Sony Vaio PCG-FX401 Instruction & Operation Manual page 25

Using your notebook
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If you attach two battery packs, you can remove one of them without shutting down the computer. When removing one battery pack, make sure that the other
battery pack indicator is on. If the indicator of the battery pack your are going to remove is on, right-click the battery icon in the system tray and click the Switch
Discharge Battery option.
Charging the second battery pack
To charge the second battery pack, proceed as follows:
1 Keep the battery pack (1-3) in the computer while it is connected to AC power.
The battery indicator (2) flashes while the battery is charging.
2 When the battery indicator stops flashing, the battery is fully charged.
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