This article explains how to properly insert and install your device's back-up battery.
INSTRUCTIONS & INFO
Gently press down on the raised arrow on the back-up battery cover, then slide it in the direction the arrow is pointing.
Insert the battery connector into the3 prong jack and press in completely up to the connector collar.  You should not expect to hear a click once it is connected.
Place the back-up battery in the WHP Base Unit with the bar code side facing up, and the back-up battery wires next to the three-prong battery port.
Replace the back-up battery cover by sliding it back into place until it clicks.
Note: Remove the back-up battery if the device will be disconnected from AC power for more than 15 days to avoid possible damage.