Intelligent Battery Charger for NiCd and NiMH

The intelligent charger was designed for high current and rapid charge that is suitable for automatically charging wide range of batteries and applications such as model racing cars and cordless power tools.

The charger uses the recommended charge method of the cell manufacturer to quickly and safely charge the batteries since these battery packs are expensive and sometimes hard to purchase. A battery consists of two or more cells and for design purposes, the NiCd/NiMH cells are rated at 1.2V although they normally develop about 1.25V. They require 1.5V to 1.6V under full charge. Very large amounts of current can be supplied and a flat discharge characteristic is displayed.

As required by a microcontroller, a constant current supply is switched ON and OFF. The battery voltage is sensed by the microcontroller and uses an analog to digital converter internally to read the battery voltage. The two LEDs display the current charging status while the microcontroller requires its own 5V regulated supply. To reduce ripple under a full 1.5A load to about 1V, the circuit uses a bridge rectifier and capacitor. A stable and accurate time base is provided by the crystal.

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