Sony PCG-XG18 Primary User Manual page 57

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 21 for more information.
There is a RELEASE lever on the bottom of the weight saver. To
remove the battery pack, slide the RELEASE lever on the bottom of
the computer in the direction of the arrow. See also "To remove the
floppy disk drive" on page 32.
Remove the weight saver, and then pull out the battery pack.
Reinsert the weight saver.