I don't know what's wrong with my phone...?

take it back again, sounds like a software fault, not a battery fault. Could be the batter terminal too, im still guessing software. Make them send it in for repair. they will treinstall the software. Ask them for a loan handset, just tell them that you are unhappy that you have had to bring it back and that you deperately need a phone and they will loan you a handset whilst it is in repair (all phone stores do this)