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One thing to note re engine failure vs battery failures. In the 787 the electrical power generation for hydraulic systems has been replaced by electrical systems powered by these batteries.

If an engine fails, you can still land the plane. If these batteries fail, you cannot. You have no power to guide the plane to a safe landing.

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If the battery fails, you still have power from the engine, APU generators, and if all else fails the ram air turbine, no? According to [1], one battery is used only for ground operations or backup braking power (so the primary braking ought to still be operational if the battery fails) and the other powers the APU starter motor and provides temporary power in the event of a failure of engine generators until the RAT is operational.

[1] - http://www.aviationweek.com/Article.aspx?id=/article-xml/AW_...

A link with more about the RAT, for those who are curious -- it's a neat bit of aviation engineering: http://en.wikipedia.org/wiki/Ram_air_turbine

You may not need power from the failed battery, but the thermal runaway scenario sounds rather terrifying. Sort of like a nuclear power plant in meltdown. Having read that Aviation Week article, my inclination would be to use nickel-cadmium and just deal with the extra weight, which is only a factor of two. But my job is not to design airplanes.

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