Temperature Controlled NICD Charger IC LM311

Posted on Feb 5, 2014

This circuit is for a temperature controlled constant current battery charger. It works with NICD, NIMH, and other rechargeable cells. The circuit works on the principle that most rechargeable batteries show an increase in temperature when the cells becomes fully charged. Overcharging is one of the main causes of short cell life, hot cells pop the

Temperature Controlled NICD Charger IC LM311
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ir internal seals and vent out electrolyte. As cells dry out, they lose capacity. The transformer, bridge rectifier, and 1000uF capacitor provide around 22 Volts of DC power to run the rest of the circuit. The 7812 regulator drops this to 12V to run the 311 comparator and 4011 nand gates. The start switch is pressed to start the charging cycle. This causes the two 4011 nand gates, which are wired as an r-s flip-flop, to go into the charging mode. The Red LED is lit, and the VMOS FET current switch is turned on. Charging current runs though the battery pack. If the battery starts out warmer than the reference temperature, the circuit will not switch into charging mode. Let the pack cool down. When the battery pack reaches a full state of charge, the differential temperature sensor causes the flip-flop to switch off, turning off the VMOS current switch, and lighting the Green LED. The 7805 voltage regulator is wired as a constant current regulator. This provides a safe maximum charge current for a number of different cell types. The 500 ohm resistor across the VMOS FET sets the trickle charge current which flows through the battery pack after the bulk charging is finished. The differential temperature sensor circuit works by presenting two voltages to the input of the 311 comparator. The comparator output switches on or off depending on which input is at a higher voltage than the other. As the thermistors warm up, their...

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