When the battery is fully charged the transistor is fully switched on and diverts all of the 1 mA from the current source to ground and the LED is off as the voltage across C & E ( less than 1 V ) of the transistor is well below the LEDs forward voltage ( about 1.8 volts ) However as the battery voltage falls to around 7.5 volts the transistor begins to switch off and the voltage across the LED becomes high enough for it to begin to light. With the pot set as it is the transistor is fully turned off and hence the LED is fully turned on by the time the battery voltage falls to 7 Volts. The threshold can be varied by adjusting the pot. The 1 mA current source can be made with a junction FET ( not available on this simulator ) with an Idss of around 1mA.
