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So what's the motivation for using these batteries in the 787 anyway? Can they just switch to whatever they've used before?

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Quick, get ahold of the engineers at Boeing! Tell them to switch to whatever they used on their older planes! Surely they haven't considered that idea yet!

Have you ever been in a situation where a manager proposes a technical solution that everyone else has already dismissed as preposterous? Now you know how it feels to be on the other side of that.

With that mockery out of the way, I'll give my understanding of the situation: The lithium ion batteries on the 787 are much smaller and lighter than their predecessors. They also take up less space and require different input and output voltages and currents. They have different charging profiles. Swapping them out with an older battery technology is infeasible. It would require redesigning other components to make room, building new transformers and voltage regulators, and writing new software to charge the batteries safely. It would also reduce the range, capacity, and/or efficiency of the 787.

I do apologize if I went overboard with the mockery. Please don't take it personally; it's fun to write.

You certainly do live up to your name there, Parsley.

The new engine requires much higher-capacity batteries to start them. A battery with the old tech would be huge and heavy. The new battery technology is much lighter than the old ones.

That's interesting - why are the batteries onboard and not ground equipment? Obviously restarting a stalled engine would be handy, but is this something that actually happens? And if it is, couldn't the running jets provide the power somehow? I seem to remember that some old WW2 prop aircraft had ground equipment requirements to start them from cold (power or warmed air or something?) Obviously this isn't an area I know about but I am curious about it. Thanks.

I found a Boeing technical publication that discusses the power architecture of the 787 and how the battery comes into play. Kind of interesting:

http://www.boeing.com/commercial/aeromagazine/articles/qtr_4...

Page 4 says this:

"The power source for APU starting may be the airplane battery, a ground power source, or an engine-driven generator. The power source for engine starting may be the APU generators, engine-driven generators on the opposite side engine, or two forward 115 VAC ground power sources. The aft external power receptacles may be used for a faster start, if desired."

So the battery seems to be very important when you're somewhere with no ground generator. The battery starts the APU, then the power from the APU starts the engines.

So the battery seems to be very important when you're somewhere with no ground generator.

Like in the sky? After your engines have stopped for some reason? That could be exciting!

That case is already addressed by ram-air turbines on many aircraft, including some from Boeing: http://en.m.wikipedia.org/wiki/Ram_air_turbine

That's what I was wondering, but does this happen? And if the other engines can start it as Joezydeco suggests, does not having them matter? Surely the number or airports without decent facilities to help start them must be small?

http://en.wikipedia.org/wiki/British_Airways_Flight_9

Its not without precedent...

I was wondering about that. They could probably fly the things without APU start batteries, as long as they avoid having to do a cold start at a grass strip. So, new AD, no landing 787's at podunk facilities with no ground carts, or, electrical service.

On a large jet airliner, the engines are not started by battery power; they're started by the APU, which is basically a much smaller engine (if you look at videos of airliners, you can see it venting out the tip of the tail). The APU also provides emergency backup power and cabin pressurization in-flight if the engines fail.

The 787 is unusual in that it's a "bleedless" aircraft; it uses other systems -- some of them electrically-powered -- to supply compressed air for various purposes, rather than "bleeding" compressed air from the engines. The batteries are part of that, and have to be onboard because the plane needs compressed air while in flight.

Not exactly. The larger battery is because the APU is larger. The engines are started on APU power, not directly from battery power. You are correct that Lithium Ion batteries are significantly lighter than older NiCad batteries.

Do you know if there's anything unique about the electrical system that would cause a greater load than a comparable aircraft?

I'm thinking high-tech radar, wifi, photon torpedoes, etc.

No bleed air from the engines. Pressurization, hydraulic actuators, air conditioning and accessories are all electrical. That uses 1.45MW on a 787

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