Only turns on via mains, instant off when unplugged
My friend gave me his 3GS which is a few years old now, he's trying to make it last until the upcoming release but I can't diagnose the issue.
Currently, it'll only turn on when plugged into a mains outlet (Plugging it into my Macbook Pro does nothing.. very confused?). As soon as it is unplugged it will turn off and the screen ghosts pretty badly.
Whilst it's on it shows the "Connect to iTunes" symbol, and if I click the "?" it says the IMEI and ICCID are unknown.. Every few minutes it will reboot. I did put it into DFU mode but kind of useless on a wall outlet.
I've tried replacing the battery but I only had an iPhone 3G battery lying around, I thought it would be ok as they're both 3.7V but it made no difference at all.
Does anyone have any theories, suggestions or tips? They'll be really appreciated!
Is this a good question?