Sony PCG-F420 Primary User Manual page 20

You can use one or two battery packs as a source of power. The second
battery pack inserts into the multipurpose bay on the right side of the
computer. (See "Using Power Saving Modes" on page 36.) 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.
Insert the battery pack into the battery bay on the left side of the
computer.
Close the cover of the battery bay until it clicks.
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