"There has been some battery leak, but quite modest"
Hi Iggy_Drougge,
The first thing you MUST do is remove the old battery even if it still is working (you do not need to replace it with anything right away). You then need to clean up the area around it with a Q-tip and some rubbing alcohal. Make sure that you look closely to make sure that all the traces on the mb are clean. Once done test with a tester to see if you have any broken traces anywhere around the affected area. Battery leakage no matter how small can cause serious dammage if not cleaned up right away.