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I have a mid 2012 unibody MBP with some odd power issues. It started when one of my chargers stopped working and I started noticing that the battery indicator would show an unusual amount of charge left such as %30 when I knew it should have close to %100, or the other way around. Then it began having red battery icons, and the laptop would intermittently just go into sleep or shut down completely when I would unplug my charger. I did an SMC reset and it seemed to help for a bit, but would start up within a week or two and eventually got to the point where the SMC reset wouldn’t do anything and being able to get my laptop running solely on battery power (being able to wake it after unplugging, or having it just startup on its own when opened) was pretty rare. Finally the battery icon disappeared completely. I tried getting it back in system preferences and it will show up as charging for a second then disappear right away and uncheck the box. When checking my battery it seemed to indicate it was fine, but it was a little high on charges and I figured I’d try a battery swap to see if that would fix it. I swapped the battery and Coconut is essentially seeming to say it’s not there. Manufactured in 1979, power adapter connected, but then saying not connected in battery info, no charge capacity, etc. Similar to when the previous battery was connected, when i try to do an SMC reset nothing seems to happen, magsafe adapter just stays a solid green (no matter what charger I use), battery icon never appears. I know it seems like the I/O but it seems strange as with the old battery it would still occasionally wake up with just the battery.
Lets do this pop the bottom cover off and disconnect the battery charger ribbon cable. Did that fix things? Here’s the IFIXIT guide: MacBook Pro 15" Unibody Mid 2012 Logic Board Replacement follow to Step4 then jump down to Step21. You may want to inspect the both eat MagSafe adapter and connector on the system to see if it’s damaged review this Apple T/N: Apple Portables: Troubleshooting MagSafe adapters
