Sony PCG-F420 Primary User Manual page 48

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 percent 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 floppy disk drive" on page 25.
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.
There is a RELEASE lever on the bottom of the weight saver. Slide the
lever toward the front of the computer to lock the weight saver. See
also "Using the Weight Saver" on page 41.