ANALYSIS: Another incident puts spotlight back on 787 batteries

Washington DC
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For Boeing, it’s another new year – and another new lithium ion battery malfunction on the 787.

Details are still sketchy about the full scope of the malfunction and the damage aboard the Japan Airlines 787 main battery today in Tokyo, but the released information so far indicates critical differences with the twin battery failures within eight days last year that grounded the fleet.

A technical refresher on the 787 electrical system and the role played by the lithium ion batteries may be helpful.

The 787’s “more electric” architecture simply means that it uses electric power to pressurise the cabin and de-ice the wing rather a bleed air-driven pneumatic power supply.

Such an architecture does not require Boeing to use lithium ion batteries as the main back-up supply for the cockpit avionics and to start the auxiliary power unit. More traditional – and less powerful – nickel cadmium batteries could have been used. However, the 787’s electrical demands are higher than any other commercial aircraft, so Boeing decided to use a powerful lithium ion chemistry to minimise the weight of the battery system.

The precise chemistry of the 787 battery system also has been controversial. Auto manufacturers use a less potent and combustible lithium chemistry for hybrid cars called iron phosphate and lithium manganese. The 787 battery, however, is based on lithium-cobalt-dioxide, a formula that when significantly overheated releases oxygen that makes the fire burn longer and with greater intensity.

Inside each of the 787’s two, 32V lithium-ion batteries are eight cells, each capable of a 4V charge. The chemistry within the cells allows each to pack an incredible 72amp hours of power storage, compared to 3-4amp hours for a consumer lithium-ion battery. It was this raw power that provoked one supplier to describe the 787 system as the “Ferrari of batteries”. That supplier, Securaplane, would know, as a malfunctioning 787 battery in 2007 led to a fire that destroyed its test facility in Tucson, Arizona.

The latest malfunction means these rechargeable lithium ion batteries in the 787 fleet have experienced three cell venting events since entering service about 27 months ago. A fire caused by a non-rechargeable lithium ion battery on a 787 severely damaged an Ethiopian Airlines 787 last July, but this involved a Honeywell emergency locator/transmitter system that was not unique to the 787.

Of three confirmed cell-venting events, the previous two last year appear to be far more serious.

In those two events spaced within eight days a year ago, at least one and perhaps more than one – the exact sequence is still being investigated – of the cells within the batteries overheated.

As temperatures rose within the tightly packed battery case, adjoining cells began to also over-heat and “vent”, spewing smoke and boiling electrolyte out of a melting aluminium battery case.

Fortunately, neither of the two batteries last year burst into a sustained fire, although a firefighter in the Boston event saw two small jets of flame emitting from the battery box. Investigators are still scrutinsing the facts. However, Boeing believes the batteries overheated, yet stopped short of a true thermal runaway.

Thermal runaway events in certain kinds of lithium ion batteries like the ones found on the 787 are feared because they create their own oxidizer, so therefore are difficult if not impossible to extinguish if there is a fire until they run out of fuel. On an aircraft at cruise altitude that can be disastrous.

Not surprisingly, the US Federal Aviation Administration (FAA) grounded the 787 fleet for 3.5 months last year until Boeing came up with a safer battery system.

Boeing decided to leave the configuration and the chemistry of the 787 battery the same, and instead focus on improving how each of the cells are arranged within the case. Instead of two rows of densely packed rows, Boeing added space between each cell. Within that space Boeing inserted ceramic plates to act as heat shields. If any cell vents, the additional space and shielding should prevent the heat from moving from one cell to the next. Boeing also replaced the aluminium battery case with a stainless steel box capable of absorbing an explosion of a battery in the event of thermal runaway.

The 787 fleet returned to service with the new battery installation in May. For eight months, Boeing did not receive a report of another venting by one or more cells within the 787’s lithium ion batteries, notwithstanding the unrelated event on the Ethiopian 787 in July.

That unblemished streak ended on 14 January, or two days before the one-year anniversary of the 787 grounding.

Japan Airlines reportedly said they discovered that one of the eight cells inside the APU battery vented smoke. Maintenance workers also found an unidentified “liquid” in the battery box – most likely, electrolyte that boiled out of the cell during the venting process.

A battery cell that vents when it overheats is malfunctioning, but it is doing so correctly. A battery that fails to vent as it overheats could explode and trigger a harmful chain reaction in other damaged cells. Rather than dismaying safety investigators, the safe venting of a malfunctioning battery cell is more likely to impress them.

Boeing released a statement that only one of the 787 main battery’s eight cells vented in the latest incident. That is also a fact that investigators will appreciate, if true. It means that the elaborate layers of protection that Boeing installed during the grounding last year worked as designed. A malfunction in one cell did not cause a chain reaction in the adjoining cells.

The lack of damage within the 787 battery could be a boon to safety investigators. The US National Transportation Safety Board is still seeking a probable cause for a 787 battery malfunction in Boston more than a year ago. Its task was made more difficult because the overheating cells incinerated most of the evidence. The Japan Transport Safety Board may now have the opportunity to inspect a lesser-damaged battery that is possibly more useful for discovering a root cause.