Sony PCG-F420 - Manual (Page 49)

Using a Second Battery Pack !'
To charge the second battery pack
Keep the battery pack in the computer while it is connected to AC power.
The battery indicator flashes while the battery is charging. See also “To
charge the battery pack” on page 11 for more information.
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To determine the amount of time the battery lasts, see “Displaying Battery Information” in
the online document Customizing Your VAIO ® Notebook.
Second battery pack
(not supplied)
Battery indicators
Battery pack (supplied)