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Comment by jlgreco | Hacker News Reader

Much smaller than I would have expected. That thing can start an engine that large?

These batteries (or more often, ground power) are used to first start a small turbine engine in the tail (APU) which generates the compressed air used to start the main engines.

Further reading: http://en.wikipedia.org/wiki/Auxiliary_power_unit

Edit: after further research, it seems compressed air is not used on the 787. It's all electric (two generators on the APU and on each engine):

"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."[1]

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

Ahh, that makes more sense. Thanks!

Early jet aircraft used a ground cart to provide power to get the engines started.

Evidently adding the APU saves enough money to justify its significant weight.

I think it's less about money and more about stuff like:

- backup power for emergencies - allowing attempted engine restarts in the air - allowing the airplane to be self-sufficient should it be forced to land somewhere without ground equipment (or big enough ground equipment)

From the captions, this battery starts the APU (and in an emergency, critical systems). The APU starts the main engines http://en.wikipedia.org/wiki/Auxiliary_power_unit

As I understood it, these batteries power auxiliary systems.

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