3GS battery problems

Ok so I am running iOS 5.0.1 on a 3GS. I never had battery problems before and all of a sudden my phone will go black and turn off. When I press the power button it flashes the no battery thing and says I need a change. This cannot be the case be cause I know it had a 86% charge. It did this 7 times to me yesturday. Is my phone dead? I got this phone as a referb from AT&T a year ago and if I read their site correctly my warranty is more than up. I am a college student and cannot afford to replace it as at&t want $250+ for a upgreade phone on my plan because it's not been two years yet. Is it just the battery going out? Can that be replaced? Is it expencieve? I don't have an apple store around where I live so are they the only ones who can fix it? HELP!

Restore a older firmware from pwaned dfu, but try restoring a custom firmware without baseband update because will not be able to restore to a normal one, if you have any shsh saved restore to 4.1. If still not working probably is a battery problem