17
Mar
13

Did Boeing take too many risks with the Dreamliner batteries?

Boeing had quite a few risks when it embarked on it’s Dreamliner project.  Several pieces of the technology it put in place were an industry first, and seldom would a company debut them all on the same platform, such as the Dreamliner.

 

 

Most of the airplane was constructed out of Carbon Fiber, over a more traditional use of aluminum.  It is well known that Carbon Fiber is a very strong composite, but is it stiff as well as flexible as required by a plane flying in turbulence?  This was yet to be identified in testing. Also numerous pieces of the assembly were completely outsourced to other companies, instead of being developed by Boeing in-house.

Now comes the biggest concern over the design – Utilizing electric controls over traditional hydraulic systems typical of most all other planes.  Boeing knew they needed a rechargeable battery solution in place to power these controls in the event of failure of the electric generator system, and that is when they turned to lithium-ion.  The plane simply required more reserve power and additional lead-acid batteries would add too much weight to the plane.

As reported by the Economist:
Relying more heavily on electrical power than any other commercial jet, the 787 Dreamliner uses two 32-volt battery packs, containing eight lithium-ion cells apiece. These are not employed during normal flight, but are kept fully charged by the plane’s main generators ready to step in when needed.


Apart from being lighter than other rechargeable cells and able to operate at a higher voltage, lithium-ion batteries have no “memory effect” (the tendency to accept less and less charge each time they are recharged). They can also be charged faster than most other cells, and they hold their charge far longer.

The downside is that, if overcharged, physically damaged or allowed to get too hot, lithium-ion cells may experience thermal “runaway”—generating heat faster than it can be dissipated. A cell may then rupture, releasing inflammable gases that ignite and cause a fierce fire or an explosion.

However, it was determined after the 2 Dreamliner planes were examined, the charging systems were working as designed, and battery voltage was spot on.  Manufacturer problems on the assembly line were also ruled out, as the batteries were from different lots.

So far, air-safety investigators in both Japan and America agree that, in neither case, was there evidence of the batteries being overcharged. The flight recorders show their voltage was correct before the fires broke out. That would seem to rule out the charging system as the source of the problem. By the same token, it would suggest the battery-management system, which is used to keep the voltage within its prescribed limits, was working properly.

This leaves the problem potentially being in the actual wiring.  However as the cabling literally went up in smoke, it could not be properly examined.

The investigators also agree that the fires cannot be put down simply to a faulty batch of batteries. Their serial numbers suggest these came from different lots. It is therefore unlikely that manufacturing defects caused the short-circuits that made them overheat and catch fire. Unfortunately, with the fires having been so intense, any evidence of a fault lying in the actual wiring of the battery-management systems went up in smoke.

So we may never know what really happened, but all Boeing can really do is keep adding layers of system security to try to prevent future issues.

Or perhaps simply go back to using traditional rechargeable batteries is the best solution until safety measures can be insured, especially when the total weight savings of Li-ion over standard NiMH batteries is about the same as an extra piece of luggage.
 
By Michael Nace


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