Nicad batteries require charging at a constant rate of current.
This can be achieved by using a constant current source, irrespective of the voltage, although the voltage does need to be appropriate for the cell or battery pack being charged.
The charging current depends on the mAh rating of the cell, and is expressed as a C/n rate. Example a nicad cell has a capacity of 500mAh then the appropriate charge rate would be C/10 (the 10 rate is the preferred time period) or 50mA...If the cell is capable of being fast charged then the C/n rate could be C/1 or greater....2C or more....
I use nicd packs a power source for electric R/C and use special microprocessor based chargersfor fast charging my packs usually in under 30 mins....but these are special packs, not your average domestic nicad.......
HTH Chip