Whether or not the battery exceeded its design voltage, however, experts believe a cooling system was critical. Lithium-ion battery chemistries in general are "energetic," they said, and the cobalt oxide varieties of lithium-ion are particularly so.
"Not all lithium-ion batteries are created equal," Cosmin Laslau, a research analyst for Lux Research, told us. "None of them should fail. They are all essentially safe. But in the event of a failure, lithium cobalt oxide would fail earlier than the other types. Chemical bonds in lithium cobalt oxide will release oxygen earlier." Experts say the release of that oxygen can, in rare cases, lead to fire.
Many engineering teams around the world choose cobalt oxide chemistries, however, because it offers energy densities that can be up to 25 percent higher than other types of lithium-ion, such as manganese spinel (used in the Chevy Volt) and phosphate-based systems.
To counteract the higher energies, big, lithium-ion batteries in general are often used in conjunction with cooling systems, no matter whether they are cobalt-, manganese-, or phosphate-based. The Chevy Volt, for example, employs liquid coolant that circulates through 1-mm thick channels machined into 144 metal plates sitting between its lithium-ion manganese spinel cells. Similarly, the Prius PHV plug-in hybrid uses specialized fans, intake ducts, and 42 temperature sensors to actively monitor and cool its lithium-ion battery.
To be sure, the 787's 63-lb battery pack is smaller than those of today's typical electric cars, which can often exceed 400 lb. But experts said that lithium-ion batteries of all types need ways for heat to get out. "Size does make a difference," Cairns told us. "But the size of that (Boeing) battery is still substantial. If the cell casings are touching one another or have inadequate space to allow for natural convection cooling by air, then you're in for trouble."
Cairns said that he hadn’t personally seen the Boeing battery pack, however, and didn't know if Boeing engineers had provided any means for the heat to escape.
Battery experts who spoke to Design News repeatedly stressed the fact that all types of lithium-ion batteries can be safe and successful, if engineered properly. The question still being answered is whether Boeing engineers did that. “They should have stress-tested the battery with charging system as it it is installed in the 787,” Sadoway said. “I myself wouldn't fly in a 787 at this point."
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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