Design News, March 2013

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and over-discharging." Boeing representatives did not know whether the battery packs included cooling, however. And cooling was not mentioned in a five-page transcription of a Boeing media call explaining the incidents. The 787's use of lithium-ion batteries for the auxiliary power unit is said to be a first, which is one of the reasons why the batteries are being scrutinized so heavily. The National Transportation Safety Board (NTSB) X-rayed batteries from a January 7 fire aboard a Japan Airlines Boeing 787 at Logan International Airport in Boston. The NTSB team also did CT scans, disassembled the battery, and examined flight data recorders to determine if it exceeded its design voltage of 32V. On January 20, investigators said that the battery did not exceed its prescribed voltage. Since then, the agency has continued to look for the root cause of the problems, which have occurred on two Japan Airlines flights and one United flight. Whether or not the battery exceeded its design voltage, however, experts believe a cooling system was critical. Lithium-ion battery chemistries in general are "energetic," they said, and the cobalt oxide varieties of lithium-ion are particularly so. "Not all lithium-ion batteries are created equal," Cosmin Laslau, a research analyst for Lux Research, told us. "None of them should fail. They are all essentially safe. But in the event of a failure, lithium cobalt oxide would fail earlier than the other types. Chemical bonds in lithium cobalt oxide will release oxygen earlier." Experts say the release of that oxygen can, in rare cases, lead to fire. Many engineering teams around the world choose cobalt oxide chemistries, however, because it offers energy densities that can be up to 25 percent higher than other types of lithium-ion, such as manganese spinel (used in the Chevy Volt) and phosphate-based systems. To counteract the higher energies, big, lithium-ion batteries in general are often used in conjunction with cooling systems, no matter whether they are cobalt-, manganese-, or phosphate-based. The Chevy Volt, for example, employs liquid coolant that circulates through 1-mm thick channels machined into 144 metal plates sitting between its lithium-ion manganese spinel cells. Similarly, the Prius PHV plug-in hybrid uses specialized fans, intake ducts, and 42 temperature sensors to actively monitor and cool its lithium-ion battery. To be sure, the 787's 63-lb battery pack is smaller than those of today's typical electric cars, which can often exceed 400 lb. But experts said that lithium-ion batteries of all types need ways for heat to get out. "Size does make a difference," Cairns told us. "But the size of that (Boeing) battery is still substantial. If the cell casings are touching one another or have inadequate space to allow Design News | march 2013 | w w w. d e s i g n n e w s . c o m –31–

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