The design of the charger is similar to that

of many commercially available chargers. The charger consists of a

mains adaptor, two resistors and a light-emitting diode (LED).

In practical use, this kind of charger is perfectly all right. Resistor

R1 serves two functions: it establishes the correct charging current

and it drops sufficient voltage to light the diode. This means that the LED

lights only when a charging current flows into the battery. The

charging current is about 1/4 of the battery capacity, which allows a

slight overcharging, and yet the charging cycle is not too long (4–5

hours).

#### NiCd Battery Charger Circuit Diagram

The value of the resistors may be calculated as follows, for which

the nominal e.m.f. and the capacity of the battery must be known. Adjust

the output of the mains adaptor to 1.17 times the nominal battery

voltage plus 3.3 V, which is the potential across R1. Note that the

adaptor must be capable of supplying a current of not less than half the

battery capacity. The value of R1 in ohms is equal to 3.3 divided by

1/4 of the battery capacity. The value of the resistors for various

battery voltages is given in the Table. The battery capacity is taken as

1 Ah.

#### Values for various cell counts

The rating of R1 should be 5 W. If the battery to be charged has a

different capacity, the theoretical value of R1 in the table must be

divided by the battery capacity. Its actual value is the nearest one in

the E12 series. For instance, if a 6 V battery with a nominal capacity

of 600 mAh is to be charged, the value of R1 must be 20/0.6 = 33R.