I second the extended battery if you live in a dead zone. I live in a low/no signal area and pay the price as well in battery life. But the extended helps mask it.
As far as your original question, If your rooted run a battery calibrator from the market.
If not wait till the phone says "Charged" then unplug for 15 seconds and plug back in. It will top off the battery and should push it to 100%. I have this problem with my batteries too. Extended and standard.