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

Silicon Power Transistor Datasheet Download - 2N3055 | MJ2955