In application that demand for high current, Nicad (NiCd) rechargeable battery still has the unbeatable performance compared to NiMH and Lithium battery. The charger circuit here is smart in the way it handle the wrong polarity of the battery placement.
The core of this battery charger circuit is the Q2, configured a constant current source for the battery. LED D3 and zener diode D2 make the voltage applied to Q2 base constant, therefore the voltage and the current across R1 is constant. Using the value shown in the schematic diagram, the current flow through R1 will be constant at around 50 mA.
You can see that the base of Q2 is supplied by R3, which is controlled by Q1. Look at the Q1 base, it’s controlled by the battery voltage, so Q1 won’t works if the battery is placed in wrong polarity. Just note that although this battery charger could charge four Nicad battery (in series), it can’t detect if one battery placed in wrong polarity while two or three other batteries are in right polarity. A small transformer, a bridge diode, and an electrolytic capacitor is needed to supply this circuit. Remember to provide a sufficient heat sink for Q2. This circuit will work as long as the battery is not completely discharged, because its weak voltage is needed to trigger the Q1 transistor.