BB Style :: Even Though 100% Battery, Dead Without Warning

For starters, my battery indicator does not seem to work. Even taking that into account, I manually check it constantly and it's at 100%. It never goes below that because the phone just suddenly goes black (and dead) every single day. There is no warning, nothing. So, I will recharge it and it shows that it IS charging though it will again be fully charged in about two hours. It is on the charger overnight, but will suddenly go totally black/dead every late afternoon without fail. Today, it happened even while I was typing. No, I haven't bought a new battery - I have taken it out and put it back in. I'm not convinced the battery is the problem.