Volume button (low) did not work correctly after water damage 3g
Phone lights up every 30 secs and loses battery power fast.
The new batter did not solve the problem (every 30 secs lights up).
After the new battery I did install also new cabling around the volume button. This solved the problem.
iPhone 3G Replacement Battery
iPhone 3G and 3GS Volume Button
SIM Card Eject Tool