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.
Since the electrolyte of Lithium Ion batteries is flammable, fire emission is common during an extreme venting failure. That is why the battery is in a metal box that is supposed to contain any fire. The box is not designed to contain smoke. This is vented to the outside while in flight.
The FAA accepted this solution assuming that it would only happen once in 10 million hours according to the model. Since this happened twice in about 100,000 hours there is an obvious issue with the earlier assumption.
There is also some controversy as to whether the fire was actually contained in the box as it was supposed to be.
The power control unit that this battery is tied to has current control for charging and discharging or so the initial post in DesignNews said. So that controls the charging. If you limit the power in (or out) then you can have static cooling. Basically because the current draw is limited just thermal conduction of the solid material is sufficient to cool the battery. It seems to be the case here. If i was to take a gues i would say it was quality control, the design should have been tested and was most likely tested during certification of the plane and the component.
Thanks g_ost for the link to a presentation made by Yuasa that shows their products used in satellites...
It is the typical business presentation, proudly showing their modern producton installations and many achievements... but, wait. Did I see the word "FIRE" in page number 46 (slide 41 of 49) where in paragraph where they report the test named "Heat to vent" , they state the following phrase:
Cell vented when cell temperature reached to approximately 145 degrees C.
A small amount of gas was emitted from the cell. Approximately 10 minutes after the venting, fire emission was monitored."
At least they know that under overheating, the cell did catch FIRE.
Grrrreat!
Let´s now tell Boeing they need to add a note to the 787 FOM (Flight Operations Manual), requiring pilots to land within 8 minutes after battery problem, assuming 2 minutes for proper passenger evacuation, that is.
Of course, that's why the root cause is of critical importance. It does no good to cool a battery that is being damaged by overcharging. Since the pack that's failing is used for back-up avionic power, it sounds like the pack is being damaged by overcharging and cooling wasn't included in the design because the discharge level is so low. Mind you, I said "it sounds like".
Published information has indicated that there are 2 sets of Lithium ion batteries on a 787. One to start the APU and the one that caught fire that is designed to operate the aircraft flight systems in case of an engine or system electrical failure. If accurate, this suggests that the battery under discussion is constantly charged and only load tested to verify capacity prior to each flight. Other published data indicates that Li-ion batteries do not tolerate overcharging so each cell is monitored and the charge current varied based upon temperature and voltage. A shorted component within a cell in a series stack with no immediate temperature rise would be trying to drive the normal charging current through a short circuit. Heat rise would be extremely rapid, if the localized heat exceeded the maximum before it was detected, the fire would possibly have started before the charging current was removed by the monitoring circuit. The fact that Boeing has been replacing batteries is not in of itself an indication of a malfunction. Removal and analysis of in-service batteries provides a timely opportunity to correlate performance with expectations. The NTSB coming-out with this report should serve to quell public fears about the mystery of the fires. Now we need to know why did a short circuit happen and how do we prevent it in the future.
The article implied that the NTSB is done with their investigation. I certainly hope this was not the final report.
Although they were able to rule out a few potential causes, they DID NOT rule out an inherant design flaw (either inside the battery or outside in the charger or the load). Will Boing be required to follow up with a report of its own along with mitigation? Or will Boing put some extra sensors on the battery unit and continue on as if that solves the problem (that remains undefined?)
I like how active cooling is mentioned again? obviously it was not the cooling that caused the thermal runaway. It was an electrical failure. Would active cooling help? or would it put other systems in danger? say systems more critical than the battery? I don't believe that active cooling is the solution... the miniscule cost that is so readily referenced is not so miniscule. If Boeing paid the miniscule increases in cost to every component that needed to be boosted to unrealistic standards they would have to lift up a tank into the air not an airplane and development costs might double or triple.
Let me explain:
The cost is not miniscule its actually quite large. Development of such a system alone will run in the hundreds of thousands and then the fire tests that it has to go through will be in the same range.
The weight added will be significant. Remember this weight costs you every time that plane lifts up flies and lands that's a lot of money...
and last but not least "It's not needed"
However I do agree with the general consensus that it would be nice if batteries did not short during operation that's bad mojo.
Actually there are a number of ways to keep the internal series connection intact and still monitor and charge each cell separately! You can do so with floating chargers/voltage sensors that measure and send charging current across each cell individually.
This is obviously a more expensive approach, but doable. And if you want fewer connections outside the battery box you put the charging and monitoring electronics inside but design it in a fail safe mode that will cut off charging if the circuitry gets too warm. Look at some of the smart camcorder batteries. They have no more than 4 terminals to the outside world, charging current, common, output current and data.
It's all rather perplexing given that the technology is pretty well understood. It makes me wonder if something else is going on. If you start with the thought of what might be different about an airplane, pressure cycling comes to mind.
I suppose that that there might be other issues outside the normal concerns that were not considered.
Well written! Boeing's engineers no doubt picked up on potential problems with the battery system, and Liaison Engineers who are involved with nonconformance would have written up a disposition to look into the situation. The problem is that the disposition may have been transferred by A.N. Other to the next higher assembly for resolution until it reached flight test. By that time the original concern which raised the nonconformance would be a faded memory. Flight testing would address the perceived problem, find nothing untoward during flight testing and sign off the paperwork without necessarily contacting the original Liaison Engineer who wrote up the disposition...they may not be with the company anymore.
This is a common scenario. Only Liaison Engineers with a roving commission can actually check up on a travelling nonconformance to ensure that it is actioned within a sensible time period. With the large number of nonconformances that emerged during the DreamLiner episode paperwork could get buried quite easily.
With regard to the comment about bloated cells...that's likely to be on the broad face of the prismatic casing...#6 is sandwiched between #5 and #7 so it has nowhere to go....the vent would normally open to relieve the bulging pressure...did this happen?
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