Boeing improves its Li-ion batteries

Boeing has initiated a comprehensive set of improvements that will add several layers of additional safety features to the lithium-ion batteries on 787 commercial jetliners that are in production and could be ready for initial installation within the next few weeks. New enclosures for 787 batteries also are being built and will be installed in aeroplanes in the weeks ahead.

The improvements include enhanced production and operating processes, improved battery design features and a new battery enclosure.

The enhancements to the battery system address causal factors identified by the Boeing technical team as possible causes of battery failure. The technical team’s findings also were verified by an independent group of lithium-ion battery experts from a number of industries, universities and national laboratories.

“We’ve come up with a comprehensive set of solutions that result in a safer battery system,” said Mike Sinnett, vice president and chief project engineer, 787 program, Boeing Commercial Airplanes. “We have found a number of ways to improve the battery system and we don’t let safety improvements go once they are identified. We incorporate them into our processes and products.”

The first layer of improvements is taking place during the manufacture of the batteries in Japan. Boeing teamed with Thales, the provider of the integrated power conversion system, and battery maker GS Yuasa to develop and institute enhanced production standards and tests to further reduce any possibility for variation in the production of the individual cells as well as the overall battery.

“We’ve all developed a better understanding of the sensitivities of this technology to variations during the manufacturing process,” said Sinnett. “And we all feel the need to increase monitoring of this process on an ongoing basis.”

Four new or revised tests have been added to screen cell production, which now includes 10 distinct tests. Each cell will go through more rigorous testing in the month following its manufacture including a 14-day test during which readings of discharge rates are being taken every hour. This new procedure started in early February and the first cells through the process are already complete. There are more than a dozen production acceptance tests that must be completed for each battery.

Boeing, Thales and GS Yuasa have also decided to narrow the acceptable level of charge for the battery, both by lowering the highest charge allowed and raising the lower level allowed for discharge. Two pieces of equipment in the battery system - the battery monitoring unit and the charger - are being redesigned to the narrower definition. The battery charger will also be adapted to soften the charging cycle to put less stress on the battery during charging.

Changes inside the battery will help to reduce the chances of a battery fault developing and help to further isolate any fault that does occur so that it won’t cause issues with other parts of the battery.

To better insulate each of the cells in the battery from one another and from the battery box, two kinds of insulation will be added. An electrical insulator is being wrapped around each battery cell to electrically isolate cells from each other and from the battery case, even in the event of a failure. Electrical and thermal insulation is installed above, below and between the cells to help keep the cells from impacting each other.

Wire sleeving and the wiring inside the battery will be upgraded to be more resistant to heat and chafing and new fasteners will attach the metallic bars that connect the eight cells of the battery. These fasteners include a locking mechanism.

Finally, a set of changes is being made to the battery case that contains the battery cells and the battery management unit. Small holes at the bottom will allow moisture to drain away from the battery and larger holes on the sides will allow a failed battery to vent with less impact to other parts of the battery.

The battery case will sit in a new enclosure made of stainless steel. This enclosure will isolate the battery from the rest of the equipment in the electronic equipment bays. It also will ensure there can be no fire inside the enclosure, thus adding another layer of protection to the battery system. The enclosure features a direct vent to carry battery vapours outside the airplane.

New titanium fixtures are being installed in the electronics equipment bays to ensure the housing is properly supported.

During engineering testing, which occurs prior to certification testing, the team demonstrated that the new housing could safely contain a battery failure that included the failure of all eight cells within the battery. The ‘ultimate’ load is the equivalent of 1.5 times the maximum force ever expected to be encountered during a battery failure. The housing easily withstood this pressure and did not fail until the pressure was more than three times the ultimate load.

Through another test, the team demonstrated that fire cannot occur within the new enclosure. Its design eliminates oxygen, making the containment unit self-inerting. Inerting is a step above fire detection and extinguishing as it prevents a fire from ever occurring. The design also vents all vapours by venting directly outside of the aeroplane rather than into the equipment bay.

“We put this new design through a rigorous set of tests. We tried to find a way to introduce a fire in the containment but it just wouldn’t happen. Even when we introduced a flammable gas in the presence of an ignition source, the absence of oxygen meant there was no fire.