The National Transportation Safety Board (NTSB) believes that a single cell failure, a short circuit in one cell, on board the Boeing 787 Dreamliner airplanes is the culprit in the battery pack fires that have grounded those airplanes. The problem began occurring about a month ago, several planes had fires that were eventually determined to have been in battery packs in the tail section. We are of course concerned because these are lithium batteries of some kind, and electric cars use lithium ion batteries, and possibly these battery pack fires might cause spillover worries that electric car battery packs are also susceptible to fire.The latter point is unlikely. My understanding is the battery packs in question use a chemistry that’s known to be susceptible to fire – while electric car battery packs use chemistries that are not.
In any case – the NTSB held a press conference today and released some findings.
|Battery pack from JAL airplane that caught fire
They studied a battery pack from a Japan Air Lines 787 and found “that the majority of evidence from the flight data recorder and both thermal and mechanical damage pointed to an initiating event in a single cell. That cell showed multiple signs of short circuiting, leading to a thermal runaway condition, which then cascaded to other cells. Charred battery components indicated that the temperature inside the battery case exceeded 500 degrees Fahrenheit.”
Why did the cell short circuit? Investigators “ruled out both mechanical impact damage to the battery and external short circuiting.” And “It was determined that signs of deformation and electrical arcing on the battery case occurred as a result of the battery malfunction and were
not related to its cause.”
This much makes a lot of sense – a short circuit would try to release all the energy at once, which is going to cause heating and arcing. That can cause enough heat to cause a fire, and lithium batteries can in the right conditions make spectacular fires.
Causes for the short circuit being investigated include “battery charging, the design and construction of the battery, and the possibility of defects introduced during the manufacturing process.”
As for Boeing’s design to include such batteries, the NTSB claims that Boeing ran risk analysis assessments and testing and found there to be no opportunity for cell-to-cell propagation of fire.
That piece I find to be astonishing to the utmost. A lithium cell on fire gets hot, and the heat can be hot enough to cause other cells to catch fire. Okay, maybe Boeing designed a battery pack containment they felt would limit the spread of heat from one cell to another. Did they not pay attention when a Chevy Volt battery pack caught fire in 2011? That fire started by one or two cells short circuiting because of physical damage, and the fire then spread to the other cells in the pack.
The NTSB also says Boeing’s risk assessment determined “that the likelihood of a smoke emission event from a 787 battery would occur less than once in every 10 million flight hours.” But in the field there were two fires with less than 100,000 hours of flight time.
Additional material including pictures and documents and videos: http://www.ntsb.gov/investigations/2013/boeing_787/boeing_787.html
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