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.
We're working at the grass roots level with lithium systems in E-Scooters and we're having our problems although not quite as dramatic as Boeings. The battery management system does what it's supposed to do where temperature increases beyond a certain level opens the circuits...somewhat inconveniently after about 3 blocks of hard acceleration. Troubleshooting indicates that by disconnecting the battery, pausing, then reconnecting resets the system, and as long as a reasonable acceleration is the input the system works continuously. As soon as a higher torque (hence amperage increase) is applied to the drive system over a certain length of time the system cuts out. Our solution to this irritation is in work consisting of a hybrid battery system using lead acid for acceleration and lithium for cruise, the controller being rigged to accomodate the changes. Boeing's solution may be the same in the long run...cooling systems are just another aggravation, but hybrid battery systems rigged through a reliable controller may be slightly heavier but likely not as heavy as a cooling system and have the advantage of no dead weight (all batteries being functional)
Not so puzzling. Before you design in a cooling system, you ask how high can you go without it. For example, if a transistor is rated with a junction temperature of 150 degrees, I would let it heat up to 100 degrees before I put a fan in. Thats a 33% margin. Cooling at sea-level is not the same as cooling at 8000ft equivalent alitude in a pressuized airliner, though.
Boeing will suffer politically, regardless of the decisions they make now. Their best option is to have a replacement strategy defined and a timeline for implementation & FAA approval before the end of this week. Any more delay will cripple their Dreamliner, giving the A380 a huge advantage.
It's possible the cooling was 'built-in', i.e. the design current draw and subsiquent heat load was lower than the thermal capacity of the unit as installed. That's how zillions of the small and ubiquitous lithium batteries consumers use every day work. It'll be interesting to see how this story plays out. Certainly some mistake was made somewhere, but we seem to be jumping to conclusions
Good point William. I wonder if the energy density of the lithium-ion battery system with an additional cooling system would now be less-favorable than the energy density of the traditional/proven battery technology (without the additonal cooling system).
Chuck, do you get a sense that perhaps Boeing moved to the cobalt oxide cathode lithium-ion batteries to shed weight and then made a decision to not include a cooling system that would have added net weight? It is feasible that this type of failure is only found after the 787 is used in process and not in testing and that Boeing can find a safe fix. Gosh, I sure hope that it does not turn out to be known, debated cost-saving management decision that turns political...
The lack of a cooling system would be very puzzling. According to a friend of mine who is a battery guy, lithium ion batteries are the highest energy density and are one of the most volatile chemistries, so correct charging and cooling are of paramount importance.
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