I don't think smartphone batteries are supposed to, I heard its bad for them to go all the way to 100%. Mine will show 100% then drop to 97 as soon as its unplugged (sgs2).
There was an app called Autolock which fixed this, however it doesn't work past eclair (2.1), a lot of people waiting for a new app to do this, apparently its proving tricky to make for un-rooted phones