I think my battery needs to be replaced

[Dustin Lane]

Whenever I try to take my phone off the charger when it says 100%, it will only work for about 3 minutes then just shut off with no "goodbye" message thing, the screen just goes black. So I turn it back on, and it'll go through the whole beginning sequence, but the screen will just flash on for a second, than turn off instantly. I managed to get it to stay on for about a minute after it would turn off initially, and when I turned it on initially, it had been on the charger for a few hours and said it was at 100% again. So, after the initial turn on, it shut off, so I tried turning it on again, and it stayed on for about a minute and said battery critically low and the battery symbol up top showed no charge, than it just shut off instantly again. Can somebody please tell me whats going on? One more thing to note, is that it doesn't turn off if it's on the charger. I really need to know what's going on so I can get it fixed. Could it be the circuitry screwing up reading it wrong making the phone think it's dead, so it just shuts off? I really don't know much about it, but I think I need a new battery. I just want to make sure before I go spending money on a new battery.