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Simple Battery Charger Circuit Charges Upto 12 NiCD Cells

This handy circuit can be used to charge from one to 12 NiCd cells from a car battery. Up to six cells can be charged with switch S1 in the "normal" position. The LM317regulator operates as a simple current source, providing about 530mA when R1 = 2.35O (two 4.7O resistors in parallel). For more than six cells, S1 is set to the "boost" position. This applies powers to IC1, a 10W (or 20W) audio power amplifier. Positive feedback from its output (pin 4) to non-inverting input (pin 1) causes IC1 to act as a square wave oscillator. This square wave signal is coupled to the junction of Schottky diodes D1 and D2 via a 330µF capacitor, forming a conventional charge-pump voltage doubler. Over 20V (unloaded) appears at the input to REG1 - enough to charge a maximum of 12 cells!