Hiya i have a slight problem with my iphone 4

Hy all

I have a little problem, my iphone 4 is water damaged and i know i need to get the dock unit and speaker changed as they dont seem to work cos theres no sound but it does activate the phone when plugged into a power source. I have changed for a new battery and the phone works fine but when the battery is connected and the phone is either on or off it gets extremely hot by the battery connector. As soon as i unplug the battery and turn the phone on by plugging into a power source the heat dissapears. I have 2 iphone 4 batterys and cant imagin they are the problem but could well be, was just wondering if anybody has had a similar problem. And yes iv done all the resetting and closing programs and things.

Could it be the motherboard? Although iv had that out and its clean as a whistle

Many thanks for any help