Right - Sony PCG-F420 Primary User Manual

If you attach two battery packs, you can remove either of them without
shutting down the computer. When removing one battery pack, make
sure that the other battery pack's indicator light is on. If the indicator light
of the battery pack you are removing is on, right-click the battery icon on
the task tray and click the "Switch discharge battery" option.
There is a Release lever on the bottom of the weight saver. To remove
the battery pack, slide this lever in the direction of arrow 1, then slide
the RELEASE lever on the bottom of the computer in the direction of
arrow 2.
Remove the weight saver, and then pull out the battery pack.
Reinsert the weight saver. See "Using the Weight Saver" on page 41.