This is a NiCd battery charger circuit. This circuit can charge 12V nicd battery pack. But you can also charge 6V and 9V battery packs by arranging the input power to get the desired ouput for different battery packs. The circuit has a built in automatic current regulator due to which
the charging currnent is regulated to 4 amperes so as the charging current get 4 ampere, the 0.15 Ohms 3W resistor voltage will become 0.7V as a result the transistor 2SC36522 becomes in ON state and supplies the current to transistor 2N1711 which will short the base of 2SC2270 to ground and cutt off the base current of 2N3055 hence the process of charging will start. Use a good heatsink with transistor 2N3055.