The Boeing 787's high-profile battery fire may have been the result of an engineering double-whammy: an energetic battery chemistry combined with a possibly inadequate cooling system.
Battery experts who spoke to Design News this week said that the 787's lithium-ion batteries employed a cobalt oxide cathode, which is known to be more prone to overheating than other lithium battery chemistries. If that chemistry was used without extra measures to draw heat away from the pack, it could be a problem, experts said.
"It's a no-brainer," Elton Cairns, a professor of chemical engineering at the University of California and a nationally known battery expert, told us. "If they used a cobalt oxide chemistry, then the battery should use a cooling system."
An NTSB engineer examines the casing from the battery involved in the JAL Boeing 787 flight in Boston. (Source: NTSB)
Although Boeing has not said whether the 787's lithium-ion battery packs use any kind of active cooling system, experts who saw photos of the packs said it looked unlikely. "The images I saw indicated that there was no active cooling system and this battery pack has many cells stacked close together," Donald Sadoway, the John F. Elliot Professor of Materials Chemistry at the Massachusetts Institute of Technology, wrote in an email to Design News. "So you need active thermal management."
Boeing representatives told Design News that their lithium-ion battery pack used specific measures to prevent overcharging. "There are multiple back-ups to ensure the battery system is safe," a Boeing spokesman told us. "That includes protection against over-charging and over-discharging."
Boeing representatives did not know whether the battery packs included cooling, however. And cooling was not mentioned in a five-page transcription of a Boeing media call explaining the incidents.
The 787's use of lithium-ion batteries for the auxiliary power unit is said to be a first, which is one of the reasons why the batteries are being scrutinized so heavily. The National Transportation Safety Board (NTSB) X-rayed batteries from a January 7 fire aboard a Japan Airlines Boeing 787 at Logan International Airport in Boston. The NTSB team also did CT scans, disassembled the battery, and examined flight data recorders to determine if it exceeded its design voltage of 32V.
On January 20, investigators said that the battery did not exceed its prescribed voltage. Since then, the agency has continued to look for the root cause of the problems, which have occurred on two Japan Airlines flights and one United flight.
Ougassing should have been observed during tests. True, low atmospheric pressure might accelerate outgassing but the atmospheric pressure inside a (pressurized) commercial aircraft cabin is generally in the range of normal.
I also think vibration could be the culprit. (see my comment, above).
That's a scary thought, Bill. It's not hard to imagine Boeing deliberately choosing cobalt oxide for the higher energy density. That's its chief advantage over other lithium-ion chemistries and it's the reason many engineers choose that chemistry. But as for their alleged lack of a cooling system: It's anybody's guess. I think a lot of engineers are still climbing the learning curve when it comes to all the lithium-ion chemistries.
I can't believe Boeing would use a battery design that was not so carefully bench tested that there was absolutely no way it could overheat no matter what happened to the charging circuit and no matter how little cooling was available.
On the other hand it is quite possible that due to vibration in the aircraft, the electrodes might go into a vibrational resonance allowing a couple to touch each other and cause an internal short. This could be difficult - but not impossible - to simulate on the bench.
Very good point, Paul. Elton Cairns of the University of California agrees with you. He told us that the higher, colder altitudes were a detriment, not an advantage in this situation. Cairns, by the way, should know: He designed the PEM fuel cells for the Gemini spaceflights in the 1960s.
Given the importance of this issue to Boeing, I would be shocked if it kept the 787 down until 2014. That would be a major setback to a very high profile program. But I also think their engineers will also be very careful in avoiding missteps in implementing a solution to this problem. Not an easy thing to have the world watching while you solve a complex technical issue.
Well, good! If that's the way it has to be, that's the way it has to be. Better to wait until the design is completely safe to fly the fleet than risk human life.
They will get it right eventually, Liz. But it could take a while. Over the weekend, CNET published a story in which they, too, interviewed Donald Sadoway of MIT. Sadoway told them that the problems could keep the 787 fleet grounded until 2014.
If this is true, then it's a bit scary to think this could happen again. Let's hope engineers get the battery chemistry right next time so something like this doesn't happen again and cause an even more dangerous situation.
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