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Electronics & Test

Slideshow: Boeing Underestimated Possibility of Battery Fire

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Arby Bernt
User Rank
Iron
No Mechanical Containment!
Arby Bernt   3/14/2013 2:20:34 PM
NO RATINGS
I cannot help but wonder how much experience the battery box engineers have with lithium cells. Without mechanical endplates and tiebars, the batteries will physically swell at about 90SOC, gradually fracturing the internal structure.

Eventually they will short, typically during the end of the charge cycle. This feature is well known, yet completely lacking in the sheetmetal Boeing housing. 

Maybe change the name to "Bowing"...

 

Arby

Charles Murray
User Rank
Blogger
Re: Troubling news
Charles Murray   3/14/2013 2:37:20 PM
NO RATINGS
You (and the NY Times) are correct, ragnar175. Boeing worked with the FAA on the approval.

g_ost
User Rank
Gold
Re: Troubling news
g_ost   3/15/2013 4:23:43 AM
NO RATINGS
I'm(was) an aircraft design engineer. In my old times we didn't implement systems not able to work at -50 deg C. the planes are not flying or landing only in California....

PDR
User Rank
Iron
Reliability 101...
PDR   4/23/2013 12:56:46 PM
NO RATINGS
Lots of misunderstandings here (the dangers of a little knowledge, and all that jazz). Firstly the certification data does not cite the RELIABILITY of the battery system, it cites the HAZARD RATE of the battery system - the instantaneous probability of failure at any moment in time (which is why it's quoted in a "per flying hour" figure rather than an MTBF). These are very different characteristics and cannot be related to eachother directly.

Secondly two arrisings in 52,000 hours does not mean that the MTBF is 26,000 hours; it means that the MTBF probably lies somewhere in the range 1,200 hours to 10^10 hours. Assuming the batteries had been through burn-in/PRAT programmes (which it's rather difficult to NOT do for aircraft equipment) then they'd be in the "random" or (as often misnamed) "constant failure rate" region of the bathtub curve. In this region it's the spaces between the failures that are random, and you need a decent population of samples to draw any conclusions about failure rates. Two is far too small a number  from which to deduce ANYTHING.

Finally there's comment about the "driving the nail through test" as if it was the only test performed. Again, I don't know about this specific system but I *do* have experience of qualifying equipment onto aeroplanes, and I would be very surprised if the battery system didn't get exactly the same set of basic qualification tests as every other significant part of the avionics - thermal, vibration, shock, bump, humidity, mould etc etc.

All batteries re dangerous because they store energy, and if the energy gets out through an unintended back door that leads to "bad stuff"(tm). Aircraft batteries tend to be big, and so there is a lot of stored energy, that's all. Now the only difference between the LithiumCobalt (as these are) technology and the previous Nickel or lead-based technologies is that the Lithium cells have a self-oxidising electrolyte that will burn in the absence of air, so it's difficult to extinguish. So the trick is to stop them getting hot in the first place. For this reason all non-trivial lithium charging systems use single-cell charging techniques (monitoring the voltage of each cell throughout the charge). This is proven technology, and we really need to wait to find out what the ACTUAL failure mode was, rether than getting hysterical about the subject before we know what really happened.

£0.02 supplied,

PDR

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