Individual component tests (including those for the BCU, SPU, and APU controller) and
integrated system tests, which included a BCU and other battery-related components, found no
evidence indicating that external components were related to the battery failure. Vibration testing
was performed at levels that exceeded in-service levels without damage that could lead to battery
failure. A battery-level test showed that condensation could occur within the battery and result in
a shorting path between the cell and battery cases, and a cell-level test with moisture showed that
a cell failure could occur and result in arcing, shorting, and heat damage internal to the cell
header. However, the damage observed during the cell-level test was not found in the cell header
from the incident APU battery. In addition, airplane flight test data did not show any abnormal
electrical transients that could lead to battery failure.
External observations of the battery case showed more thermal damage on the right side
of the case than the left side. The right aft corner of the battery case showed the most thermal
damage and the most damage to the case lid and its fastening points. The right aft area
corresponded to the positions of cells 5 and 6.