This Motion Sensor takes 1 CR123A battery and the typical battery life for the sensor is approximately 5 years.
Press on the round button on the top of the Motion Sensor while pulling the sensor away from the rear cover to access the battery.
Replace the CR123A battery on the inside of the sensor.
After replacing the battery, the sensor's red LED will starting blinking, indicating the sensor is warming up.
Close the sensor by inserting the front of the sensor into the rear cover of the Motion Sensor until you hear a click.