An alternative method Boeing investigated but did not use for certification involved a heating element that could be used to increase internal temperatures to produce a thermal runway, but Sinnett says the method could mask the results of the test by making it too conservative. “We heated batteries, but in no case did we get fire or flame from the battery,” he says. “We got thermal runaway inside the cell, but no cell-to-cell propagation.”
Boeing, however, did report propagation of thermal runaway between cells, as well as fire, when it performed overcharging tests, a key failure mode for lithium-ion batteries, but one that is controlled by the battery charging system. Sinnett says there is no evidence from the Boston incident that the battery was overcharged.
“The first thing that strikes me is that the overall system worked,” says Sinnett of the incident. “We didn’t have a catastrophic outcome. Clearly the event that happened was a serious event and we need to make sure it doesn’t happen again, but the added protection and layers of protection performed their function. From that perspective, it validates our process to a great degree.”