This reverse polarity protection NiCd battery charger can charge up to 7 NiCd batteries connected in series. This number can be increased if the power supply is increased with 1.65V for each supplementary battery. If T2 is mounted on a proper heatsink, the input voltage can be increased at a maximum of 25V.
Unlike most of comercial NiCd chargers available on the market, this charger has a reverse polarity protection. Another great quality is that it does not discharge the battery if the charger is disconnected from the power supply.
Usually , NiCd batteries must be charged in 14 hours at a charging current equal with a tenth percent from battery capacity. For example, a 500 mAh is charged at 50 mA for 14 hours. If the charging current is too high this will damage the battery.
The level of charging current is controlled with P1 between 0 mA – 1000 mA. T1 is opened when the NiCd battery is connected with the right polarity or if the output terminals are empty. T2 must be mounted on a heatsink.
If you cannot obtain a BD679, then replace it with any NPN medium power Darlington having the output parameters at 30V and 2A. By lowering R3 value the maximum output current can be increased up to 1A.