When the batteries are almost empty, the LED indicator on the sensor will blink briefly when the sensor detects motion.
When the LED flashes orange, the batteries are getting low, but the sensor is still fully functional. The batteries will need to be replaced soon.
When the LED flashes red, you urgently need to replace the batteries, as your sensor can stop working very soon (Note: a long red flash of the LED (2 seconds) is an indication that the sensor has communicated with the bridge, only short red flashes are related to the state of the battery)
The sensor uses 2 AAA-batteries (LR03 Alkaline, non-rechargeable).