Pretty theoretical.... It's much more simple and effective to read the manual !!!
www.apexhk.com
Nicad voltage range:
4,8-10,8 volt for input voltage over DC 13,5 volt.
4,8-9,6 volt for input voltage under DC 13,5 volt.
These voltages are nominated, NOT peak voltage. So 9,6 volt corresponds to 8 cells.
And hereby, I correct myself, when I claimed that charging 8 TX cells requires 15 volt.