A fast charger for NiCd batteries can be made using this circuit diagram. This electronic circuit is built using 555 timer circuit connected as astabil.
Battery is charging with a current (in mA) ten times higher than the rated capacity (in mAh).
If the integrated circuit output is "1", charging process will start. Voltage across it is then compared by IC1a with maximum battery voltage, fixed with P1. Comparator output is integrated with R3 and C2 and then used to impose timer period. If not reached maximum battery voltage, load is about 90% in this period. If the maximum voltage is reached, charging is only 1% of the time (pulse load).
LED lights when the battery is fully charged and must be disconnected from the circuit.
Proposed charger is designed for charging a battery of 1.5 V in 8 to 10 minutes. Charge current for a battery of 500 mAh is about 5 A, which does not require stabilization, as will be limited by R1.
The value of R1 is given by Ohm's law. If, for example, the load current is taken from a source of 8 V, and assuming a power on the battery and T1 2 V, the voltage across the resistor is 6 V. Its value will be: 6 / 5 = 1.20. Note that the power dissipation is 6 x 5 = 30 W (therefore will connect a number of resistors in parallel).
Voltage of the charger should always be 2 V higher than the level set by P1.