The regulated power supply and the four identical current sources, shown in the diagram, enable the simultaneous charging of four 9-volt Ni-Cd batteries. The potential at wiper of P1 determines to what voltage the batteries will be charged; an unusual but effective method. The voltage at the wiper is also applied to the non-inverting inputs of four comparators, IC-2a and IC-2d via 100 kΩ resistors. When the battery voltage is too low the relevant comparator toggles which results in the associated transistor being switched on, whereupon the battery is charged. The rate at which the comparators can toggle is slowed down by capacitor shunting the Op-amp (when a battery is being charged, its emf rises).
The charging voltage, resulting from the current through the battery or batteries can rise to the level set with P1. When that level is reached, the relevant circuit is switched OFF and the emf of the battery drops instantly. This might cause charging current to be switched ON again resulting in a charging voltage rising to the level set with P1. To prevent hid oscillatory action, the capacitor across the Op-amp enables the battery to stabilize. If, after a short delay, the battery voltage proves to be too low, the current is switched ON again. The capacitor then ensures that the current will flow for a while irrespective of the battery emf (after all the battery was not found to be fully charged).
NiCd Battery Charger Circuit Diagram:
LEDs in the emitter circuits of the transistors give a virtual indication of the ON and OFF switching of the charging current. When the battery is nearly flat the LED will be ON continuously when it is about fully charged, the charging current will be interrupted more and more frequently so that the LED begins to flicker. The more nearly the battery is charged, the faster the LED will flicker when the rate is about 1 Hz, the battery is fully charged.
The circuit requires an alternating voltage of 15-18 V, it draws a current of about 150 mA.