Properly maintained rechargeable batteries can provide good service and long life. Maintenance involves regular monitoring of battery voltage. The circuit in Figure 1 works in most rechargeable batteries. It comprises a reference LED, LEDREF, which operates at a constant current of 1 mA and provides reference light of constant intensity regardless of battery voltage.
It accomplishes this task by connecting resistor R1 in series with the diode. Therefore, even if the battery voltage changes from a charged state to a discharged state, the change in current is only 10%. Thus, the intensity of LEDREF remains constant for a battery state from a fully charged state to a fully discharged state.
The light output of the variable LED changes with respect to changes in battery voltage. The side-by-side-mounted LEDs let you easily compare light intensities and, thus, battery status. Using diffused LEDs as crystal-clear LEDs can damage your eyes. Instead, mount the LEDs with sufficient optical isolation so that the light from one LED does not affect the intensity of the other LEDs.
The variable LED operates from 10mA to less than 1 mA as the battery voltage changes from fully charged to fully discharged. Zener diode DZ in series with resistor R2 causes the current to change with battery voltage. The sum of the zener voltage and the drop across the LED should be slightly less than the lowest battery voltage. This voltage appears across R2 . As the battery voltage varies, it produces a large variation of current in R2 . If the voltage is approximately1V, then 10 mA will flow through LEDVAR , which is much brighter than LEDREF . If the voltage is less than 0.1V,then the light intensity of LEDVAR will be less than LEDREF , indicating that the battery has discharged.