Seattle news just showed XRay's of one of the toasted batteries that showed 6 of the 8 cells badly defromed. They are saying it may have been caused by "Thermal Runaway". http://en.wikipedia.org/wiki/Thermal_runaway
Today's 'Taipei Times" has an article saying that nobody has reached any conclusion about the cause of the fires, but that Boeing may have started some test flights. There were mentions of having done CT scans on the cells as well.
I've been pondering the problems with Lithium cells in relation to the charge cycle.
If you go too high a voltage you damage the battery and have a thermal runaway.
On the other hand, if you go to low on discharge, the cell goes into a failure mode that is effectively a short. Then in subsequent attempts to charge, that cell may create a thermal runaway due to near zero resistance ( if it is near zero - other cells may be getting an over-voltage on the charge cycle).
The balancing can easily be controlled electronically with one and maybe two cells in series, but it gets to be more difficult as you chain more and more cells into a series. Internal battery resistances are dynamic and cannot be easily monitored to compute and insert a moderating influence. I am not sure that inserting a moderating scheme is feasible.
I suspect that because the cells are in series to establish higher voltages, it is easier to have one cell become out of balance with the rest. The actual position of the first failure cell in a series arrangement may be very telling about why the control circuits do not catch a defective cell before a thermal runaway occurs. Cells at the tail ends of a series may go to greater extremes due sudden demands.
Everything I read last year about large series arrays of Lithium cells seemed more theory than practical. Many chip manufacturers created chips to monitor series of 8 to 12 cells and then discontinued the products - apparently nobody had a good design thesis. Individual thermal sensing each cell should be a redundant fail safe network to monitor the cell's health. And temperature should be logged in relation to charge or discharge to determine which mode causes a thermal runaway.
There has been sales literature that claims that matched lithium cells are a must for safe series operation and other literature that claims that matching lithium cells is not feasible. Statistical performance analysis of production batches show large variance in performance within one batch. And test procedure to match internal resistances are rather dubious. So having a completely separate thermal monitoring scheme might be the only way to monitor battery stability.
These days we have a lot of electric bicycles that have been sold with 8 or more cells. The charge control boards have tried to create advanced balancing schemes, but every balancing scheme made the cells more prone to being locked out as one cell fell out of spec, and the balancing schemes actually shortened the overall capacity of the cells and or lengthed the charging time.
So the bicycle manufactures seem to have accepted that a series of lithium cells is best balances by letting the batteries seek their own via the chemistry with a simpler control and monitoring circuit.
I think the topic could make a good master's thesis for an EE. Lots of items are still unresolved.
What ever the characteristics of the batteries, this was supposed to be covered in test: Whats the maximum voltage the batteries will see? What the minimum voltage to which that batteries will be run? If these are exceeded, flag an error. Put it through its paces.
This part is simple. The battery guys know the expected limits of the design. They know the causes and consequences of these limits. They are supposed to check this by putting it though its paces during testing; THAT was the time to discover is something needed more analysis or change, not when MY flight is in the air. Either the limits are wrong, or the method for detecting exceeding the limits is wrong, or logging and reporting is wrong.
To say this is a new tech, and we don't know enough about it, so we skipped the proper testing and put it in an airplane full of people, if not acceptable.
There's temperature, vibrations of different kinds, possibly shock from touchdowns.
There may or may not be an issue with low pressure, too.
(Shouldn't be, as most pressurised airliners keep the pressure 'normal' inside the entire fuselage, but then again... Also, they typically keep the pressure to about 900mBar to reduce weight)
In any case: One must ask, what's new in the battery requirements of the Boeing 787? If there are genuine technical issues not still resolved w.r.t. airplane batteries, why isn't this a problem for every other commercial airliner? I mean, there are brand new Airbus plastic planes around.. and even those with more metal are constantly manufactured and put into service, and there are no battery issues (or certainly not enough to ground the whole fleet). To me it looks like braino assessed the problem correctly in post #3.
Well, the limits for voltages in batteries are statistically determined, while a shorted battery is a deterministic failure. There is a big difference in how reliable the engineering is and in testing procedure. Statistics can come back an bite you. Japan was never supposed to have 9.0 earthquake, it was a statistical outlier.
Added to that that statistics collected for single cell behavior may vastly differ from the statistics for a long series configuration of cells due to relying on the aggregate chemical behavior of all the cells in use.
Good testing procedures are based on experience, without experience everything is a best guess or an interesting thesis.
New battery requirements? Double the batteries on board and have the second set remain unused but ready in case of catastrophic failure. You would still have less weight for power than lead acid, and you would be able to more freely explore reconfiguration while collecting data in actual use.
There are unique issues as Boeing went to an all electric control and quit their previous hydraulic system. So even if the airplane has no jet engines, it needs the batteries to operate the controls.
Of course, this presumes that the problem will be cheaper to resolve if the airplanes are not grounded while seeking a final solution.
