A week after the Federal Aviation Administration initiated a grounding of the Boeing 787 that spread worldwide, there is still no indication of when the Dreamliner fleet may return to the skies. After some initial hope that the airplane could begin flying again over the weekend, investigators in the U.S. and Japan continue to focus on the pair of cooked lithium-ion batteries that led to the first grounding of an American commercial airplane fleet since 1979.
The National Transportation Safety Board is examining the 63-pound lithium-ion battery from the 787 that caught fire in Boston on January 7, along with stored data from the airplane, and currently believes the battery did not suffer an overcharging. Japanese investigators came to a similar conclusion after looking at the 787 that made an emergency landing last week in Japan. At a press conference today, NTSB Chairman Deborah Hersman emphasized the seriousness of the two events affecting the 787.
"This is an unprecedented event," Hersman said, "and this is a very serious air-safety concern."
Hersman said the NTSB is concerned that there were two separate battery failure events involved in such a new aircraft. The NTSB is working with Boeing, Japanese investigators along with other investigators from around the world to try and determine why the battery failures occurred.
The NTSB has two shifts of investigators working in both the U.S. and Japan. Hersman confirmed the battery involved in the 787 that caught fire in Boston did show evidence of a short circuit in one of the eight cells that make up the battery. No information was given about how long the investigation may take or what may need to be done to the grounded 787 fleet before it can return to flight.
"If we find that there are any vulnerabilities, we will make recommendations" Hersman said.
The lithium-ion batteries used in the 787 are relatively new batteries made of relatively large cells compared to those used in most consumer devices. The history of lithium-ion batteries has many thinking the problem might not be a "teething problem" with the airplane, but instead an issue with the batteries.
Unlike the well-proven, and relatively small bundles of battery cells used in consumer devices or even the Tesla electric car known as "18650s," the batteries in the 787 made by GS Yuasa of Japan are produced in low numbers and are not used in many applications. And as the focus on the battery continues, one lithium-ion expert says the large batteries used by Boeing simply increase the potential for failure.
"As the size of the cell increases, the chance of something happening increases because you have an increased amount of material being exposed" says Dr. K.M. Abraham. "[Lithium-ion batteries] are not as forgiving as far as design and construction are concerned. If you have quality control issues, it can be very bad."
Dr. Abraham is a battery consultant and professor at Northeastern University in Boston and has been researching lithium-ion batteries since 1976.
Lithium-ion batteries are extremely power dense, delivering a lot of electricity from a relatively compact package. To do this they need extremely thin sheets of the plastic material to separate the cathodes and anodes inside the battery. The large 32 volt battery like those found in the 787 is made up of eight 3.7 volt cells. Dr. Abraham says the design and construction of a battery is critical so that these separators are not damaged during manufacturing or during a battery's use.
"I always consider the separator as a major source for a problem" Dr. Abraham says of the sheets that are 25 microns thick, around the same thickness as cellophane.
Inside the batteries used on the 787, each of the eight cells consists of three separate foil windings that are made up of copper and aluminum electrodes separated by the thin film separator. Each of the three windings is approximately ten meters long providing more than 35,000 square centimeters of separator surface area where a single hole could lead to a short circuit between electrodes.
"Once the polypropylene is breached you have an internal short" says Dr. Abraham. "Once they internally short, there is little you can do to stop it, it goes off like a rocket."
Another possible source of failure is still an overcharging, but it may be in just a single cell rather than in the battery as a whole.
"Each battery is made of large number of cells to get 32 volts" Dr. Abraham says. "Each is protected for overcharge and over discharge. It is possible that one cell is overcharged, even when the total voltage is only 32."
Dr. Abraham says that after many cycles, it is possible that the electrodes in a cell can be used differently. If this leads to an imbalance, there can be an overvoltage in just one cell while the battery as a whole does not indicate an overcharging as appears to be the case in the batteries on board each of the affected 787s.
The NTSB says it is still investigating whether or not a single cell could have been overcharged, and did not have an answer at this time to whether that may be a possibility.
The rigors of actual in-flight use is widely known to lead to many of the teething problems experienced in new airplanes. The day-to-day flights can often uncover problems not found during the testing of individual components such as the batteries and the flight testing of the airplane.
Right now all of the evidence does point to a problem with the batteries and not with the design of the 787 as a whole. Chairman Hersman did say the NTSB is also looking into why the several safeguards designed into the battery system did not prevent the thermal runaway event. It appears the design of the area where the batteries are located did do its job in limiting further damage to the aircraft and venting smoke out of the airplane, and not in the cabin. Though since fire fighters were on the scene in about ten minutes to extinguish the battery that caught fire in Boston, it is unknown how much damage may have occurred had the battery burned out on its own. Boeing says the electronics bay is designed to contain such a fire.
With the investigations both in the U.S. and Japan focused on the batteries, a possible reason for the events that led to the 787 grounding could be manufacturing problems with the battery itself. Each Dreamliner carries a pair of the 32 volt lithium-ion batteries made by GS Yuasa.
The battery in the Japan Airlines 787 in Boston is located towards the tail of the airplane and caught fire while the airplane was sitting at the gate after passengers had disembarked. The All Nippon Airways 787 was forced to make an emergency landing in Japan after pilots smelled smoke and indicators in the cockpit showed a problem with the battery located in the front of the airplane. Both batteries are in sections of the airplane that are temperature controlled and pressurized, though at a different level than the main cabin to minimize the chance of smoke entering the cabin according to a Boeing spokesperson.
The batteries provide electrical power for starting the auxiliary power unit in the tail of the airplane, as well as backup power for several systems on board the Dreamliner.
Lithium-ion batteries have been used in consumer devices since the 1990s and they are manufactured in highly automated factories which have been refined for many years. Dr. Abraham says today batteries such as the 18650 used in laptops and flashlights have extremely low failure rates, less than one in a million. But this wasn't always the case. In fact he says it was more than a decade before the manufacturing process was refined and improved to give the reliability the small lithium-ion batteries provide today.
And in larger scale applications such as the Tesla electric cars, the thousands of 18650 batteries used are controlled by electronically to provide the power needed as well as to protect the bundles of batteries. Tesla founder and CEO Elon Musk even offered to help in a Twitter message after the fleet was grounded.
"Maybe already under control, but Tesla & SpaceX are happy to help with the 787 lithium ion batteries" Musk messaged on Friday.
Boeing has stood by its decision to use the energy dense lithium-ion batteries, and any change to another type of battery would require significant changes to the overall electrical design on board the 787. There is currently no indication of when the Dreamliners may be flying again, though production continues on the Boeing 787 assembly lines near Seattle and in South Carolina. But with the grounding affecting flight testing as well, airplanes are simply being parked outside the factories.
