Circuit :  Abu-Hafss, Pakistan
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Description
There are many circuits available on the net, for indicating battery status. The circuit presented here features a single LED which:
a) Lights constantly to indicate that power has been switched ON and the battery charge level is good.
b) Flashes to indicate battery voltage is low.
Notes
The design is pretty straight forward, consisting of two parts. In the green part is the battery monitor. The op-amp 741 configured as comparator, along with its corresponding components compares the battery voltage with the reference voltage preset using a zener diode. If the voltage is higher than the threshold level, the output of 741 remains low; there is no change in the status of the relay. The LED and the main circuit remain connected to the battery through the NC contacts of the relay.

As soon as the battery voltage level falls below the threshold level the output of 741 goes high. This causes the Q1 to conduct and consequently the relay coil is energised and the main circuit and LED is disconnected from the battery. At the same time, the circuit shown in blue is now connected to the battery through NO contacts of the relay. The blue circuit is a flasher, built around a pair of an NPN and a PNP transistor. This flasher is connected to the same LED. The flashing rate can be adjusted by varying the values of R8 and/or C1. D3 and D4 are used for isolation.

NOTE:
For the flasher, Q2 should be rated 300mA or more otherwise the flasher won't work. Please email me if you don't want to disconnect the main circuit when the battery level falls.