There are unique issues as Boeing went to an all electric control and quit their previous hydraulic system. So even if the airplane has no jet engines, it needs the batteries to operate the controls.
New to Boeing perhaps, but not unique.. Airbus has been fly-by-wire since the A320. But maybe the 787 is even more electric - there are still some hydraulic pumps in the Airbus planes. But whatever differences there may be, I'm convinced braino hit the nail of the problem in post #3.
Well, the limits for voltages in batteries are statistically determined, while a shorted battery is a deterministic failure. There is a big difference in how reliable the engineering is and in testing procedure. Statistics can come back an bite you. Japan was never supposed to have 9.0 earthquake, it was a statistical outlier.
Added to that that statistics collected for single cell behavior may vastly differ from the statistics for a long series configuration of cells due to relying on the aggregate chemical behavior of all the cells in use.
Good testing procedures are based on experience, without experience everything is a best guess or an interesting thesis.
New battery requirements? Double the batteries on board and have the second set remain unused but ready in case of catastrophic failure. You would still have less weight for power than lead acid, and you would be able to more freely explore reconfiguration while collecting data in actual use.
There are unique issues as Boeing went to an all electric control and quit their previous hydraulic system. So even if the airplane has no jet engines, it needs the batteries to operate the controls.
Of course, this presumes that the problem will be cheaper to resolve if the airplanes are not grounded while seeking a final solution.
Any airliner is at risk of control system loss if the main engines fail. The old hydraulic systems needed just as much energy to run. But the hydraulic systems have the added risk of running dry. Backup power is what the APU if for, and if that fails, the ram air turbine is deployed. The battery that failed in the 787 was mostly a glorified starter battery, If it could power the control systems for more than a 5-10 minutes I'd be amazed. (i.e. I suspect most of the 787's about 2 megawatt electrical capacity goes to climate control with only a few kilowatts used for flight control.)
Anywho, best guess I've seen so far for a failure cause is pressure cycling during operation. The cells used are large and rectangular, thus the walls will move some as the pressure changes. This movement is likely to cause internal damage, and there are plenty of YouTube videos showing how lithium batteries react to internal damage. (hint: fire!)
@Lawson
Interesting info. The starter system would be huge power drains on the lithium, maybe pulling cells destructively below their low voltage limits for mere moments. But it does imply that the overall safety regime of the 787 remains intact.
Pressure flux is an interesting issue. The solution would be to simply encase all the cells in a pressurized housing with a stable pressure -- maybe a nitrogen atmosphere would also limit combustibility in a thermal runaway.
I need to read the big report about. At least the USA is willing to let the public read things like this rather than put up a wall of secrecy. As I keep saying, this is a very interesting topic for anyone starting a career in EE. We seem to have a world that is committed to more and more batteries in the future.
The batteries are ruggedised and supposedly rated for aviation use. I would be surprised if it was anything more than charge/discharge issues. This alone is bad enough. No need to look elsewhere given how easy they are to destroy this way.
A. While the NTSB is indeed investigating a 'fire event' in the APU battery (which is an auxillary battery used for starting the jet), in Japan the JTSB is investigating a 'smoke event' in the MAIN battery that occurred a week later.
...So it seems that any and all power on the 787 might be at risk
B. In an 8 cell battery pack, cell #5 has been found shorted and others swollen. The NTSB say a short circuited cell alone is enough to cause a thermal runaway. Cells #8, #7, and #6 seem more damaged that the ones below #5. Not sure where the ground is, but this may be important in terms of which cells suffer more stress due to their position in a series.
My own observations...
The NTSB avoided some of the details and talked mostly about the 8 cells as a whole, rather than one cell failure having a knock-on effect.
The JTSB investigation is ongoing and the NTSB said nothing about what they are finding. It seems that where the incident takes place determines who is the lead investigation and so there are two investigation - one in the US and another in Japan.
The chicken and the egg....
Did a shorted cell cause the over-heating that caused the thermal runaway or vise-vera?
From what I have read it is dropping below the low voltage boundary caused the cell to short, while exceeding the high voltage boundary causes a thermal runaway.
So do the two happen in rapid succession?
In other words, a battery is taxed by a high load and one cell drops below the low voltage boundary, and then the charge circuit tries to recharge the cell and due to the shorted portion exceeds the safe high voltage limit for an Individual cell and thus causing a thermal runaway. That is what I suspect.
While the sensors might read safe overall voltage levels, in the low voltage situation the drop may be too quick to catch. In the high voltage situation, the charger may be watching the aggregate voltage to the cells and not the individual cells.
Thermal sensors on each cell might independently call attention to a cell that may be getting over-voltage due to an adjacent shorted cells and enable a safe shutdown. Did they have on-going thermal monitoring of all cells?
But you still need to get electricity from somewhere...
