February 1, 2012, 5:15 AM

by Matt Thurber

The photo of a badly burned Apple iPhone that circulated after the phone caught fire during a Regional Express flight has raised important questions about lithium-ion battery safety among a wide aviation audience. The incident occurred after the Regional Express Saab 340B landed in Sydney, Australia, on Nov. 25, 2011, according to the airline, “when a passenger’s mobile phone started emitting a significant amount of dense smoke, accompanied by a red glow.” Although the airline didn’t identify the specific procedures the flight attendant used to contain the smoking battery, the crew “carried out recovery actions immediately and the red glow was extinguished successfully.”

The Australian Transport Safety Bureau (ATSB) provided more information, noting that the flight attendant used a fire extinguisher to cool the phone. But interestingly, this appears to be the only case in Australian aviation of a lithium-ion-powered device spontaneously combusting. According to the ATSB, “There is no previous record in theATSB’s databases of spontaneous self-ignition involving smartphones or other portable electronic devices on an aircraft in Australia. However, the ATSB is keen to fully understand the nature of this event given the increasing and widespread carriage and use of such technology on passenger-transport vehicles.”

Lithium-ion batteries power nearly every portable electronic device that pilots and passengers carry on aircraft. “The fact that batteries can fail on rare occasions in an uncontrolled manner has brought an increased public awareness for battery safety, in particular as a result of some large product recalls of portable notebook computer and cellphone batteries,” according to a July 2011 study conducted by Exponent (Failure Analysis Associates) for the Fire Protection Research Foundation (FPRF).

Dangers of Thermal Runaway

With an energy density as high as six times that of a lead-acid battery, lithium-ion batteries are somewhat sensitive to design and manufacturing flaws. Failures are categorized as non-energetic (meaning loss of capacity, activation of a disabling mechanism, electrolyte leakage) or energetic, which is the kind that can cause smoke and/or fire. It is the energetic-type failure–thermal runaway–that most concerns flight crews.

A thermal runaway means that the battery cell releases stored energy rapidly, and lithium-ion batteries with higher energy density can release a lot more energy more quickly than other battery types. Lithium-ion batteries contain a highly flammable electrolyte, so they store both electrical energy from the normal battery chemistry and chemical energy from the lithium-based electrolyte.

Thermal runaway can occur when the battery self-heats, which can happen when electrolyte reaches temperatures as low as 158 to 194 degrees F (70 to 90 degrees C), according to the FPRF report. Runaway accelerates quickly at higher temperatures, and the greater the charge in the battery, the faster runaway happens. Temperatures during a runaway can reach 1,110 degrees F (600 degrees C). The battery cells will also experience increased pressure, venting or popping of the cell, possible ignition of cell gases, possible ejection of cell contents and propagation to adjacent cells.

According to the FPRF report, “Venting of isolated small cells (cellphone cells and smaller) seldom results in flame ignition. This is likely due to the limited volumes of vent gases released from these cells–that is, the gases become diluted before ignition can occur. In comparison, ignition of vent gases from 18650 and larger cells [used in some laptops] is fairly common: these cells contain more electrolyte (more fuel), and are usually used in multi-cell battery packs. If the flow of vent gases is ‘restricted’ due to the configuration of a vent port (typical in hard case cells), flames emanating from the cell will be highly directional (flames from 18650 cells are often described as ‘torch-like’).

“Propagation of cell thermal runaway has significant implications for fire suppression and fire protection. A fire suppressant or low-oxygen environment may extinguish flames from a battery pack, but the thermal runaway reaction will propagate if heat is not sufficiently removed from the adjacent cells. Responders to fires involving lithium-ion battery packs have often described a series of re-ignition events. Typically, responders report they used a fire extinguisher on a battery pack fire, thought they had extinguished the fire, and then observed the fire re-ignite as an additional cell vented.”

The ways that lithium-ion batteries can fail are numerous, but the report noted, “It has been observed that the vast majority of thermal runaway reactions that occur in the field occur during or shortly after cell charging.” One way of lowering risk may be to avoid charging of lithium-ion devices while in flight. It should be noted, however, that reports of spontaneously combusting lithium-ion devices that burned their owners included devices that were not being charged.

Anyone flying with lithium-ion-powered devices might want to know of these hazards and take steps to minimize the risk of a thermal runaway from an onboard device, especially now that many pilots have adopted tablet computers for chart and document viewing.

 © copyright aljacobs Stardate 10-18-2012