Typical operating voltage of Physiotel HD transmitters is between 1.57V and 1.85V. The voltage will be consistent throughout the life of the transmitter. Toward the end of the transmitter battery life the voltage will drop to a lower level, and the software will alert the user that the voltage has dropped.
HD-X02, HD-S02 and HD-XG transmitters have a low voltage alarm of 1.5V. All Other HD transmitters have an alarm at 1.55V.