I was using a battery grinder which is the highest current of all power tool that you can have
I was using this tool very hard the battery pack had got very very warm almost to warm to hold on to it (one end of the battery pack) for very long but what happen next was surprising that the pack was hissing I took the pack apart after the pack cool down and found 2 cell that paired together there voltage when to 0 Volts
So would not be surprised if this is not part on the problem with the 787
Thermal sensors on each cell might independently call attention to a cell that may be getting over-voltage due to an adjacent shorted cells and enable a safe shutdown. Did they have on-going thermal monitoring of all cells?
But you still need to get electricity from somewhere...
About a week ago I bought a lithium polymer battery power supply that charges it own battery it has three cell and two thermal sensors in this pack normally you find only one thermal sensor so maybe they will get this right
With the new 18650 Lithium cells, each and every cell is supposed to have a chip that monitors temperature and voltage in relationship to charge and discharge. When the parameters are exceeded, the cell shuts off -- at least for a period of time, but maybe forever.
For the ultimate in safety, that probably is the best, but there are a couple of flaws with the approach.
a. When one cell shuts down in a series, you are left without power... not even partial power
b. Only some makers of the 18650 are including the safety feature, others are still selling 18650 without any protection.
There is a real possibility that the airplanes might go back to earlier battery technology just to be sure that they can fly safely. I don't think they will roll all the way back to lead acid, but NiCd cells are a lot less problematic than Lithium ion or NiMH.
The newer technologies have come to rely on plates and poles with thinner and thinner materials. In the case of the NiMH, one of the poles relies on a thin coating to gain the extra power. It seems to me that it doesn't take much energy to damage materials that are layered in micros or mills and the distances between plates are equally small. The designers presume that current and voltage are always smoothly and even distributed throughout the internal matrix. Is that possible in real use in the real world?
Yasusa may think these are rugged batteries, but in compared to what? Cell phone batteries or diesel truck starter batteries?
Looking at the NTSB photos of the exemplar batteries, it does seem that each cell had thermal and voltage monitoring capability, maybe current discharge as well. But was it being well used? There certainly was no way to isolate a failed cell as the power link between the cells was a heavy bar of metal, likely copper or brass. So the charge and control system might have presumed that a heat problem might lock out charging the whole system, but still allow the battery to be used.
It may be that just using a lithium battery with a failed cell is subject to the other cells trying to rebalance the internal resistance changes and providing enough energy for a thermal runaway event. And it may just be too late to rest the device and to regain any use.
The NTSB issued another press release today and it sounds like any batteries that are capable of catastrophic failure (explosion and/or fire) are going to be ruled out of operation on passenger planes. That would likely push the Boeing 787 back to NiCd as Lithium ion and Nickel Metal Hydride... unless someone comes up with a way to provide there won't be any fires.
The decision seems to be based on having a pressurized cabin and limited amounts of oxygen.
Because FAA regulations didnt cover aspects of the new design, the plane was certified with special conditions that allowed use of the lithium-ion batteries, Hersman said.
Boeing received regulators permission to use the batteries in 2007, three years after the FAA barred passenger planes from carrying non-rechargeable versions of that type of battery as cargo because of fire concerns.
Boeings chief engineer, Mike Sinnett, said Jan. 9 that the batteries were designed so that failure of one cell wouldnt cascade to the others, and that the plane would be safe even if it did. He said there had been no issues with the battery cells over 1.3 million operating hours.
Looks like another case of the FAA taking Boeing's word for it. Couldn't be bothered to even test any scenarios even though such technology is well known for it's volatility. Not even a second opinion it seems.
FAA has failed as badly as Boeing. I wonder what other "special conditions" have been applied in the certification?
The airplane that failed in Boston has less than 200 hours of flying time, something like 11 trips. It seems the failure of one cell did cascade into a block that might have had the 8 cells completely combust if fire fighters hadn't stopped it.
The battery packs don't look that big in comparison of the whole airplane, so I don't think that reverting to older technology and heavier batteries is going affect the fuel consumption very much.
The problem with lithium is that the metal burns in air once it get hot enough. Nickel, Lead, and Cadmium do not. There has been a lot of very strong statements by the NTSB that there should NEVER be a fire inside the airplane and these batteries are within the interior.
While other Lithium ion chemistries might be workable, it looks as though Boeing may just opt to blacklist Lithium for the sake of assuring everyone that the problems won't resurface. They have a lot of these planes on order and cancellations would be a huge disaster.
If Boeing doesn't nix Lithium, the NTSB or the FAA might do it for them.
There was a little irony in the last article sited by Loopy...that airlines were barred from transporting Lithium cells as cargo not long ago.
Also comical is the fact that the FAA bought the notion that there was a lithium technology so reliable that it would belch smoke less than once every 10 million hours of use.
There are too many well-dressed people sitting in luxurious offices making important decisions, who are actually clueless about important things.
