The Boeing 787 battery fire in Boston last month was caused by a battery short circuit that led to a thermal runaway condition, the National Transportation Safety Board (NTSB) has concluded.
NTSB investigators who did forensic analysis of the Boeing battery said the charred components indicated the temperature inside the 63-pound pack had exceeded 500 degrees Fahrenheit during the January 7 incident at Logan Airport. Evidence showed that one of the pack's eight cells short circuited, causing the temperature to soar.
"Our review of the flight data recorder data shows that the voltage of the battery dropped from a full charge of approximately 32 volts to approximately 28 volts," said NTSB chairman Deborah Hersman in a press conference on Thursday. "This drop is consistent with the charge voltage of a single cell."
NTSB chairman Deborah Hersman said last week that investigators have concluded that a short circuit occurred in cell number six of the pack's eight cells. (Source: NTSB)
NTSB investigators ruled out mechanical impact damage and external short circuits as the reason for problems inside the battery, Hersman said. They did not know, however, whether the short circuit was caused by battery charging or by defects in the design and construction of the unit, or some other problem.
Experts contacted by Design News said a temperature of 500 degrees can create serious problems for a lithium-ion battery pack. "It's high enough to cause the cells to bloat and possibly to burst into flame," Donald Sadoway, John F. Elliott professor of materials chemistry at MIT, told us in an email. "Remember, the electrolyte is volatile and flammable."
Boeing said it still has not announced any specific ways to fix the 787 batteries, despite reports in various news media that it has already settled on a fix. The Wall Street Journal reported on Friday that the company "is looking at increasing the separation between cells in the lithium-ion batteries to reduce the potential hazards from heat or fire spreading within the batteries and adding enhanced heat sensors."
"There has been no public decision by the company on what these answers might look like or, specifically, what our teams have been working on," Boeing spokesman Marc Birtel told Design News. "We're not going to talk about a fix until we have something to share that's at the point of approval by the FAA."
Improvements like the ones mentioned by The Wall Street Journal have been the subject of much speculation over the past month. Two weeks ago, Tesla Motors CEO Elon Musk publicly stated that the 787 battery pack design was unsafe, and experts such as Sadoway and Elton Cairns of the University of California have suggested that an active cooling system would provide an additional layer of safety for a cost that would be miniscule compared to other 787 sub-system costs.
Sadoway again said last week that the Boeing batteries would be safer with active cooling. "That's what GM does in the Chevy Volt," he told us.
First, It could very well be that the cells were OK, but were badly installed, or badly monitored, or badly handled or used outside their true limits... all by others and not by the original maker. Or it could have a cell internal failure, which could be blamed directly on their manufacturer. At this moment, we DON'T know.
But what we do know is that the Lithium chemistry in general (with its different and different behaviour varieties), is certainly not intrinsically safe, as is has much more energy density and requires considerable care in its use and implementation. There have been numerous fires caused by them in many places, some aboard planes.
On the other side, the airplane manufacturer has the absolute responsibility of releasing a throughly tested and safe design. I respectfully disagree with those believing that a given "advance" or "huge leap" is the responsibility of a commercial plane manufacturer. At least not on airplanes meant to cross oceans between continents with hundreds of human beings aboard. Would the 787 fire happened on a long ocean crossing and causing a downed plane, would the urge of "progress" justify that?
The battery was not made by Boeing-- it was made by Yuasa.
I wonder how they have been able to keep their name out of the news so succesfully? As a Japanese company shouldn't their president resign and publicly apologize-- or isn't that done any longer?
As someone has said above, the 787 battery that failed was made by several cells connected in series in order to provide the required useful voltage. Those cells are hardwired. If you want to be able to electrically separate any of the cells, you would need to take the terminal wires to a selector switch, togehter with the wires of any extra cell. Those extra connections would reduce the reliability and the output current capacity, demand a comparatively large additional space and would have a sizable weight increase, largely negating any weight/space advantages of the Lithium chemistry. In regards to your statement about "smart" batteries, they are already "smart", as all cells are monitored individually with thinner wires in order to be able to be "balanced" by a dedicated circuitry during recharge. Cooling will only solve a high temperature problem if it is inside the thermal runaway limit, but won't help at all if the cell goes bad by itself. Trying to keep the resulting flame contained or conducted to a safe place is feasible in theory, but will again reduce the weight-size savings of the selected Lithium chemistry... so, was it a sensible selection to start with?
As any model aircraft amateur who has been into electric powered models can attest, Lithum batteries are a peculiar animal. They need dedicated chargers, a lot of Tender Loving Care and some luck too. They can go crazy from time to time. A heavy walled ceramic pot is not out of place when charging them. (May I suggest you to take a look at some photos of completely burned down cars when someone left a small battery pack charging inside).
I'm surprised that some overly 'creative' engineers at Boeing went on using this technology on a plane, when everyone that has some real-life experience with Lithium cells has developed a lot of respect and a healthy dose of distrust too. Subcontracting/Outsourcing this set of components is another reason for failure, as responsibility is thrown around and diluted all too easily. It will not be easy to precisely know what happened, as bad reputation is going to be "managed" in order to perform a crisis handling that will pretend that the original battery chemistry selection was either very "advanced", or "bold" or a "breakthrough"...
Like I've said before, if the battery is prone to runaway thermal failure, it really needs to be designed to be jettisoned like a fuel tank when it gets into trouble, or insulated and isolated in such away that it can burn itself out without smoking up the aircraft or risking further damage. Additional thermal sensors alone are not going to fix this problem! I'm also concerned that charging is not done on a cell by cell basis as is the case with all of the consumer Lithium Ion battery packs I'm familiar with. This is the only reliable way to catch faulty cells before they fail catastrophically or to insure proper charging.
Charging the cells in series configuration requires perfect matching of each cell. And if they don't track with age, serious overcharging of some cells could result! One of the benefits of individual cell charging is being able to measure cell temperature and control the charge cutoff accordingly. In a series strung pack, the weakest cell, the one that attains end of charge temperature first, would have to force charge termination for the entire pack. There is no way to use the voltage curve to safely terminate the pack's charge if one cell starts to go bad. In fact, if a cell's voltage gets depressed due to a partial internal short it might result in a prolonged charging cycle which would further damage the bad cell.
With Boeing indicating they want to add more temperature sensors to the battery, I wonder if they presently only monitor the entire packs temperature and not individual cells.
Great point, plasticmaster. Adding an active cooling system would indeed add weight. Depending on the type of cooling system used, the weight of the battery pack could jump from 63 pounds to a little over 100 pounds. As you point out, an additional 40 pounds tends to make aircraft engineers unhappy.
I'm confused. Cooling can be achieved using liquid, gas, or air. VW did it with the Beetle (using air) to cool the engine. A transatlantic flight will see outside temperatures of -65°F. Couldn't this air be harnessed in some way to act as a cooling agent on these batteries? (providing that's the real problem)
Wouldn't be possible to put some intelligence into these batteries?
I can see having a simple monitor for voltage and temperature of each cell. If one cell got hot or reduced it's voltage a redundant cell could be switched in. Of course the monitor would also report status upstream to the aircraft electronics.
Surely this has been considered in the past. Does anyone know why it is not practical?
Now you're talking! Excellent observation AREV. At the very least, this incident/challenge is keeping Boeing foreward moving down the innovative and technological path.
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