Here is an Unusual LED indicator for the Battery. It turns on fully when the battery voltage is above 13 volts and starts to blink faintly when the voltage drops below 12 volts .Finally the LED turns off when the voltage drops below 10 volts. Its design is unusual because the LED driver transistor is connected in an unusual way.
No doubt, everybody knows that an NPN transistor conducts in the collector-emitter direction, if its base is above 0.6 volts. But here the NPN transistor BC 547 is connected in the reverse direction with the base left open. In this mode, the p-n junction of the transistor works like a reverse biased diode. So its Emitter-Collector path polarizes in the reverse direction and the transistor starts oscillating depending upon the voltage across the p-n junction. When the voltage level is above 12 volts, the oscillations are very fast and the LED appears as fully lit. When the voltage level drops below 12 volts, frequency of oscillation drops and faint blinking can be observed. Brightness of the LED also decreases. When the voltage drops below 10 volts, transistor turns off and LED turns dark. Connect it with the 12 volt battery observing polarity.
No doubt, BC547 can tolerate maximum 45 volts and I have observed its behavior for 24 hours. Nothing happened. Heating, burning, puncturing, LED breakdown or anything. Still it is monitoring my Inverter battery.
Try it and see its functioning. If it breakdown after few days, don’t worry you lose only Rs.3
These images reveal its working