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.