A very simple battery monitor can be made with a dual-colour LED and a few surrounding components. The LED produces orange when the red and green LEDs are illuminated.
The following circuit turns on the red LED below 10.8v
The orange LED illuminates between 10.8v and 13.2v.
The green LED illuminates above 13.3v
Just adjust source voltage between 9V and 14V. If you want other voltage thresholds adjust the values of zener diodes
