bubbers44
NiCads are touchy beasts. What with their memory effect and flat voltage profile, verifying their state of charge and fully charged capacity is problematic. The only way to ensure a 'good' battery is to follow its maintenance and operations procedures carefully. Parking a plane (batteries installed) for months probably qualifies as being outside the envelope of normal operation. So the batteries would be suspect and not relied upon for capacity to support critical functions.
misd-agin
The 30 minute capacity is a minimum requirement. The systems design assumes some very conservative (worst case) electrical load profiles and a battery at the end of its uesful life. So its not unheard of to have the standby system remain operational for much longer. Just don't count on it.