How do I know when to change the batteries in the PV Analyzer Wireless sensor transmitters?

The temperature and irradiance sensors have wireless transmitters powered by a standard AAA battery. If the battery inside one of the wireless sensor transmitters is low, the ?LOW BATT? LED on front of the transmitter will be illuminated.     To replace a transmitter?s AAA battery, unscrew the two screws on the back of the transmitter using a small flat-head screwdriver, then remove and replace the AAA battery.