The LCD displays battery voltage when the button is pressed or when initially plugged in.
Display voltage range is 3 to 16V, resolution is +/-0.1 volt.
The red LED flashes when voltage is low.

Green: Battery Charged (Greater than 12.0V)
Yellow: Battery is Low (11.6 to 12.0V)
Red: Battery is Discharged (Less than 11.6V)