Why buy a battery that is not really designed for aviation ?
A NiCad has a memory effect so should not really be trickled charged, which is exactly what you do with your starter gen after a start
I would stick with sealed lead acid, means if you do flatten it you can take it out and charge it from a normal 24 volt cat battery charger.
Unless you can get a NiMh battery or a LiOn battery which do not have memory effects !