If you pushed the APP switch in cruise, initiating bus isolation.... would the battery go flat before you got to the runway?
Well, Bus isolation only happens when valid approach data is available to each FCC. (don't think there would be valid glideslope and/or localiser signals available in cruise)
In any event, when isolation happens (valid triple autoland) and the centre DC bus is powered from the battery bus - the main battery charger goes into TR mode (transformer/rectifier) to supply the centre bus which prevents any real draining of the battery !
