Compaq Armada M300 Series Reference Manual page 44

The primary battery pack is the battery pack attached to the
computer.
For information on using a battery pack in the mobile
expansion unit MultiBay, refer to Chapter 5.
1. If the computer is on, save your work and initiate Hibernation
or shut down the computer.
2. Turn the computer bottom side up and tilt the battery pack so
it lies flat (covering all ports on the rear of the computer).
3. Slide in the two battery latches toward each other.