burners, allowing the unit to completely burn itself out. The team then tracked temperature, energy and gases released during the fire.
“The maximum HHR [heat release rate] measured during testing was approximately 700 kilowatts, at test time 17 minutes and 30 seconds (about 3 minutes prior to the burners being turned off),” said Blum and Long.
“After the 36 minute mark, flaming had subsided significantly on the battery to a point where the HHR was essentially zero. Small localized flames on the battery continued for approximately another hour, at which time all visible flames ceased.”
At that point, the exterior temperature of the battery was 750 F. Even three hours later the pack was still extremely hot at 310 F.
How to Put Out An EV Fire
The study reported that plain water, which is also typically used to put out fires on conventional vehicles, will also suppress an EDV fire.
“However, the amount of time required applying water and the total volume of water necessary for extinguishment was significantly larger than what is typically required for extinguishing a traditional ICE vehicle fire,” Blum and Long said.
Significant levels of carbon monoxide and carbon dioxide gases were released during the EDV fires, and water samples had 100 times more fluoride than normal. Chlorine levels were two to three times greater, and no hydrogen cyanide or hydrogen fluoride gases were found.
“None of the batteries tested ‘burst’ or ‘exploded,'” said the researchers, though “popping” and “arcing” sounds were heard and white gasses were given off.
In comparison to ICE vehicle fires, the report noted that EDV battery fires carry additional chemical, high voltage and thermal hazards for firefighters. However, the researchers said that there wasn’t enough data to recommend any protective gear above what is normally worn.
Two of the biggest differences with EDV fires – and a considerable hazard – was found after the fire was put out. First, the researchers found that sometimes the batteries stayed hot for quite a while afterwards.
“In some cases” … batteries cooled down after an hour, but other times “it took as long as 12 hours” before temperatures returned to normal, the report said.
It was also discovered that the fire could return hours later.
“In one test, the battery reignited 22 hours after it was extinguished,” said the researchers, adding that no flames or smoke was seen before the fire flared up.