There are too many well-dressed people sitting in luxurious offices making important decisions, who are actually clueless about important things.
That was one of Richard Feynman's observations when he sat on the Challenger investigation board (quote taken from here):
It appears that there are enormous differences of opinion as to the probability of a failure with loss of vehicle and of human life. The estimates range from roughly 1 in 100 to 1 in 100,000. The higher figures come from the working engineers, and the very low figures from management. What are the causes and consequences of this lack of agreement? Since 1 part in 100,000 would imply that one could put a Shuttle up each day for 300 years expecting to lose only one, we could properly ask "What is the cause of management's fantastic faith in the machinery?"
We have also found that certification criteria used in Flight Readiness Reviews often develop a gradually decreasing strictness. The argument that the same risk was flown before without failure is often accepted as an argument for the safety of accepting it again. Because of this, obvious weaknesses are accepted again and again, sometimes without a sufficiently serious attempt to remedy them, or to delay a flight because of their continued presence.
His conclusion at the end of the article:
For a successful technology, reality must take precedence over public relations, for nature cannot be fooled.
A. Lithium is a metal that burns in air, but it does require higher temperatures
B. The electrolyte for these lithium batteries produces oxygen when it breaks down
C. The NTSB does NOT want any fires inside the airplane and that is where the batteries are for in flight inspection or service.
Airbus is just happily letting Boeing do the trailblazing and suffering the negative media exposure. The batteries on either airplane are not a huge portion of the total weight. Going all the way back to lead acid would not be a big cost factor.
So it seems this is mostly a battle to save the lithium battery from suffering a bad reputation. The US Airforce very recently announced that the F35 fighter jets were going to all lithium batteries.
In sum, it is certainly involved in some degree of PR as Boeing and Airbus would love to make the other one be perceived as less safe by the public.
Nonetheless, there is something counter-intuitive about having a metal that burns in air in use in a passenger jet.
I doubt that Airbus did extra test. This is a policy decision. That's what executives do and engineers don't understand.
Boeings chief engineer, Mike Sinnett, said Jan. 9 that the batteries were designed so that failure of one cell wouldnt cascade to the others, and that the plane would be safe even if it did. He said there had been no issues with the battery cells over 1.3 million operating hours.
There are;
Lies,
Damn Lies, and
Statistics...
How many flight-hours has each aircraft flown?
How many of them still fly with the ORIGINAL battery pack?
There is at least one liar in every corporation. The NTSB stated the 787 was under 100,000 hours of total flight time when this crisis arose... only 50 airplanes is actual operation. Mr. Sinnett may have just found a path to early retirement.
Just looking at the 8 cells of the battery, one can see that they are connected in series with heavy bus metal. There is no way to isolate a failed cell from causing electrical imbalance being passed on to others. And then, the fact that they are tightly packed in one box shows a complete lack of any fire or thermal barrier when one cell goes into runaway and catches fire.
At the very least, if one cell short circuits, you no longer have a battery that produces its full voltage. The knock-on is that the charger might engage and actually provide a net over-voltage charge that could begin a thermal runaway. And that is without considering the senario that a shorted lithium battery is capable of a thermal runaway on its own.
There may be a thermal runaway hazard of just running a full load of current throuigh a shorted lithium cell.
And everybody knows that if you want to start a fire, you create kindling by making material with more and more surface area.
From the looks of it, the fire damage occurred only on the high voltage side of the shorted cell. The lower voltage side was stable. So it seems that the short-circuit began a destructive process, charging or discharging stress after that imbalance did the rest.
It appears the that shorted cell was the 'event trigger'. What isn't clear to me is which cell was the first to engage in thermal runaway.. the short-circuited cell or one of the other high side cells. It may be that the high side cells and the short circuit cell rose to failure temperatures in unison. Seems that thermal sensing of the battery pack didn't pull the battery off line, maybe just chose to cease charging.
Do your own post-mortem from the information on line, especially the photos. Compare the Japanese failure to the US failure.
There is at least one liar in every corporation. The NTSB stated the 787 was under 100,000 hours of total flight time when this crisis arose... only 50 airplanes is actual operation. Mr. Sinnett may have just found a path to early retirement.
Battery operating hours, not flight hours, I would suggest (more than one battery on each plane?)... And that includes commissioning/
testing/maintenance - all of which is logged and recorded so a concrete figure ought to be available from Yuasa's Boeing's databases.
It shows how much testing and experience you need with new technology before rating it for flight - the reliability is actually pretty high,
it's the severity of the failure-mode that's an issue - otherwise one would simply use redundancy to increase the confidence.
When is a battery a cell and a cell a battery? The hours may be actually inflated by 8x if you count each cell as a separate battery. The testing should be hours as a complete unit as the testing context is important. 8 cells in series obviously behave differently than one cell in independent use. That's why cell phone lithium cell safety is not an analogy for safe blocks of series cells.
