Sony PCG-XG18 Primary User Manual page 56

You can purchase a second rechargeable battery pack. With two battery
packs, you can extend the time that you use your computer with battery
power. When using two battery packs, the battery pack you insert first
charges first. The battery pack you insert second charges when the first
battery pack is 85% full. You can monitor which battery pack is charging
by observing the battery indicator lights.
If another device is in the multipurpose bay, remove the device. See
"To remove the DVD-ROM drive" on page 35.
Insert the battery pack into the multipurpose bay.
If the battery is being used, the battery 2 indicator light turns on.
Attach the weight saver to the computer to stabilize the battery pack.
Insert the tab of the weight saver into the hollow of the multipurpose
bay slot, and then push the weight saver into the multipurpose bay
until it clicks.