Your microphone sensitivity is set at maximum. This forces continues transmission, which consumes much energy from the batteries.
Solution: Set the microphone sensitivity to a value below max.
Programming actions at the parent unit:
press MENU button
press “-” button twice to go to “Sensitivity”
press OK button (sensitivity blocks appear in display)
press “-” button to adjust the sensitivity from the shown 5 blocks to any lower level
press OK button (message SAVED appears)
press MENU button to leave setup menu