Regardless of how it is counted and how it is represented, it seems that 1.3 million hours wasn't enough.
Extensive testing and experience are indeed important. And how you count things is too.
Counting each "cell" as a "battery" is just not correct.
@Heater
I am with you on this, but that seems to NOT be the way Boeing chose to get their 1.3 million hours.
I am still curious if this was a short-circuit followed by an over-charge failure, or if this was an initial short-circuit that lead to a cascade of short-circuits and over heating in other cells.
A lot remains to be discovered about the behavior of these cells in long chains of series. Motors want higher supply voltages than a single cell can provide as the control circuits have built-in voltage drops. So there are some serious design parameters that are in conflict.
Besides that, while the Lead Acid cells have never been a great performer at low temperatures.. they do work. The Lithium cell not only has a high temperature limit, it has a higher lower limit and that makes in NOT ideal for sub-zero weather. Lithium may at best be assigned to transportation in the sub-tropics.
Yeah, well that may be corporate speak misdirection.
I could claim I have tested a bolt for a million hours and leave it to you to assume that was real flying time.
In actuality those bolts have never flown they were just repeatededly stressed and temperature cycled in some test rig which we hope simulates the conditions the bolt sees in actual use. Of course we might have forgotten to simulate the corrosive environment it will experience or the latteral stresses we forgot about....
I have seen chips and electronic devices that claim their Useful life is in the range of centuries, and yet we all know that such devices haven't existing for a fraction of one. These mathematical projections seemed to take off when boom boxes began to claim watts of output in ways that didn't conform to traditional audio power rating. Some of the problem may have begun with required mileage ratings for automobiles.
We have gone from facts to factoids. If you are going to claim 1.3 hours of operation, maybe you should have to also mention when you started counting. After all, it isn't just the testing that is important, but the amount of knowledge acquired over a period of time that needs to be looked at. 1.3 million hours of wrong headed testing isn't going to present meaningful data. Neither is a useful life of a device that is longer than a human life time (unless it is really capable of being exploited by successive users).
This Boeing engineer claimed there were not issues found with the batteries over 1.3 million hours, but we now know that Yuasa and Japan Airlines were frequently swapping batteries in use during the 100,000 hours of actual in flight use. He may not have been in the loop, but there was a problem and he denied such.
Any engineer worth his salt knows that observation of behavior in actual use provides the best data, sometimes along with destructive testing. There is an illusion that one can predict the future, but in use experience really defines a well-engineered product. Everything else is an educated guess.
Comments
I've been pondering the problems with Lithium cells in relation to the charge cycle.
If you go too high a voltage you damage the battery and have a thermal runaway.
On the other hand, if you go to low on discharge, the cell goes into a failure mode that is effectively a short. Then in subsequent attempts to charge, that cell may create a thermal runaway due to near zero resistance ( if it is near zero - other cells may be getting an over-voltage on the charge cycle).
The balancing can easily be controlled electronically with one and maybe two cells in series, but it gets to be more difficult as you chain more and more cells into a series. Internal battery resistances are dynamic and cannot be easily monitored to compute and insert a moderating influence. I am not sure that inserting a moderating scheme is feasible.
I suspect that because the cells are in series to establish higher voltages, it is easier to have one cell become out of balance with the rest. The actual position of the first failure cell in a series arrangement may be very telling about why the control circuits do not catch a defective cell before a thermal runaway occurs. Cells at the tail ends of a series may go to greater extremes due sudden demands.
Everything I read last year about large series arrays of Lithium cells seemed more theory than practical. Many chip manufacturers created chips to monitor series of 8 to 12 cells and then discontinued the products - apparently nobody had a good design thesis. Individual thermal sensing each cell should be a redundant fail safe network to monitor the cell's health. And temperature should be logged in relation to charge or discharge to determine which mode causes a thermal runaway.
There has been sales literature that claims that matched lithium cells are a must for safe series operation and other literature that claims that matching lithium cells is not feasible. Statistical performance analysis of production batches show large variance in performance within one batch. And test procedure to match internal resistances are rather dubious. So having a completely separate thermal monitoring scheme might be the only way to monitor battery stability.
These days we have a lot of electric bicycles that have been sold with 8 or more cells. The charge control boards have tried to create advanced balancing schemes, but every balancing scheme made the cells more prone to being locked out as one cell fell out of spec, and the balancing schemes actually shortened the overall capacity of the cells and or lengthed the charging time.
So the bicycle manufactures seem to have accepted that a series of lithium cells is best balances by letting the batteries seek their own via the chemistry with a simpler control and monitoring circuit.
I think the topic could make a good master's thesis for an EE. Lots of items are still unresolved.
This part is simple. The battery guys know the expected limits of the design. They know the causes and consequences of these limits. They are supposed to check this by putting it though its paces during testing; THAT was the time to discover is something needed more analysis or change, not when MY flight is in the air. Either the limits are wrong, or the method for detecting exceeding the limits is wrong, or logging and reporting is wrong.
