You mean running it on battery only? How old your notebook is? If it's older than 6 months, and you've been using it on mains with battery inserted, then probably your battery is dead, and warranty is for 6 m. only. You probably need to replace it for new one, before you may want to read this:
http://forum.notebookreview.com/showthr ... ost1696311
and google for "repairing battery" or "reviving battery". You'll find there some informations about repairing it by yourself. If you don't want to "mess with stuff" you'll also find information about companies specializing in such repairs.
REMEMBER! Original equipement guarantee you compatibility, safety for your device (using OEM product you don't void warranty) and intensive wallet drain. On the other hand, home methods and non OEM products, although cheaper, may leed to damage to your device or may not work properly.
FSC Lifebook E6595 PIII M 850MHz, 256 MB, ATI M4 16MB, HDD 20GB, _NEC DVD+-RW ND-6500A; and an original FSC mini RF mouse ;P