This is one of my work made this weekend. :-)
I made battery voltage checker designed by JA1XRQ. LED color is changed according to the voltage variation.
And LED flicks >15V, alarm buzzer sounds <10V.
For more details are shown in JA1XRQ's webpage
[Addition on Feb/28]
I made mistake on selection of legs of color LED.
The connection of red leg and blue leg were reverse.
I fixed and put it in a small case.