MurphyWasRight,
For example what % of the APU battery charge is used by a single start cycle? How long does it take to "top off" after that and how many start attempts must be avaialable in non-nominal flight?
The only thing I found on this is that the APU battery is designed for two consecutive start attempts, after which a 5 minute cooling period must be observed before a third starting attempt. I don't know if that means 5 minutes from then on after every start attempt, or if a maximum of 3 attempts is permitted, and if the limiting factor is only the battery or perhaps also the starter motor. (Starting from other sources, the starter motor used can be alternated, but the APU battery can only power the right starter motor.)
RR_NDB,
I agree that a redesign for Ni-Cd would almost certainly be cheaper than another (serious) incident or accident.
Bernd