How it works : When the battery voltage is 11.5V or less transistor Q1 is turned on and the LED D1 will be bright. When the battery voltage is between 11.5 and 13.5 V, the transistor Q2 is turned on and the LED D2 will light up. When the battery voltage is 13.5V transistor Q3 will be on the D3 and the LED will light.
