Typical low-battery indicators simply turn on an LED when the battery voltage drops below a certain value. Such circuits actually accelerate the battery’s extinction by drawing excessive current. This circuit draws a mere 1 mA in its standby mode and averages only 20 m A when activated. The low current imposes a minimal burden on any battery circuit. source

The circuit is inserted between the supply and the load, therefore, until a trickle-charging current of at least 100µA is flowing towards the load, D1 and D2 will conduct. The forward voltage drop (about 1V) available across the Diodes, drives Q2 into conduction and, consequently, Q1 will be cut-off.If no appreciable load is connected across the circuit’s output, Q2 will become cut-off, Q1 will conduct and the Piezo-sounder will beep.source

This circuit is used to monitor the battery voltage, using a bi-color LED to indicate the state of the battery. When the LED is “GREEN” the battery voltage is above 11.9 volts. When the LED is “YELLOW”, the battery voltage is between 11.9 and 11.5 volts. When the LED is “RED” the battery voltage is below 11.5 volts. You can of course, modify the trigger points by using the trimmer resistors and/or changing the value of the dropping resistors in the divider.source

This circuit is Undervoltage indicator.The green led light up whenever Vin is larger than approx. 12.5V, while a red led flashes when Vin is below 12.5V. The circuit draws current approx. 15mA.source