Sony PCG-XG18 Primary User Manual page 22

You can use one or two battery packs as a source of power. The second
battery pack inserts into the drive bay on the right side of the computer.
(See "Using Power Saving Modes" on page 42.) Additional battery packs
are available as a separate option.
The battery pack that comes with your computer is not fully charged at
the time of purchase. Follow the steps below to insert and charge the
battery pack.
Open the cover of the battery bay on the left side of the computer.
Insert the battery pack into the battery bay.
Close the cover of the battery bay until it clicks.