Thursday, February 7, 2013

Investigators Pinpoint a Short Circuit Within a 787 Dreamliner Battery | Autopia |

Investigators Pinpoint a Short Circuit Within a 787 Dreamliner Battery | Autopia |
Boeing, along with investigators in the United States and Japan, have focused on the lithium-ion battery from the start. And today’s announcement that the problem appears to have started with a short circuit within a cell is exactly what battery expert Dr. K.M. Abraham suggested was the problem when we spoke with him last month. The lithium-ion cells within the 787 batteries use a graphite-coated copper anode and a lithium cobalt oxide-coated aluminum cathode. The anode and cathode are separated by a very thin polyethylene film known as the separator.
The separator is roughly the same thickness as cellophane and behaves in a similar way. There doesn’t need to be a tear or a hole to create a short circuit that can cause thermal runaway. The material is very thin – typically around 25 microns, according to Abraham – and small irregularities in the thickness can be enough to lead to problems. A section of the separator that is just 20 microns thick might be enough.
“It could be a stretch, it doesn’t necessarily have to be a big hole, just a weak point where you have low resistance,” Abraham said. “It can be a problem when you have such a very large surface area electrode where there is a lot of inhomogeneity in the current distribution.”
The variable thickness of the separator material could be a result of manufacturing, but also could occur during charging and discharging of the battery. A very small short might lead to the growth of a lithium crystal within the battery cell.
“Sometimes what happens is you start with a very small dendrite growth due to an internal short,” Abraham says of the small fibers of lithium metal that can grow in the cell, “but it gradually heats up because gas can pass through it and heat up that location.”
And just like cellophane, the separator can shrink when it is heated, Abraham says, “once it starts heating up slowly it will shrink and then a small short will become a massive short.”
Abraham, agreeing with comments made by Tesla Motors CEO Elon Musk, said the relatively large cells in the 787 battery pose a problem. The large surface area of each cell increases the chance that an irregularity could lead to a short, Abraham said. The problem of the separator changing thickness due to heating is something addressed in the batteries used in the Chevrolet Volt. The separator is less likely to change thickness due to heating, according to Abraham.
“That was overcome in the Chevrolet Volt separator where they reinforced the separator with ceramic particles to mitigate the shrinking problem,” he said.
According to the NTSB, the separators used in the 787 batteries are not reinforced with ceramic particles.

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