Battery charger shown in this circuit diagram can be used to charge one or more batteries with a total nominal voltage of 12 V (ie ten 1.2V or six 2V NiCd batteries or lead acid )
Misuse connection is impossible, because the batteries connected with incorrect polarity, output short circuit terminal or network loss have no effect on the charger or batteries.
Power is taken from the network through a transformer secondary voltage of 18 V. The output voltage from transformer is rectified by diodes D1 - D4 and filtered by C1, resulting in a voltage of 22 V across its C1.
Exhausted batteries are charged in advance with a current of about 6 mA through R2-R4-R6, D5 and D8. Once the batteries have reached a voltage around 0.3 + / - 0.5 V, base-emitter voltage of T1 is large enough to bring the transistor into conduction.
Charge indicator, D6, lights and also opens T2. Through R5-R6 pass a charge current of 60 mA.
If the battery is connected with reverse polarity or shorts power transistor T2 remains blocked and the current can not exceed 6-12 mA.