To say this is a new tech, and we don't know enough about it, so we skipped the proper testing and put it in an airplane full of people, if not acceptable.
There's temperature, vibrations of different kinds, possibly shock from touchdowns.
There may or may not be an issue with low pressure, too.
(Shouldn't be, as most pressurised airliners keep the pressure 'normal' inside the entire fuselage, but then again... Also, they typically keep the pressure to about 900mBar to reduce weight)
-Tor
Added to that that statistics collected for single cell behavior may vastly differ from the statistics for a long series configuration of cells due to relying on the aggregate chemical behavior of all the cells in use.
Good testing procedures are based on experience, without experience everything is a best guess or an interesting thesis.
New battery requirements? Double the batteries on board and have the second set remain unused but ready in case of catastrophic failure. You would still have less weight for power than lead acid, and you would be able to more freely explore reconfiguration while collecting data in actual use.
There are unique issues as Boeing went to an all electric control and quit their previous hydraulic system. So even if the airplane has no jet engines, it needs the batteries to operate the controls.
Of course, this presumes that the problem will be cheaper to resolve if the airplanes are not grounded while seeking a final solution.
-Tor
Kind of long but lots of technical information. Some interesting comments beginning at 24:45
"...we do not expect to see events like what we saw on the 787 with the battery system..."
Any airliner is at risk of control system loss if the main engines fail. The old hydraulic systems needed just as much energy to run. But the hydraulic systems have the added risk of running dry. Backup power is what the APU if for, and if that fails, the ram air turbine is deployed. The battery that failed in the 787 was mostly a glorified starter battery, If it could power the control systems for more than a 5-10 minutes I'd be amazed. (i.e. I suspect most of the 787's about 2 megawatt electrical capacity goes to climate control with only a few kilowatts used for flight control.)
Anywho, best guess I've seen so far for a failure cause is pressure cycling during operation. The cells used are large and rectangular, thus the walls will move some as the pressure changes. This movement is likely to cause internal damage, and there are plenty of YouTube videos showing how lithium batteries react to internal damage. (hint: fire!)
lawson
Interesting info. The starter system would be huge power drains on the lithium, maybe pulling cells destructively below their low voltage limits for mere moments. But it does imply that the overall safety regime of the 787 remains intact.
Pressure flux is an interesting issue. The solution would be to simply encase all the cells in a pressurized housing with a stable pressure -- maybe a nitrogen atmosphere would also limit combustibility in a thermal runaway.
I need to read the big report about. At least the USA is willing to let the public read things like this rather than put up a wall of secrecy. As I keep saying, this is a very interesting topic for anyone starting a career in EE. We seem to have a world that is committed to more and more batteries in the future.
http://www.ntsb.gov/investigations/2013/boeing_787/FAA_Special_Conditions_LIB.pdf
A. While the NTSB is indeed investigating a 'fire event' in the APU battery (which is an auxillary battery used for starting the jet), in Japan the JTSB is investigating a 'smoke event' in the MAIN battery that occurred a week later.
...So it seems that any and all power on the 787 might be at risk
B. In an 8 cell battery pack, cell #5 has been found shorted and others swollen. The NTSB say a short circuited cell alone is enough to cause a thermal runaway. Cells #8, #7, and #6 seem more damaged that the ones below #5. Not sure where the ground is, but this may be important in terms of which cells suffer more stress due to their position in a series.
My own observations...
The NTSB avoided some of the details and talked mostly about the 8 cells as a whole, rather than one cell failure having a knock-on effect.
The JTSB investigation is ongoing and the NTSB said nothing about what they are finding. It seems that where the incident takes place determines who is the lead investigation and so there are two investigation - one in the US and another in Japan.
The chicken and the egg....
Did a shorted cell cause the over-heating that caused the thermal runaway or vise-vera?
From what I have read it is dropping below the low voltage boundary caused the cell to short, while exceeding the high voltage boundary causes a thermal runaway.
So do the two happen in rapid succession?
In other words, a battery is taxed by a high load and one cell drops below the low voltage boundary, and then the charge circuit tries to recharge the cell and due to the shorted portion exceeds the safe high voltage limit for an Individual cell and thus causing a thermal runaway. That is what I suspect.
While the sensors might read safe overall voltage levels, in the low voltage situation the drop may be too quick to catch. In the high voltage situation, the charger may be watching the aggregate voltage to the cells and not the individual cells.
Thermal sensors on each cell might independently call attention to a cell that may be getting over-voltage due to an adjacent shorted cells and enable a safe shutdown. Did they have on-going thermal monitoring of all cells?
But you still need to get electricity from somewhere...
