The figure below shows a very useful project of battery level monitor using TL071 IC. The circuit is simple and easy to build and using only few discrete components with the opamp IC. The circuit can be used to indicate the required low level of batteries. The LED will glow and show a visual
indication when your battery voltage level will reach to your preset low voltage level. For sound indication you can also connect a small buzzer parallel to the LED.
The heart of the circuit is a TL071 IC that is used in comparator mode here. The 10K variable resistor is used to set the voltage level on which you want to activate the LED. A 5.1V zener diode is used as a voltage reference.
The given circuit is adjusted to monitor 12 volt batteries. But it can be easily adjusted to monitor any voltage battery. For example to monitoring a 6V battery use a 3.1V zener diode in the place of 5.1V zener.