The high-powered lithium-ion batteries used in Boeing's 787 Dreamliner planes have come under a harsh spotlight in recent weeks; a pair of battery-related fires have resulted in the grounding of fleets across the globe. Yet while Japanese investigators said this week that they are yet to find any factory-level problems with the batteries, SpaceX and Tesla Motors head Elon Musk — who competes with Boeing in commercial space ventures — believes that there is a more fundamental issue in play.
"It is simply a matter of time before there are more incidents."
"Unfortunately, the pack architecture supplied to Boeing is inherently unsafe," Musk told Flightglobal. "Large cells without enough space between them to isolate against the cell-to-cell thermal domino effect means it is simply a matter of time before there are more incidents of this nature."
Unusually, Tesla cars use lithium-ion batteries with thousands of tiny cells versus the eight-cell packs used on the 787. Though most electric automobiles use far fewer cells, Tesla says that the small cell size improves reliability and enables efficient heat transfer.
"When thermal runaway occurs with a big cell, a proportionately larger amount of energy is released and it is very difficult to prevent that energy from then heating up the neighboring cells and causing a domino effect that results in the entire pack catching fire," says Musk.
"I would have used the same words," says Donald Sadoway, a MIT Professor of Materials Chemistry and battery expert who also spoke to Flightglobal. "I'm glad someone with such a big reputation put it on the line. He's engineered [Tesla's battery] to prevent the domino effect, while Boeing evidently doesn't have that engineering." Sadoway recently suggested Boeing implement an active cooling system or switch to a different battery technology such as nickel metal-hydride.
"I design a cell to not fail and then assume it will."
However, Boeing's chief project engineer on the 787 believes that the batteries were designed with such concerns in mind. "I design a cell to not fail and then assume it will and the ask the next 'what-if' questions," says Mike Sinnett. "And then I design the batteries that if there is a failure of one cell it won't propagate to another. And then I assume that I am wrong and that it will propagate to another and then I design the enclosure and the redundancy of the equipment to assume that all the cells are involved and the airplane needs to be able to play through that."
Musk, whose SpaceX competes with Boeing and Lockheed Martin's United Launch Alliance for government space contracts, maintains that the 787's battery structure is likely to cause problems. "They [Boeing] believe they have this under control, although I think there is a fundamental safety issue with the architecture of a pack with large cells." While most lithium-ion batteries use large cells, the Dreamliner uses batteries to replace hydraulic systems on a scale not seen before in an airplane. Musk doesn't appear to be proposing a specific solution, but he says that Boeing has refused assistance from both Tesla and SpaceX. Investigators, however, say that the cause of the recent issues is still unclear.