This happen in a rebuilt tool battery pack
I was using a battery grinder which is the highest current of all power tool that you can have
I was using this tool very hard the battery pack had got very very warm almost to warm to hold on to it (one end of the battery pack) for very long but what happen next was surprising that the pack was hissing I took the pack apart after the pack cool down and found 2 cell that paired together there voltage when to 0 Volts
So would not be surprised if this is not part on the problem with the 787
About a week ago I bought a lithium polymer battery power supply that charges it own battery it has three cell and two thermal sensors in this pack normally you find only one thermal sensor so maybe they will get this right
For the ultimate in safety, that probably is the best, but there are a couple of flaws with the approach.
a. When one cell shuts down in a series, you are left without power... not even partial power
b. Only some makers of the 18650 are including the safety feature, others are still selling 18650 without any protection.
There is a real possibility that the airplanes might go back to earlier battery technology just to be sure that they can fly safely. I don't think they will roll all the way back to lead acid, but NiCd cells are a lot less problematic than Lithium ion or NiMH.
The newer technologies have come to rely on plates and poles with thinner and thinner materials. In the case of the NiMH, one of the poles relies on a thin coating to gain the extra power. It seems to me that it doesn't take much energy to damage materials that are layered in micros or mills and the distances between plates are equally small. The designers presume that current and voltage are always smoothly and even distributed throughout the internal matrix. Is that possible in real use in the real world?
Yasusa may think these are rugged batteries, but in compared to what? Cell phone batteries or diesel truck starter batteries?
Looking at the NTSB photos of the exemplar batteries, it does seem that each cell had thermal and voltage monitoring capability, maybe current discharge as well. But was it being well used? There certainly was no way to isolate a failed cell as the power link between the cells was a heavy bar of metal, likely copper or brass. So the charge and control system might have presumed that a heat problem might lock out charging the whole system, but still allow the battery to be used.
It may be that just using a lithium battery with a failed cell is subject to the other cells trying to rebalance the internal resistance changes and providing enough energy for a thermal runaway event. And it may just be too late to rest the device and to regain any use.
The decision seems to be based on having a pressurized cabin and limited amounts of oxygen.
http://www.bloomberg.com/news/2013-02-08/ntsb-s-787-findings-may-lead-to-boeing-battery-redesign.html
http://news.yahoo.com/ntsb-787-battery-approval-reconsidered-164544964--politics.html
FAA has failed as badly as Boeing. I wonder what other "special conditions" have been applied in the certification?
The battery packs don't look that big in comparison of the whole airplane, so I don't think that reverting to older technology and heavier batteries is going affect the fuel consumption very much.
The problem with lithium is that the metal burns in air once it get hot enough. Nickel, Lead, and Cadmium do not. There has been a lot of very strong statements by the NTSB that there should NEVER be a fire inside the airplane and these batteries are within the interior.
While other Lithium ion chemistries might be workable, it looks as though Boeing may just opt to blacklist Lithium for the sake of assuring everyone that the problems won't resurface. They have a lot of these planes on order and cancellations would be a huge disaster.
If Boeing doesn't nix Lithium, the NTSB or the FAA might do it for them.
Also comical is the fact that the FAA bought the notion that there was a lithium technology so reliable that it would belch smoke less than once every 10 million hours of use.
There are too many well-dressed people sitting in luxurious offices making important decisions, who are actually clueless about important things.
That was one of Richard Feynman's observations when he sat on the Challenger investigation board (quote taken from here):
It appears that there are enormous differences of opinion as to the probability of a failure with loss of vehicle and of human life. The estimates range from roughly 1 in 100 to 1 in 100,000. The higher figures come from the working engineers, and the very low figures from management. What are the causes and consequences of this lack of agreement? Since 1 part in 100,000 would imply that one could put a Shuttle up each day for 300 years expecting to lose only one, we could properly ask "What is the cause of management's fantastic faith in the machinery?"
We have also found that certification criteria used in Flight Readiness Reviews often develop a gradually decreasing strictness. The argument that the same risk was flown before without failure is often accepted as an argument for the safety of accepting it again. Because of this, obvious weaknesses are accepted again and again, sometimes without a sufficiently serious attempt to remedy them, or to delay a flight because of their continued presence.
His conclusion at the end of the article:
For a successful technology, reality must take precedence over public relations, for nature cannot be fooled.
http://www.bbc.co.uk/news/business-21477126
Perhaps Airbus took the extra time to do more tests? Or just PR?
A. Lithium is a metal that burns in air, but it does require higher temperatures
B. The electrolyte for these lithium batteries produces oxygen when it breaks down
C. The NTSB does NOT want any fires inside the airplane and that is where the batteries are for in flight inspection or service.
Airbus is just happily letting Boeing do the trailblazing and suffering the negative media exposure. The batteries on either airplane are not a huge portion of the total weight. Going all the way back to lead acid would not be a big cost factor.
