I don't know, Charles - first, this article seems nothing more than a rehash of stuff we've been seeing for weeks; second, it shows a lack of sophistication about how MTTFs (Mean Time To Failure) are determined and what they mean.
It's only possible to calculate a meaningful MTTF when a statistically significant number of cases have been run through the period in question - then you have real failures over real time to base the number on. In this case, where an unusual technology (lithium-cobalt) is being applied in a completely new application, all that can be done is to try to guesstimate an MTTF by imagining some specific failure modes, how often they might occur, and cranking to get a number. Would a company with integrity do this, you ask? Sure, if they were compelled to do the impossible by some inane government bureau to satisfy some inane regulation! If you need an example, look at our EPA requiring fuel providers to blend in ethanol in amounts that don't exist! Your government hard at work in fantasyland.
In other words, these MTTFs are often fantasy numbers, and we shouldn't be surprised when the reality turns out far different.
If some fault happens that was not considered in the calculation, then of course the actual failure will occur much sooner. This doesn't mean there was anything wrong with the calculation itself. Take the suggestion by an EE on this blog some days ago - that the problem was that individual cells shorted to the battery enclosure. As an EE myself, I can tell you that we can really build perfect insulation systems - that is, that their MTTF would be ~infinite. But if one is installed badly - or designed poorly - it might fail in a few hours. The original analysis might assume - duuh - that the cells would be insulated. But if this unanticipated problem occurred, a failure would happen much earlier than the calculated MTTF. Of course, this would also be a very easy problem to correct, which is why I doubt it's the cause; this would have been triumphantly announced already.
I think you have hit it one the head. As a veteran road warrior I have followed this subject with a combination of consternation and amusement.
Since their inception LiPo cell batteries have been variously considered
too hazardous for passengers to carry in their laptops
unsafe to carry spares
unsafe to carry spares unless especially caped
Some of the makers have had severe problems with battery failures resulting in burned users.
Yet suddenly (when the airplane makers want them) they are safe to use even when vital for flight.
I think both Boeing and NTSB have badly missed the mark on the hazard analysis here. One thin spot in the polymer spells a cascading short. Electable batteries might be a good solution if and only if the plane can fly safely without them. The military can supply the technology for the ejectors.
Sorry if I seem too firm here but we are dealing with a lot of people's lives, including mine.
I can't imagine the number of ways a battery can be induced to malfunction, let alone create a test to demonstrate the batteries vulnerability. A brief perusal of on-line battery certification references suggests the air-worthiness qualification requirements for any system is extensive. For Lithium-ion batteries it would be doubly so. These incidents seem to represent an aspect of this technology not previously seen which explains why there is so little information being presented to the press.
About a year ago, I was scheduled to fly back to Melbourne, FL from Jackson MS in a 7 am flight. We had boarded the plane and were sitting comfortably when the pilot came on the intercom and stated that the plane had a problem and would not be departing as scheduled. After about 45 minutes, he announced we should deplane and make other arrangements. After I rescheduled for a late afternoon flight, I was walking out to catch a ride. I ran into the pilot and crew. He stated that someone had left some switches in the cockpit in the wrong position and discharged the batteries. As we chatted I discovered they had to replace the ones in the plane which turned out to be Lithium Ions. Since they could not fly the replacement batteries on passenger aircraft, they had to send them to New Orleans via a cargo flight and then a technician had to drive them up to Jackson and replace them. I laughed because they could fly the INSTALLED batteries but could not fly the "loose" replacement batteries on commecial passenger airplanes. When I departed that evening about 5 pm the aircraft was still sitting there and as I understood it, it departed the next day, empty, back to Atlanta...
When our Li-Ion battery failed in our aftermarket radio several years ago, many of the symptoms were the same as with the 787... much heat, vapor, venting, smoke... These were NOT the result of poor charging, overloading of the circuit... none of that! You say that shorting the cell with a nail is not a satisfactory test... What do you suggest is a more appropriate test? Easy to armchair quarterback this... For us, it turned out that the nail test was a fairly accurate predictor of the results of a short circuit... And gee, guess what? Our issue turned out to be the result of an internal cell short circuit... Really, the 787 issue sounds exactly like ours, only a scale difference... I would be looking very carefully at dendrite growth causing out-gassing internal to the cell, making the cell swell. internal parts can then slide and cause unforseen shorts and it is all downhill from there!
I think the NTSB and Boeing would get a whole lot more frequent flyer mileage with the general public if they gave just a bit more disclosure on the cause and effect of the fire. Driving nails thru cells to sim shorts and desktop analysis of fault probabilities doesn't go too far with road warriors. Either excess load or unregulated charge made those batteries fail unless they really are unreliable chemistry. They weren't that old. So either the electrons going inny or outy weren't being properly managed and that's outside the battery; not where the fire happened. The longer the answer isn't focused on, the more people lose confidence in the whole design. Not fair but reputation and confidence in equipment is 98% perception and only 2% direct experience.
Most machine design engineers will survey existing component manufacturers for standard linear guide products, limiting what they can do with their designs. Using extruded aluminum profile guides can customize machine designs while shrinking the bill of materials.
Practically all electronic devices today contain metals that may
be coming from conflict-ravaged African countries. And political pressures will increasingly influence how these minerals are sourced and used in products.
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