The Japan Transport Safety Board (JTSB) is broadening its ongoing investigation into the cause of the Boeing 787 battery problem to include support system components after finding no evidence of issues with the batterymaker, G S Yuasa.

As with the U.S. National Transportation Safety Board (NTSB) inquiry into the battery fire on board a Japan Airlines 787 at Boston on Jan. 7, the JTSB probe into the issue, which struck an All Nippon Airways-operated 787 the following week, appears to be no closer to finding a prime suspect.

JTSB officials report that among the ancillary systems suppliers, they are planning to inspect Kanto Aircraft Instrument, the maker of the battery monitoring unit which tracks the charging, voltage level and temperature of the lithium-ion units at the center of the mystery. However the NTSB, in its latest update, released on Jan. 27, indicates that as part of its investigation into the earlier JAL event, it already has visited Kanto, based in Fujisawa, and “found no significant discoveries.”

The NTSB says its investigation team “cleaned and examined both [Kanto] battery monitoring unit circuit boards, which were housed in the APU battery case. The circuit boards were damaged, which limited the information that could be obtained from tests; however, the team found no significant discoveries.”

The NTSB says disassembly is now under way of the last of eight cells from the APU battery involved in the JAL incident. “Examinations of the cell elements with a scanning-electron microscope and energy-dispersive spectroscopy are ongoing,” says the agency, which adds that a cursory comparison with the undamaged main battery revealed “no obvious anomalies.” A more detailed forensic examination will be conducted as the main battery undergoes a tear-down and test sequence series of non-destructive examinations, say investigators.

The NTSB’s airworthiness group undertaking tests of related battery system elements in Arizona also came up short of answers. The team tested the APU start power unit at the manufacturer Securaplane in Tucson, as well as at the supplier of the APU controller at UTC Aerospace Systems in nearby Phoenix. “Both units operated normally with no significant findings,” it says.

NTSB investigators also have been sent to Seattle to take part in FAA’s comprehensive review. One will focus on testing efforts associated with Boeing’s root cause corrective action efforts, while the other will take part in the FAA’s ongoing review of the battery and battery system special conditions compliance documentation.