Original post by: James Soulsby ,
Quite often water damage will ruin the battery and you will need to replace it. You could try that if you haven't already. You mention you spotted corrosion near the battery connector. Unfortunately though with water damage it is hard to know exactly where the problem would be. To find out is very time consuming and involves taking off all the shields looking and testing the components on the board. Even after doing all this sometimes it is impossible to get it working 100% again. Not 100% sure how Apple warranty works but you could give it a try if there is no evidence of water damage.