12 volt NiCd battery charger design circuit diagram for your DIY


12 volt NiCd battery charger circuit diagram
This simple schematic shows how to design a 12 V Nicad (nickel cadmium) battery charger circuit by using 555 timer and 78L05 voltage regulator circuit.

If you attempt to charge a 12 V NiCd battery from a 12 V lead -acid car battery, you will soon find that that is not really possible: the charging voltage should be somewhat higher than the nominal bat-tery voltage.

A 12 V battery should be charged froma source of about 14 V.

The present circuit is, therefore, a voltage doubler based on the well-known 555 IC. The IC oscillates, which means that output 3 is connected alternately with earth and the +12 V supply voltage.

When pin 3 is logic low, C3 is charged via D2 and D3 to almost 12 V. When pin 3 is logic high, the voltage
at the junction of C3 and D3 becomes almost 24 V, because the negative terminal of C3 is at + 12 V and the capacitor itself is charged to about 12 V.

Diode D3 is then reverse biased, but D4 conducts, so that C4 is charged to just over 20 V, which is ample for our purposes.

Function of 78L05 in 12 volt NiCd battery charger circuit diagram

The 78L05 in the IC2 position functions as a current source, which tends to keep its output voltage, Uri, appearing across R35 at 5 V. The output current, hi, is therefore easily calculated from Ill = U1/R3= 51680 = 7.4 mA.

The 78L05 itself also draws current: the central ter-minal (normally earthed) delivers about 3 mA.

The total load current is, therefore, of the order of 10 mA, which is a good value for continuously charging NiCd batteries. The LED has been incorporated to indicate that charging current flows.

The characteristic of the charging current versus battery voltage in figure 2 shows that the circuit is
not perfect: a 12 V battery will be charged with a current of only about 5 mA.