Cells inside the Boeing 787’s lithium ion batteries get “significantly” hotter when tested in cold temperatures like the conditions experienced by three aircraft that suffered battery issues, according to the US National Transportation Safety Board (NTSB).
“During the cell-level testing, the cells heated significantly more when discharged at temperatures between 0ºF and 32ºF than at temperatures above 32ºF,” says the NTSB in a report released today.
The report, the board’s response to a 7 January 2013 battery fire aboard a Japan Airlines 787 at Boston, does not attribute the fire to cold weather. Rather, the NTSB pegs blame on failures of Boeing, its contractors and the Federal Aviation Administration.
Still, the NTSB notes that three instances of major problems involving the 787’s batteries have occurred on aircraft that routinely operate in cold weather. In addition to the 7 January fire, on 16 January of that year a battery venting forced the emergency landing of an All Nippon Airways 787 at Takamatsu in Japan.
Then, on 14 January 2014, a Japan Airlines 787 suffered a battery failure at Tokyo Narita International airport.
“The airplanes involved in the [Boston, Takamatsu and Narita] events were based in Tokyo and were exposed to below-freezing temperatures during the winter months,” says the NTSB’s report. “Even though other 787 airplanes had occasionally been exposed to below-freezing temperatures while at their base location, the winter temperatures in Tokyo were the coldest of the cities where 787 airplanes were based.”
As part of the NTSB’s investigation, testing company Underwriters Laboratories conducted “cold temperature testing” of the 787’s eight-cell lithium-ion batteries.
“The NTSB was concerned that cold weather exposure could be a significant risk factor for the 787 main and APU [auxiliary power unit] lithium-ion battery," says the report.
The tests shows, that at 32ºF (0˚C), aluminum rivets on the batteries’ cells heated as high as 315ºF, exceeding the melting temperature of material that separates electrodes inside the cells.
The NTSB notes that Boeing has since “revised the low temperature charging limit for the main and APU battery and the [battery charger unit] to reduce the likelihood of an internal short circuit.”
The Japan Transport Safety Board, in its report on the 16 January incident, also raised the possibility of cold weather as a contributing factor.