Iphone 3GS won't hold charge?

I recently acquired a used Iphone 3GS from a friend, who said it worked fine. And it does, but it won't hold a charge. While it's plugged in, it works great, but when I unplug it, the battery dies within minutes, sometimes seconds. If I don't use the phone at all while it's unplugged, it'll stay on for the normal amount of time. But if I try to do anything, play games, use the internet, call, text, etc. The battery dies, and it won't turn on again until I plug it back in. The battery says it has full charge. Does anyone know of a way to fix this?

Apple purposely make their batteries only last a couple years so that you will buy the new ones. My advice is to ditch the iPhone you won't be able to fix it unless you want to pay 100$. Get an android with a removable battery and if the battery starts to suck you can get a new one for about 10$

There are a few things that could be wrong.

1. The software may not be up to date
2. The internal battery may be damaged (as i have seen in many tests that the phone won't work unless its charged, as the batteries use has deteriorated by the submergence of water)
3. The charger itself may be defective

those are just a few things, to say the least. You should check with your friend, in spite of him saying that it works fine, or it seems to work fine. You don't know what has been previously done to the device. You should also call apple if you have apple care for the device or anything similar to that.