Posted on Feb 4, 2014

This is a charger circuit for both NiCd and NiMh Batteries. Some time ago I aquired an old commercial handheld-TRX which was powered by a 12 Volt NiCd-Battery pack (10 Button Cells each having a capacity of 280mAh). In fact this (at least 10 year) old battery is best because there is a very special charger used: When the Battery voltage is low it

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is charged with the usual constant current source in the normal way. But when the battery voltage increases, the charging current will become pulsed. A completly fully charged battery only requires short flashes to maintain its charge and terminal voltage. Here is the circuit diagram: The original circuit used some old-fashioned components, so I devised a completly new design using only standard, commonly available components. The LED (green) shows that the battery is charged. A full battery is indicated when the LED is "dark" most of the time. Set the Charger to 1. 42 Volts for each cell (i. e. 5. 68V for a 4. 8V Battery-Pack) by adjusting the 47k variable resistor. The charging current is around 120mA and can be changed by adjusting the 12 Ohm resistor, or the forward votlage (colour) of the LED. At the moment I am using the new circuit for 4. 8V, 6V and 7. 2V Batteries having a capacity ranging from 500mAh to 1200mAh. 12vDC is used to power this charger. Here is a photograph of the completed project:

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