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Japan presses Boeing to redesign 787 battery
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Citat:The JCAB called for the battery redesign despite determining that two of the three protective layers from a May 2013 battery redesign worked as intended, and the safety of the aircraft was never at risk during the incident on 14 January 2014.
Citat:The 787 is designed with two lithium ion batteries made by Japan-based GS Yuasa. One battery in the aft electronic equipment (E/E) bay supplies power to start the auxiliary power unit. The other battery is located in the forward E/E bay and supports the avionics systems in case all six onboard power generators fail. Both batteries are composed of eight cells that deliver 32V nominally.
Despite having similar functions, the Boeing design contrasts significantly with the lithium ion batteries installed in the Airbus A350-900. The Airbus supplier, Saft, designed a system with four batteries, each composed of 14 cells delivering 25V nominally combined. Thus, the A350-900 uses more batteries, with less power demanded from each cell than the 787 system.
All three major investigations into the 787 battery failures have deduced that an internal short circuit occurred within a cell, but have not been able to isolate the cause of the short circuit. The reports have suggested several possible causes, including cold soaking, the formation of dendrites and manufacturing errors.
The first two battery failures on the 787 posed serious safety concerns. In both incidents, a single overheating cell vented, causing adjacent cells in the tightly packed battery box to fail and partly melting the aluminium enclosure.
In response, Boeing redesigned how the batteries were installed. More spacing and thermal shielding was added between each cell. The eight-cell pack was enclosed in a stainless steel case. If one or more cells overheated anyway, new ducts would vent the heat and smoke directly off board.
According to the JCAB, two of the three layers of protection – preventing a thermal runaway within the battery and damage to the rest of the aircraft – worked as Boeing intended. But the third cell venting incident showed that the first layer of protection – preventing a cell from overheating in the first place – failed, the JCAB report says.
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