The charging light allows you to see how much charge your sensors have left while the sensor is turned on. 
 

Light Colour	Description
Solid green	Sensor is fully charged
Flashing green	Sensor has between 50% and 99% charge
Flashing yellow	Sensor has between 20% and 49% charge
Flashing red	Sensor has between 6% and 19% charge
No light	Sensor is off