The US National Transportation Safety Board launched a two-day hearing on lithium-ion batteries today with testimony from experts about battery technology and the phenomenon of "thermal runaway", as US regulators are expected to be reviewing test data from Boeing on the results of recent 787 flights with an improved lithium-ion battery.
The US Federal Aviation Administration (FAA) grounded the 787 on 16 January, following two incidents involving the lithium-ion batteries powering the aircraft's auxiliary power unit on two different 787s.
The FAA's decision prompted a global grounding of the aircraft type across all operators.
NTSB staffers, questioning academics and industry executives, focused on short circuits and thermal runaways, a process in which unintended chemical reactions generate heat that can cause batteries to burn.
Investigators in the USA and Japan have detected signs of thermal runaway and short circuits within the 787's two eight-cell, 32V lithium-ion batteries.
Daniel Doughty, president and founder of Albuquerque, New Mexico-based Battery Safety Consulting, tells NTSB officials that internal short circuits can be caused by foreign material trapped in a battery cell, impurities in the materials used to make the batteries and repeated operation of the battery outside design limits.
Short circuits in cells can cause temperatures to increase gradually at first, Doughty says, but once the temperature hits a certain "tipping point," a thermal runaway can be triggered.
"The rate of temperature rise could go on for a day and a half," he says. "Then it rises much faster. When it reaches that [tipping point] it is very rapid."
Doughty adds that not all thermal runaways are violent; they depend on the chemistry of the cell and other conditions.
Glen Bowling, vice president of sales for Saft Specialty Battery Group, a designer and manufacturer of high-performance lithium-ion batteries, says runaways in one cell can be stopped from spreading to adjacent cells by spacing battery cells farther apart to allow heat to dissipate. Barriers between cells and ducts and baffles to expel hot gases can accomplish the same, he said.
Experts tell officials that liquid materials in the batteries could be made less flammable, but that doing so would reduce the power generated by the batteries.
Vincent Visco, senior vice president of business strategy and development at Sylmar, California-based battery manufacturer Quallion LLC, says battery cell failures are very rare and seldom occur in one-cell batteries used in mobile phones or four-cell batteries used in laptop computers.
But failures have become more common recently because newer, higher-power batteries have more cells, Visco says.
And when thermal runaways occur in batteries with many cells, the result is "more spectacular," he adds.
In March, Boeing disclosed that it redesigned the 787 battery enclosure to "eliminate" the possibility of a fire caused by an over-heating battery. The enclosure now allows the electrolytes within the battery to vent if they become overheated within the battery box.
Any fumes emitted by the electrolyte are then vented directly offboard through a newly-installed, dedicated vent line.
The redesign also features a new charger with reduced maximum charging levels, a higher maximum discharging level and a softened charging sequence.