My iPhone 3GS turns on for a second and shuts off immediately.

So I pretty much was told that my phone was "water damaged" (if it helps) but I have fixed a water damaged iPhone before. So I took it upon myself to try and fix this issue. however, there was one big thing that I noticed that happened everytime I tried to turn the phone on before I did any sort of repairs. When I plug it into the wall, it tries to turn on immediately. The problem is that it stays in the apple image but shuts off after about 20 seconds. It continues this cycle over and over. I have left the iphone charging overnight to see if it would help, but not only did it NOT charge at all, it still continued to try and boot. I dont know if this would be a battery problem or something else. Any thoughts before I go out of my way to buy a new battery??