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EDNE MARCH 2013

edn.coment Battery management matters There’s an old joke about the difference between hardware and software engineers that tells of three engineers in a car that suffers brake failure at the top of a hill. They arrive shaken but unhurt at the bottom of the hill; the hardware engineer wants dismantle the braking system and mend it; there’s a project engineer in the car and he wants to set a project framework with a scheduled time-to-fix; and there’s a software engineer who want to push the car back to the top to see if it does the same again. It’s somewhat tempting to see something of the latter approach in Boeing’s troubles with its lithium-ion battery packs, which are still causing all of the new 787s to be parked in corners of various airfields around the world. Meanwhile, Boeing is continuing to produce five new 787s each month, so the parking lot at Seattle must also be getting a bit crowded. At the time of writing this, Boeing (and its contractors) have proposed a solution to allow them to continue to use the same battery chemistry, and have applied to the Federal Aviation Authority to be allowed to flight-test it. The proposal, as far as details have been released, appears to involve placing a little more space and thermal isolation around each cell in the battery pack; enclosing it in a more fire-resistant enclosure; and arranging for that enclosure to be vented to the outside if things go wrong. What does not seem to have been gained so far is any thorough understanding of why the battery packs in two aircraft suffered “severe overheating events”. The history of lithium-chemistry battery development has been long, and a propensity to catch fire has had to be overcome at more than one stage along the way. I recall visiting a factory that was working on Li-ion technology very early, over 20 years ago. At that time the battery structures used much more metallic lithium: that factory was, literally, hard-wired to the local fire department. Later, we had the laptop-battery-fire episodes; and incidents of lithium batteries catching fire while being transported in bulk, while not even in-service. All of these, the battery technologists have overcome and in one view, Boeing’s problems will be just one more step along that path. Reports on the incident on the apron at Boston say that the battery pack was not overcharged beyond its specification. However, it seems unlikely that the pack was at that stage monitored by the aircraft’s data recorder down to the level of individual cells, and we know that cell-to-cell equalisation is very important in managing lithium-ion batteries – in fact, in the pages of this issue, you will find news of another novel chip designed to do exactly that. Boeing is opting to persist with taming the Li-ion technology: Airbus (in a not-exactly equivalent application) has said it will take a step back to NiCd batteries, and accept the lower power density. Why, some commentators have asked, did Boeing select such a radically new (for a flight-critical system) battery technology? It’s a question that does not stand scrutiny; in the course of the design, Boeing had to optimise the weight/ performance profile of every aspect of the 787: there would have been no more reason to leave the battery in an older technology than to have held back on, for example, the extensive use of composite materials in the airframe. In one sense, this is how engineering has always progressed, at its most fundamental level. Analysis is good when you can have it, but sometimes you have to resort to pushing the boundaries and experimenting. Build that arch a little higher, that bridge a little wider – if it falls down, you know your existing technology isn’t up to the task, and more research is needed. We may be sure that when Boeing resumes test flights, those battery packs will be instrumented in every conceivable parameter and, hopefully, will reveal exactly why the existing pack design failed the way it did. The least satisfactory outcome would be that one or more design changes cause the problem to go away, without revealing the fundamental reasons why the failures occurred. There are going to be many more high-density battery packs around us in the near future, and we need to understand them completely. Meanwhile, it’s down to Boeing to push the car back to the top of the hill – or in this case, get back up to 12,000m – and wait to see if it does it again. - searches all electronics sites - displays only electronics results - is available on your mobile w ww.edn-europe.com MARCH 2013 | EDN Europe 7 www.eetsearch.com


EDNE MARCH 2013
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