posted on January 17, 2013 |
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Last night the FAA announced, "The battery failures resulted in release of flammable electrolytes, heat damage, and smoke on two Model 787 airplanes. These conditions, if not corrected, could result in damage to critical systems and structures, and the potential for fire in the electrical compartment."
Engineers at Boeing likely chose lithium ion batteries because they pack the most energy into the smallest space – and greatest volumetric density
However lithium batteries can, under certain circumstances, reach thermal runaway conditions.
Here’s a you tube video of thermal runaway in a laptop. You can see the laptop virtually explode in the first minute. (sorry about the ad).
Ken Chisholm is a battery management expert who designs sophisticated Lithium ion battery management systems (“BMS”) for Vecture Inc. According to Ken, the most likely reasons for the battery failure are an overcharging on voltage or a physical vibration causing a short circuit malfunction.
The BMS is a circuit board that acts as a protection system for cell overvoltage and under voltage as well as current discharge and charge.
According to Ken, the design of the BMS should not have allowed the Dreamliner problem to occur unless the problem was due to cell internal short circuit or in the wiring on the circuits between the cells and the BMS. Cell internal short circuits, especially in two aircraft, is unlikely.
It’s more likely that the BMS was at fault and the cause was overcharging or failure to control external excessive loading. That would be because the BMS wasn’t designed or sized for the potentially adverse loading conditions presented by the new aircraft.
For example, if a load dump occured with resultant inductive kickback, the BMS electronics may have been damaged and rendered ineffective. In that case, a subsequent overcharge could happen and lead to thermal runaway.
The BMS designers may have used Mosfets or Relay contactors. Where the BMS control elements are Mosfets they require sizing for load current and thermal management. If the designers got that wrong, the N Mosfets could burn out with collateral damage to the PCB, possibly causing a short circuit on the board. Relay contactors are more robust and generally the power path is not through the BMS PCB.
BMS design for large format batteries such as those on the Dreamliner requires a conservative approach including secondary cell-by-cell overvoltage protection. According to Ken, this requires an understanding of the full aircraft system, something that may have been tough to come by before the aircraft was operational.
Boeing shares have not been impacted much on this news, as investors show confidence that the company will be able to quickly address the problem.