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 vide

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.