Universal Battery Charger for Charging up to 18V Cell

This universal battery charger has regulated output voltage which can be adjusted as you need. Also, this circuit has an adjustable constant-current charging circuit.  So, the circuit will be compatible and easy to use with the most NiCad batteries.  It capable to charge a single cell or a number of series-connected cells up to a maximum of 18V.

A series connection of power transistors T1 and T2 to manage the battery charger’s output voltage and charge-current rate. An LM317 adjustable voltage regulator supplies the drive signal to the bases of power transistors T1 and T2. R9 potentiometer sets the output-voltage level. Then, R8 is used as a current-sampling resistor and it 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 IC3. The other input of the comparator is to connect to a variable resistor (R10).

As the charging voltage across the battery begins to drop, the current through R8 decreases. Because of that, the voltage feeding at pin 5 of IC3 decreases, and the comparator output follows, turning T3 back off, which completes the signal’s circular path to regulate the battery’s charging current.

You can set the charging current output you need by adjusting the R10 potensiometer. And you can set the universal battery charger’s output voltage by varry the R9 variable resistor.