Adjustable Regulated Battery Charger

This battery charger circuit is regulated and adjustable to make this circuit able to charge the mosto NiCAD battery. This adjustable regulated battery charger circuit will work for single cell or multi battery cell which connected with series/parallel connection. The maximum voltage of the batteries should be 18V maximum.

Power transistors Q1 and Q2 are connected as series regulators to control the battery charger ‘s output voltage and charge-current rate. An LM-317 adjustable voltage regulator supplies the drive signal to the bases of power transistor Q1 and Q2.

Potensiometer R9 sets the output-voltage level. A current sampling resistor, R8 (a 0.1 ohm/5W unit), is connected between the negative output lead and circuit ground. For each amp of charging current that flows through R8, a 100mV output is developed across it. The voltage developed across R8 is fed to one input of comparator U3. The other input of the comparator is connected to variable resistor R10.

As the charging voltage across the battery begins to drop, the current through R8 decrease. 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. The circuit’s output voltage is set by R9, adjust the R9 to get the correct voltage output value as needed.