It is an ideal solution to protect rechargeable batteries from deep discharge. It gives an LED indication when the voltage level in the battery drops below 5.1 volt.
CA 3140 Op Amp is used as a comparator with a zener to fix voltage in its non inverting input. Preset VR1 set the voltage level just below 5.1 volt at the inverting input. When the battery voltage is above 5.1 volt, zener provides regulated 5.1 volt to pin3 of IC and its output goes high to light LED. It indicates that battery is full. If the voltage in the battery drops below 5.1 volt, output of IC turns low to switch off LED.