Im using a Fujitsu Siemens Amilo Li 1818. It takes about an hour and a half to charge fully and the battery life will last about 20 minutes. Is there anything I can do to make it last longer?

Sounds like the batterys shot but for now you can do this:

When on battery turn down the brightness and turn off bluetooth and wifi if you're not using them

In bios disable the infrared port and modem if you don't use them and there might be a setting to change the hard drive speed in there sometimes called silent mode

In control panel>power set the profile to portable which will set the cpu to dynamic mode (i.e it will speed up when doing big things and slow down when doing small things)

Replace the battery

Replace the battery. Then be careful not to overcharge the new one.

Before you do do that recalibrate the battery. Boot the computer in safe mode and let laptop shut down. Then reboot and let charge completely.