A battery-powered occupancy sensor for detecting an occupancy condition in a space comprises first and second batteries, an occupancy detector circuit, a controller, and a wireless transmitter for transmitting a first wireless signal in response to the occupancy detector circuit detecting the occupancy condition in the space. The controller and the wireless transmitter are powered by the first battery, while only the occupancy detector circuit is powered by the second battery, such that the occupancy detector circuit is isolated from noise generated by the controller and the wireless transmitter. The occupancy detector circuit draws a current having a magnitude of approximately 5 microamps or less from the second battery. The occupancy sensor transmits a second wireless signal is response to determining that the voltage of one of the batteries has dropped too low.