South African-born US innovator and entrepreneur Elon Musk last week asserted that the type and layout of lithium ion batteries used by Boeing in its revolutionary 787 Dreamliner is “inherently unsafe”. Musk, founder, CEO and chief designer of rocket and spacecraft company SpaceX and CEO and head of product design of electric car company Tesla Motors, was communicating with British aerospace journal Flight International.

In early January, a Japan Air Lines (JAL) 787 suffered from a battery explosion and, a few days later, All Nippon Airways 787 had to make an emergency landing because a battery overheated and sprayed burning electrolytes in a compartment beneath the cockpit. As a result, the US Federal Aviation Administration (FAA) grounded the aircraft on January 16. The FAA decision was rapidly followed by other air safety agencies around the world, as well as by airlines operating the aircraft, resulting in the grounding of all 787s in revenue service.

These failures are being investigated by the Japan Transport Safety Board (JTSB), the FAA, the US National Transportation Safety Board and by Boeing. JTSB investigations of the battery manufacturer, GS Yuasa, and of the maker of the battery monitoring unit, Kanto Aircraft Instrument (both Japanese companies), revealed no manufacturing problems.

Boeing uses lithium ion batteries that are powered by lithium cobalt oxide. These have some of the highest energy densities of any lithium ion batteries but are also amongst the most vulnerable to “thermal runaway” (overheating that can result in a fire). Tesla uses the same category of batteries in its cars, while a version has also been developed for use in SpaceX’s Falcon 9 space rocket.

“Unfortunately,” Musk told the UK maga-zine, “ the pack architecture supplied to Boeing is inherently unsafe. 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.”

Each battery is divided into cells. The batteries used by Boeing in the 787 each have eight of these, each rated at 3.7 V. In contrast, Tesla uses batteries with thousands of small cells, each separated from all the others to stop thermal runaway or fire in one affecting any of the others.

“Moreover, 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 neighbouring cells and causing a domino effect that results in the entire pack catching fire,” he affirmed. “I think there is a fundamental safety issue with the architecture of a pack with large cells. It is much harder to maintain an even temperature in a large cell as the distance from the centre of the cell to the edge is much greater, which increases the risk of thermal runaway.”

Boeing designed the 787 to use electrical systems in place of traditional hydraulic and pneumatic systems in order to save weight. In addition, lithium ion batteries are themselves lighter than nickel cadmium batteries and can, unlike the latter, be fully recharged. Moreover, lithium ion batteries provide 100% greater energy storage than nickel cadmium batteries and can release 100% more energy than a nickel cadmium battery of the same size.

The chief project engineer for the 787, Mike Sinnett, explained to Flight International the approach adopted to the design of the aircraft’s battery packs. “I design a cell to not fail and then assume it will and ask the next ‘what if’ questions. And then I design the batteries so 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.”