Water damaged iPhone 4S. Only working whilst plugged in.

I am currently working on a repair for a water damaged iPhone 4S.

Brief history: It was dropped down the toilet whilst the toilet water was clean (so less damage) about 3 days ago. The father put it in rice, which is pretty much useless but worth a try but the phone still did not turn on.

At the moment: I have cleaned it in Isopropanol alcohol after taking the cages off the motherboard. At first I did it with a tooth brush and then I put it in an ultrasonic cleaner with the alcohol in. I then went over the gold connection plates with an eraser to get all of the dirt and corrosion off them. After assessing the corrosion the motherboard seems pretty much near enough perfect (for what it is) but it still only comes on when plugged in. Obviously at first I thought this was a battery issue so I replaced the battery which should have had not much charge in it but the iPhone 4S says that the both batteries have 100% which doesnt prove true when checking with a multi metre. So both batteries are perfectly fine but the phone still only turns on when plugged in and straight off when taken off the mains. I have assessed the charging block and this is fine and also the power button is working perfectly (at first it was stiff but I have fixed this to see if it was the issue).

I think it may be a board issue in the sense that the power from the battery isn't getting to the board but the battery connections are fully intact and functioning.

Any ideas?