What prevents Boeing from switching to NiCad-batteries? Their 777 has them "ready".
Because of the size of the starter/generator units fitted to the APU, a NiCad cannot produce sufficient current, no matter how big it is.
My understanding is the main battery is required to have enough power to operate the emergency brakes in the event of an emergency landing with no other source of electrical power other than a rapidly slowing RAT.
The APU battery is the same type for the sake of commonality of LRUs.
IE. You are carrying a spare to get you home in the unlikely event the main one fails away from base.
