The NTSB has released it’s interim report on the Boeing 787 JAL battery fire and it appears that Boeing’s initial safety assessment had concluded that the only way in which a battery fire would eventuate was through overcharging.Therefore, so the logic went, Boeing needed only to prevent overcharging and it also prevented the possibility of a cell fire.
In a classic example of experimenter’s regress when Boeing engineers subsequently planned their type test program they weren’t explicitly looking for other causal factors for a battery fire because their theory informed them that only battery overcharging might cause a fire (1). And because their test program was not looking for evidence of other causes they didn’t find them, rather unfortunately as it turns out.
In contrast to Boeing’s assessment a 2011 study into the hazards of lithium batteries sponsored by the NFPA makes it quite clear that the risk of a thermal runaway resulting in a fire increases dramatically in relation to the state of charge, independent of the cause of the runaway (2). In fact they found that at 100% charge fire was quite common as the casing of the overheating cell would exceed the auto-ignition temperature of the gases.
The NFPA study also found that the heat transfer environment of a cell in thermal runaway can play a large role in the severity of the reaction. High temperatures or insulation increases the likelihood that any given internal fault can drive a cell to thermal runaway, and also increases the energy available to heat the cell to the auto-ignition temperature.
Boeing’s 787-8 electrical power system safety assessment also included an analysis of lithium-ion battery failure modes. This analysis determined that overcharging was the only known failure mode that could result in cell venting with fire.
NTSB report, DCA13IA037 dated 7 January 2013
So contrary to the Boeing safety assessment, whether a fire occurs depends on battery and environmental states, it doesn’t however depend on the cause of the thermal runaway as Boeing assumed. What the Boeing safety engineers appear to have done is confused the observation that most thermal runaways resulting in fires occur at or near a state of full charge with an act of overcharging, and in the final analysis this may have been simply due to the engineers misinterpreting the literature (3).
1. For example while Boeing engineers did perform a nail puncture test on one of the battery cells to determine the results, they didn’t recognise, as NFPA investigators did, that this sort of damage is probably only going to generate a low impedance short between current collectors, which was in turn unlikely to cause a thermal runaway especially if the state of cell charge is also low. As it turns out pinch damage in concert with a full state of charge is actually much more hazardous.
2. The likelihood that a fire will occur depends on the state of charge (most significant), the thermal environment and finally the cell chemistry used.
3. They may have done so because the majority of battery thermal runaways that occur in service do so during, or directly after a charging event. The NFPA investigators theorised that this was because:
- Lithium dendrite formation occurs during charging; thus, shorting of dendrites that cause extreme localised shorting is more likeley,
- Charging provides more energy in the cell by raising the state of charge increasing the likelihood of a subsequent thermal runaway, and
- Charging provides energy to any shorting point within the cell.