Chosen Solution
I have this MBP 15" Mid 2012 that someone spilled some water on. I have no idea how much because they removed the bottom cover and put it near a fan and then turned it on (against my advice, why do people do that?) I blew it out with a compressor and it runs fine but when the battery is installed, it won’t charge or run off the MagSafe adapter, it only runs off battery. If I remove the battery it will run off the MagSafe. I have tried some resets, etc and even a new battery. Any suggestions?
Sadly your logic board has some damage within the charging logic. You could try installing: coconutBattery which will allow you to see thats happening here. There is a set of MOSFET’s that switch the power feed from the MagSafe adapter to the system and the battery as needed controlled by SMC. You’ll need to find someone with the tools and skills to diagnose what needs replacing and has access to the needed parts.
