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."
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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