I am using windows 7.
In windows xp, when the battery of my laptop charged full, it automatically worked on direct supply of current means by cable.just by removing cable laptop worked on battery.but since I installed win 7, I have never seen this option working, although I have tried for a lot of power options.
Moreover the battery use time has reduced from 4 to 2 hours in just 6 months.I have a lenovo laptop.
Please guide me.

Sounds like your battery is on it's way out.