It may just be the % indicator not in sync with the battery. Try running the battery down as far as you can until it shuts off. Then charge it up to 100%. Keep charging it past 100% for about 30 minutes or so. Just let it charge. Don't use it while it's charging. This calibrates the % indicator with the phone. If you think the battery is sick, try this test.
Write down your usage and standby time, press the sleep/wake button (or lock button, as some call it) to put the device to sleep, and set the device down for five minutes. When you come back, take note of the change in time. If your device is sleeping properly, then the Standby time should have increased by five minutes and your Usage time by <1 minute. If your Usage time rises by more than one minute, you have a drain problem. Something is keeping your device from sleeping properly, significantly shortening the time it will last.