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Solving the 787 battery problem

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January 25, 2013 – Comments (2) | RELATED TICKERS: BA

Well, well: the entire fleet of Dreamliners have been grounded, pretty drastic even for the FAA. Batteries bursting into flames are apparently are too much for them. There are stories in the media that a “bad” batch of lithium-cobalt (LiCo) rechargeable batteries might be the cause. For Boeing's sake, let us hope that this is true, but color me skeptical.

The hot-n-spicy battery back (BP) is used to start the auxiliary power unit (APU), which in turn starts the jet's engines. So, the BP has to supply mucho amps briefly, then spends the rest of the flight recharging. Big problem: this is exactly how not to use the LiCo. High energy density does not necessarily mean high current capacity. Consumer versions (yes, they are available but they ain't cheap) come with a built-in circuit protector that cuts off the battery if you try to draw too many amps. Why? If you try to pull too much current, it causes overheating and the LiCo bursts into flame. Worse, pictures of the 787 BP show a plastic cover, which exasperates the problem of heat build up (wait, you don't suppose the solution is something stupidly simple like removing the cover and adding one of those punky little fans that laptops use?).

OK, so you fetch a clean cocktail napkin, I will get out my crayons, and let us solve Boeing's problem.

Lithium-Manganese (LiMn)

Boeing made the decision 5-7 years ago to use LiCo during the design stage. Since then, a new type of rechargeable lithium ion has been invented: LiMn. It has the same high energy density, but will happily pump out as many amps as you want until it is empty. Better, it is not subject to “venting with spontaneous combustion”, no matter how badly you misuse it. In fact, it is often used in those high-capacity battery packs used in more expensive rechargeable hand tools like drills or saws that require many amps quickly.

NiCd

This is the old-fashion solution well known for its reliability and long lifespan. It was used in the first communications satellites launched many decades ago. They will also crank out as many amps as you wish with a smile. I have some cheapie, generic NiCd D cells that will pump out more than 5 amps continuously for several minutes (at least 15) before they are empty.

Lead-Acid

Yes, the same one that is in your car. Cold cranking amps: exactly what the APU needs. I do not have the tech specs for the 787 BP, but I wonder if those high-tech gel-pack-lead-acid batteries used in high end motorcycles will supply sufficient amps, since they seem to be about the same size as the BP. In any event, it is hard to imagine that the deep cycle marine battery will not be sufficient.

Ask Elon Musk

His team of crack engineers seems to have solved the problem of a high output LiCo battery pack. Perhaps Boeing engineers can be convinced to do the right thing by swallowing their pride and contracting with Tesla for a correctly designed BP (including the issue of cooling).

Final Word

Personally, I would never fly in a plane that uses batteries whose bursting into flames is a normal part of the battery chemistry, and this includes the upcoming A350 from Airbus.

2 Comments – Post Your Own

#1) On January 25, 2013 at 6:51 PM, portefeuille (99.67) wrote:

If you want to make a "caps" game call on EADS ("Airbus") go here.

http://caps.fool.com/Ticker/EADSY/Scorecard.aspx.

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#2) On January 26, 2013 at 1:06 PM, ikkyu2 (99.27) wrote:

Written like a true engineer.  But how about this:

The 737 APU, which has never failed in an operational setting, has no APU battery at all.  It is a small rotary propellor that drops down from the fuselage, connected to a generator on its other end.  The propellor is spun up by the forward velocity of the aircraft itself; in powered flight, the jet engines provide this velocity, and in the event of a catastrophe, the glide speed (powered, eventually, by gravity) is the power source.

There was nothing wrong with this technology except that it was a hidden part that mechanics had to check before every flight to make sure it wasn't broken, sticky, etc.  It might be a lot harder to integrate one into a composite airframe/skin but it can certainly be done, and I think they should go back to it ASAP.

Agreed that for all the wonders the 787 offers, the giant burny battery pack in the avionics bay makes me very uncomfortable. 

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