There are two main types of battery charger – constant voltage and constant current. Both have their advantages and disavantages. For constant voltage, the battery cannot be overcharged but the charging rate is slow. Constant current mode can charge batteries more swiftly but there is the danger of overcharging them.
The dual mode battery charger circuit featured here was designed to combine both modes, but without their disadvantages, for use with a 6V sealed lead-acid battery. The main players of the circuit are voltage regulator IC1, which is used for constant current mode, and precision adjustable shunt regulator IC2, which is used for constant voltage mode.

This NiCad baterry charger regulator circuit can charge 6 volts as well as 12 volts NiCd batteries. It uses a transformer which can deliver 4 to 5 A current between 12.6 and 15 V. Ordinary chargers select the charging voltage through a swith but the circuit featured here uses an automatic current regulator.

The charging current is automatically regulated to 4.2 amperes. Once the charging current reaches 4A, the voltage drop at R1 will be 600 mV. In this case the transistor T1 will conduct and supplies current to T2.

The portable battery charger, car adapter and charger has been designed to provide an opportunity to load NiCd batteries even in the outdoors from a 12 V battery of a vehicle. When the circuit is connected to the vehicle battery, D2 lights only if NiCd battery was connected with the correct polarity. Since even on the discarded batteries we have a certain tension, T1 opens and D2 flashes.