A very simple battery charger project can be designed using this circuit diagram. This charger has an adjustable regulated output voltage and also has an adjustable constant-current charging circuit that makes it suitable to use for charging most NiCad batteries and some other types of batteries . This battery charger can charge a single cell or a number of series-connected cells up to a maximum voltage of 18 V.
This universal battery charger circuit use just some common electronic components like LM317 regulator , operational amplifier and some 2n3055 power transistors .
2n3055 power transistors Q1 and Q2 are connected as series regulators to control the battery charger's out¬put voltage and charge-current rate. The LM317 used as an adjustable voltage regulator supplies the drive signal to the bases of power transistors Q1 and Q2.
By turning the potentiometer R9 the output-voltage level will be modified. A current-sampling resistor, R8 (a 0.1-fi, 5-W unit), connected between the negative output lead and circuit ground.
As the charging voltage across the battery begins to drop, the current through R8 decreases. , then the voltage feeding pin 5 of U3 decreases, and the comparator output follows, turning Q3 back off, which completes the signal's circular path to regulate the battery's charging current.
The charging current can be set by adjusting R10 for the desired current.
For this universal charger circuit you will need an power transformer that will provide a secondary output voltage of 18 volt at a maximum current between 2 and 6 amperes .