Most NiCads have the required charging current printed on them.
If not use one-tenth of the capacity. For example, a 2 AH battery would need a charging current of 200 mA.

This circuit uses a 5 volt voltage regulator I.C. as a constant current device.

R1 is connected between common and the output terminals.
The I.C. will maintain 5 volts across R1.

Ohms Law is      I = V/R     or     R=V/I
If the charging current is 200mA and the voltage is 5 volts then R1= 25 ohms. Since this is not a preferred value, use the next value up, 27 ohms. This will give a current of 185 mA.

Power dissipation is V x I   which is 5V x 0.185 A = 0.925 Watts. To be on the safe side, use a 2 watt resistor.

Before connecting the battery, test the circuit by connecting a meter, on the current range, across the output. It should read about 185 mA.