The RadioBridge Sensors use a 3V battery which measures about 3.2 volts when freshly installed. Once the battery depletes to 2.5V the sensor will cease to function. The Supervisory Message periodically sent from the sensor uses the last byte to display a human-readable battery voltage.
A typical Supervisory Message might look like:
1 5 01 03 00 29
1 Protocol Version (currently always 0x1)
5 Packet Counter, increments with each message and wraps from 0xF back to 0x0
01 Message Type (Supervisory Message = 0x01)
03 Error Codes (Comm Error, Low Battery, Downlink Error, Tamper State, Tamper Detected)
00 Sensor State (defined differently for each sensor type)
29 Human-readable Battery Voltage Level
In this example the last byte is 0x29 which indicates 2.9 Volts.
Radio Bridge sensors are shipped with lithium batteries, and these are recommended when the battery needs to be eventually replaced. The battery will stay just under 3V for most of the battery's life and then experience a rapid drop near the end. Because of this, it is recommended that users rely on the "low battery" error code (bit 1 of the error code byte) within the Supervisory Message rather than rely solely on the tracking of battery voltage.