The is calling on the to “review the methods of compliance” used to certify all permanently installed, rechargeable lithium-ion batteries in the active fleet and “require additional testing” if the results indicate the design and installation do not adequately protect against cell thermal runway.
The recommendation, along with four others to the FAA, are part of the agency’s continuing investigation into the thermal runaway of a lithium-ion auxiliary power unit (APU) battery on ain Jan. 2013. That aircraft was parked on the ground at Boston’s Logan airport when cleaning crews saw smoke in the cabin and maintenance crews found heavy smoke and a small flame in the APU battery case. The NTSB later determined that one of the eight cells in the battery experienced a short circuit, which caused a thermal runaway that spread to adjacent cells. A main battery failure on an 787 later that month led to the grounding the of 787 fleet and design changes to the system.
Based on testing by battery manufacturer GS Yuasa,in the 787 design phase had determined that an internal short circuit in a single cell -- simulated by driving a nail through the cell -- would not cause thermal runaway in adjacent cells within the battery. The NTSB says Boeing’s electrical power system certification plan, which the FAA agreed to, did not include a battery cell “abuse” test, which “simulates the most severe effects of internal short circuiting by triggering thermal runaway of a cell.” In the GS Yuasa testing, the battery was not electrically grounded or mechanically integrated into a representative 787 electrical system.
Along with thermal runaway in several cells, investigators probing the Boston APU battery found evidence of electrical arcing between the battery and case and “excessive” current flow in the ground wire and battery charger connector. The NTSB says “the unintended electrical interactions,” which likely occurred after the thermal runaway in the first cell, “might have contributed to the propagation of thermal runaway in other cells.”
During battery testing in March, the NTSB found that that the configuration of the battery during testing has a significant effect on the results. A nail penetration test of one cell in an electrically grounded battery resulted in thermal runaway of just the one cell, but “other cells within the battery case began to electrically discharge at an uncontrolled rate,” leading to arcing between the cell and case.
Based on the tests and other findings, the NTSB is recommending that the FAA develop “abuse” tests for initiating thermal runaway in worst-case conditions for permanently installed lithium-ion batteries, mounted with representative mechanical and electrical connections to the aircraft. Once the tests are defined, the NTSB is asking the FAA to require aircraft manufacturers to perform the tests during certification of “any new aircraft design” with permanently installed li-on batteries, as well as to reevaluate approvals already made. The NTSB says the description of the certification testing Boeing performed on the redesigned battery for the 787 “is consistent” with the recommendations.
Along with the 787’s main and APU batteries, lithium-ion batteries are used for the model flight control electronics, emergency lighting system and the data recorder independent power supply. FAA also has approved the batteries for use in the, -300 and -300ER as well as the and the , says the NTSB.