Easy to prevent, hard to extinguish
by Jacques Gordon
Conversation with a friend about fires in electric cars prompted me to do a little research. Lithium-ion (Li-ion) batteries are being used in some electric and hybrid/electric cars, and they will be used in more cars in the future. They’re also in laptop computers and cell phones, cordless power tools, and in the Boeing 787 Dreamliner. Compared with other types, li-ion batteries have a higher energy-density, can charge and discharge quickly, deliver full power deeper into the discharge cycle, and will hold a charge about twice as long when not in use. They don’t develop a ‘memory’ after X number of charge cycles, a condition that degrades the charge/discharge capacity in other types of battery. However, they do require a control circuit to prevent over-charging and -discharging, and they often become unreliable after a few years even if they’re not used.
A typical lithium-ion cell generates a nominal 3.6 volts, ranging from 3.0 volts discharged to 4.2 volts fully charged. Repeated over-discharge or significant over charging can lead to cell failure and “thermal runaway.” That thermal event (fire) can happen even when the battery is idle.
During shipping, large li-ion batteries or large quantities of smaller ones are considered a hazardous material, and labels for battery packs are designed to help shippers determine the extent of the hazard.
Overcharging can lead to significant gas generation within a cell, causing it to swell. Sometimes this forces the electrodes apart, breaking the circuit and interrupting the charging, and therefore preventing thermal runaway, but not always. First responders have reported putting out a battery pack fire with an extinguisher, then seeing the fire re-ignite as heat causes another cell to swell and vent flammable electrolyte. Mechanical damage can cause leaks or shorting within the battery, with similar results.
The National Fire Protection Association (NFPA) commissioned a study of li-ion batteries, resulting in a very detailed report released in 2011. It’s worth a look (http://tinyurl.com/o4r84wl), especially the incident reports about 2/3 of the way down. The report points out that the working medium is ionized lithium and not natural lithium metal, so if a battery cell ignites due to either an internal fault or an external source, standard metal-fire suppression techniques are “not appropriate.” Part of the reason is that the electrolyte is flammable, and water will just make it spread out.
The NFPA report concludes that there are two major challenges in putting out a vehicle fire involving li-ion batteries: remove heat as well as oxygen, and prevent any leaking electrolyte from re-igniting. Methods and materials for doing this are still being developed. The bottom line is, while li-ion battery fires are rare and not hard to prevent, they’re very hard to extinguish. Training for first-responders is being updated now, but putting out a car fire involving li-ion batteries is a major challenge that few fire departments are prepared to handle.
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