"Unfortunately, the pack architecture supplied to Boeing
is inherently unsafe," writes Musk in an email to 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," he adds.
Both Boeing and Tesla use batteries fueled by lithium cobalt oxide,
which is among the most energy-dense and flammable chemistries of
lithium-ion batteries on the market. While Boeing elected to use a
battery with a grouping of eight large cells, Tesla's batteries contain
thousands of smaller cells that are independently separated to prevent
fire in a single cell from harming the surrounding ones.
"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 neighboring
cells and causing a domino effect that results in the entire pack
catching fire," says Musk.
An aerospace-capable version of Tesla's battery has been developed
for use in SpaceX's Falcon 9 space launch vehicle. SpaceX, also owned by
Musk, competes with Boeing/Lockheed Martin
joint venture United Launch Alliance
for customers. Boeing has thus far declined offers of assistance from Tesla and SpaceX, says Musk.
"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," writes Musk in an email. "It is much harder to
maintain an even temperature in a large cell, as the distance from the
center of the cell to the edge is much greater, which increases the risk
of thermal runaway."