NiMH Battery Charger Based on AVR

The simple project is built around an ATmega8 that utilizes PWM and ADC to control the charging current of NiMH battery cells.

One very popular energy source in portable media like digital cameras, clocks, and various players, is the NiMH cells since they provide good energy density for reasonable price. About 10% of their rated hourly capacity is the charging current of NiMH cells while 14 hours of charging should not be exceeded. The NiMH charger ensures that after the recommended charging time, it simply switches off. When charging is done, a bi-color LED indicator will start flashing green.

Before using the charger, it has to be calibrated once through a serial terminal then the potentiometers handle are set to right position. The charging current is set by the potentiometer. It simply controls a voltage on the microcontroller’s ADC unlike conventional analogue chargers.

To provide serial port I/O, a 3-pin header is added used in the once off calibration of the system. To prevent mechanical inaccuracies, the calibration only takes place after the final construction. Flattening the round POT shaft on the side of the grub screw can prevent POT knob slippage.

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