AA Battery Charger

AA Battery Charger Circuit with Auto Turn-Off Feature

This circuit is a AA battery charger that is prepared in series connected to 4-cell AA batteries. The charger is automatically disconnected from the mains when the batteries were fully charged. You can also use this charger when one or more of the 4-cell also discharged.

AA battery charger circuit is quite simple and can be divided into 3 sections, namely AC to DC converter, relay drivers, and charging.

AA Battery Charger Circuit Diagram

In the AC-to-DC converter section, transformer X1 steps down mains 230V AC to 9V AC at 750 mA. Rectifier diodes D1 through D4 rectified the voltage to a full-wave and filtered by capacitor C1. The 12V DC charging voltage provided by Regulator IC LM317 (IC1). The charger starts operating when you press switch S1 momentarily and the power-on LED1 glows to indicate that the charger is ‘on.’

You may determine the charging voltage depending on the NiCd cell specifications by the manufacturer. Here, we’ve set the charging voltage  at 7.35V for four 1.5V cells. Nowadays, 700mAH cells are available in the market, which can be charged at 70 mA for 10 hours. The open-circuit voltage is about 1.3V.

The shut-off voltage point is determined by charging the four cells fully (at 70 mA for 14 hours). After measuring the output voltage, add the diode drop (about 0.65V) and bias LM317 accordingly.  You can also  modify this automatic AA battery charger.

See also: Dual Power Supply Circuit – Ultrasonic Range Meter.