The risk assessment for the batteries, in part, came from GS-Yuasa's experience in building more than 14,000 lithium-ion cells for its industrial customers since 2001.
From the FAA's perspective, the nine special conditions were performance-based and flexible enough to cover whatever chemistry was used. Those conditions—meant to control lithium-ion-specific issues such as over-charging, over-discharging and cell flammability—cast a wide safety net with the flexibility that, so far, has covered the evolution of the battery through four design iterations.
Though there were more than 100 battery tests in the qualification/certification program, key to proving compliance with the special conditions were “abuse” tests—one of which called for a nail to be driven through a cell to force a short-circuit.
The testing, which GS-Yuasa says it performed at least 3-5 times for each generation of batteries, did not lead to thermal runaway in neighboring cells.
“There was no propagation to other cells,” says Boyd. “That was the basis of our conclusion that Boeing's analysis was reasonable.” In hindsight Boeing and the agency concede that certain battery tests to show compliance with FAA special conditions were too lenient.
“One particular test—nail penetration—resulted in a short that wasn't as energetic as we've seen in service,” says 787 chief project engineer, Mike Sinnett. “What we've since found out, in retrospect, is that we don't feel [the nail test] was conservative enough.”
En route to the new battery system being retrofitted on the grounded fleet, Boeing has made significant changes. After the Securaplane factory fire, attributed to the battery control system being disconnected, Boeing stayed with cobalt-based lithium-ion chemistry, but built-in an additional battery monitoring unit.
Three years later, while testing the electrical subsystem at Hamilton Sundstrand's Airplane Power Systems Integration Facility, one cell in a 787 battery experienced thermal runaway and vented. As a result, the third-generation battery included better sealing of the box, enhancements to the monitoring unit and a fuse-like “latch” function that requires a battery with too low a charge to be serviced by Boeing before being used again.