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

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 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_07/article_02_4.html http://www.boeing.com/commercial/aeromagazine/articles/qtr_4...