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Simple battery-status indicator uses two LEDs

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-sidemounted 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.