Bought an iphone 4 that wont turn on hoping to fix it. Previous owner tried to change the faulty home button, did, put it back together and it wouldnt power on.

So far I found out the phone wont turn on at all on battery power, I tried holding the power and home button for 15 seconds and nothing.

I then tried the USB cable and nothing on itunes

I then tried the mains charger and after holding the power button for 15 seconds I got a vibrate and the screen light up but it was black, nothing displayed but it was on.

I then turned the power adaptor off and it instantly it all went off which made me think battery problem

I then opened it up got the battery out and noticed the batteries ribbon cable / wire was split. I connected the phone up with just the mains (no battery) and it powered up again but just black screen again (but the screen was on and it would vibrate when muted)

I decided to strip it all down and it all looks fine.

Should the phone power up fully when run just on mains power only (no battery) or will it just give a blank screen like I have been getting?

Also where is the water damage indicators on these?