NiCad cells, nominal voltage 1.2V, will charge up to a terminal voltage of about 1.4V (off load) when fully charged.
So your 7-cell nominal 8.4V battery will rise to about 7 x 1.4V = 9.8V at full charge, so you need about 10V to fully charge it.
The rate of charge is proportional to the difference between the battery voltage and the charger output voltage, so the current steadily reduces as the battery charges.
However, if your charge current is too great at any time, the battery may overheat and be damaged, so a resistance in series may be necessary. Batteries should be marked with their maximum charge rate.
Modern commercial chargers monitor the battery voltage and stop charging when the battery is full. If charging manually from a power supply, regularly monitor the charging current and take the battery off-charge to check terminal voltage.
Charging time:
Basic calculation: for a flat battery, the charging time = the capacity in Amp hours / charging current.
E.g. for a 2 Ah battery, charged at 0.1 A, charging time will be 2 / 0.1 = 20 hours.
(Actually due to inefficiency losses, it will take a little longer.)
