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This is a 12v battery over voltage and under voltage indicator circuit. This circuit will work if the voltage goes about 14 v and below 10v ( you can adjust the voltage of indication using Preset ). The 12v circuit contains a single comparator ic which will detect the high voltage and low voltage. The ic is used in this circuit diagram is LM339.
Circuit Diagram
The transistor is connected to the LED. If the battery voltage goes up while charging and cross the set values that time the LED gets glow. That point of led glowing can be set using a multimeter manually. Suppose the battery voltage goes down that times the second led will glow and indicate a lower voltage in the battery.
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