iPhone 3GS turns off randomly even though it has full battery?
I own several iphone 3g and 3gs phones that I bought from friends when they got upgrades. They've just been laying around for the past couple of months because one turns off randomly, and another just won't turn on (when it's plugged in, it either says it's charging but doesn't turn on, or it says to plug the phone in even though it is plugged in). The phone that turns off randomly looks like it's in perfect condition, and upon inspection the inside looks very clean too. I've cleaned the connection between the battery and the motherboard, and tried multiple different batteries too. I've also cleaned the gold prong on the bottom of the motherboard. I've done all of these things for both with no luck. Any advice?