Low battery indicator with LED
Here is a basic circuit that you can adjust with R1 when you want your LED to light up when the voltage decrease.

D1 is a Zener diode, you can use something between 1-2.5V with low leakage.
R1 is a trim-pot that you can adjust.
OA1 needs to be chosen to fit your supply range and has enough current output to supply the led.
Use a low current LED like 1-2mA.
You can add an NPN transistor after OA1 if you need more current for the LED.
R2 has to be big enough not to deplete the battery, but small enough so that the leakage current of D1 is not significant.




simulate this circuit &ndash; Schematic created using CircuitLab