So it seems this is mostly a battle to save the lithium battery from suffering a bad reputation. The US Airforce very recently announced that the F35 fighter jets were going to all lithium batteries.
In sum, it is certainly involved in some degree of PR as Boeing and Airbus would love to make the other one be perceived as less safe by the public.
Nonetheless, there is something counter-intuitive about having a metal that burns in air in use in a passenger jet.
I doubt that Airbus did extra test. This is a policy decision. That's what executives do and engineers don't understand.
There are;
Lies,
Damn Lies, and
Statistics...
How many flight-hours has each aircraft flown?
How many of them still fly with the ORIGINAL battery pack?
How many times has each pack been 'used'?
Just looking at the 8 cells of the battery, one can see that they are connected in series with heavy bus metal. There is no way to isolate a failed cell from causing electrical imbalance being passed on to others. And then, the fact that they are tightly packed in one box shows a complete lack of any fire or thermal barrier when one cell goes into runaway and catches fire.
At the very least, if one cell short circuits, you no longer have a battery that produces its full voltage. The knock-on is that the charger might engage and actually provide a net over-voltage charge that could begin a thermal runaway. And that is without considering the senario that a shorted lithium battery is capable of a thermal runaway on its own.
There may be a thermal runaway hazard of just running a full load of current throuigh a shorted lithium cell.
And everybody knows that if you want to start a fire, you create kindling by making material with more and more surface area.
From the looks of it, the fire damage occurred only on the high voltage side of the shorted cell. The lower voltage side was stable. So it seems that the short-circuit began a destructive process, charging or discharging stress after that imbalance did the rest.
It appears the that shorted cell was the 'event trigger'. What isn't clear to me is which cell was the first to engage in thermal runaway.. the short-circuited cell or one of the other high side cells. It may be that the high side cells and the short circuit cell rose to failure temperatures in unison. Seems that thermal sensing of the battery pack didn't pull the battery off line, maybe just chose to cease charging.
Do your own post-mortem from the information on line, especially the photos. Compare the Japanese failure to the US failure.
Battery operating hours, not flight hours, I would suggest (more than one battery on each plane?)... And that includes commissioning/
testing/maintenance - all of which is logged and recorded so a concrete figure ought to be available from Yuasa's Boeing's databases.
It shows how much testing and experience you need with new technology before rating it for flight - the reliability is actually pretty high,
it's the severity of the failure-mode that's an issue - otherwise one would simply use redundancy to increase the confidence.
1.3 million hours is 148 years!
Regardless of how it is counted and how it is represented, it seems that 1.3 million hours wasn't enough.
Extensive testing and experience are indeed important. And how you count things is too.
Counting each "cell" as a "battery" is just not correct.
@Heater
I am with you on this, but that seems to NOT be the way Boeing chose to get their 1.3 million hours.
I am still curious if this was a short-circuit followed by an over-charge failure, or if this was an initial short-circuit that lead to a cascade of short-circuits and over heating in other cells.
A lot remains to be discovered about the behavior of these cells in long chains of series. Motors want higher supply voltages than a single cell can provide as the control circuits have built-in voltage drops. So there are some serious design parameters that are in conflict.
Besides that, while the Lead Acid cells have never been a great performer at low temperatures.. they do work. The Lithium cell not only has a high temperature limit, it has a higher lower limit and that makes in NOT ideal for sub-zero weather. Lithium may at best be assigned to transportation in the sub-tropics.
I could claim I have tested a bolt for a million hours and leave it to you to assume that was real flying time.
In actuality those bolts have never flown they were just repeatededly stressed and temperature cycled in some test rig which we hope simulates the conditions the bolt sees in actual use. Of course we might have forgotten to simulate the corrosive environment it will experience or the latteral stresses we forgot about....
I have seen chips and electronic devices that claim their Useful life is in the range of centuries, and yet we all know that such devices haven't existing for a fraction of one. These mathematical projections seemed to take off when boom boxes began to claim watts of output in ways that didn't conform to traditional audio power rating. Some of the problem may have begun with required mileage ratings for automobiles.
We have gone from facts to factoids. If you are going to claim 1.3 hours of operation, maybe you should have to also mention when you started counting. After all, it isn't just the testing that is important, but the amount of knowledge acquired over a period of time that needs to be looked at. 1.3 million hours of wrong headed testing isn't going to present meaningful data. Neither is a useful life of a device that is longer than a human life time (unless it is really capable of being exploited by successive users).
This Boeing engineer claimed there were not issues found with the batteries over 1.3 million hours, but we now know that Yuasa and Japan Airlines were frequently swapping batteries in use during the 100,000 hours of actual in flight use. He may not have been in the loop, but there was a problem and he denied such.
Any engineer worth his salt knows that observation of behavior in actual use provides the best data, sometimes along with destructive testing. There is an illusion that one can predict the future, but in use experience really defines a well-engineered product. Everything else is an educated guess.