In DACI’s 1st Quarter 2012 newsletter I predicted that a catastrophic safety event would eventually occur due to lithium batteries (please see “Li-Ion Battery Pack Hazards and our Psychic Prediction“). The recent fires in the initial flights of the new Boeing Dreamliner have come close to fulfilling that prophecy.
From “Detecting Lithium-Ion Cell Internal Faults In Real Time” (Celina Mikolajczak, John Harmon, Kevin White, Quinn Horn, and Ming Wu, in the Mar 1, 2010 issue of Power Electronics Technology) it is known that internal cell faults in lithium batteries can lead to thermal runaway, subsequently resulting in fires and/or explosions. Therefore the question arises: do the Boeing lithium batteries have an advanced internal construction that prevents cell faults, or mitigates thermal runaway in the event of a fault? If not, the Boeing team or vendor responsible for the battery system design is in big, big, trouble.
Although deficiencies in basic battery chemistry and/or construction appear to offer the best root cause hypothesis for the fires, there are also other possible factors. For example, it has been reported that perhaps the charging system malfunctioned, causing the batteries to overheat. However, a properly designed charger for an aircraft application would have fail-safe protection, preventing an overcharge. Plus, it was also reported that charging sensors did not detect an overvoltage. Although these factors sound reassuring, they are not sufficient to eliminate the charger from consideration. For example, one can hypothesize a charging waveform that contains spurious high frequency oscillations that create high rms charging currents. This would not necessarily result in overvoltage, but could result in overheating.
It is also possible that battery “cell defects” are nothing more than cell imbalances that vary according to production tolerances. In other words, the lithium battery, by its very nature, tends towards thermal runaway unless the internal cells are very tightly matched. This sensitivity would become more pronounced with a higher number of cells and higher mass, which would explain why no explosions have occurred in small button-style batteries, but do occur in the larger batteries.
There are other scenarios, including the thorny possibility that some combination of conditions conspired to create the failure. And, of course, the root cause may be highly intermittent, making detection extremely difficult. Such hypotheses are undoubtedly being examined by the Boing engineers. I wish them well, and hope that they are allowed to perform their work calmly, methodically, and thoroughly.
Note: Because it may take quite a long time to conclusively establish a root cause, I would suggest that Boeing immediately begin planning to retrofit the lithium system with one containing battery types that have not shown the proclivity to explode; e.g. nickel metal-hydride, or sealed lead acid gel. Heavier, yes, but in this case safety and the economic timeline indicate that it would be wise to be prepared with a retrofit design.
(For some brief guidelines on design failure crisis management, please see Scenario #6: “Coping with Design Panic,” in The Design Analysis Handbook, Appendix A, “How to Survive an Engineering Project.”