So 2 days ago I look at my battery sign and for the first time, I see a battery with an X on it. Clicking on it brings the sign "No batteries available." The battery is fine though as I can run the laptop on battery without problem. And when I try to run Coconut Battery, the program crashes. I've attached the report.

And I've tried searching for solutions online, but it seems everyone's clueless.

It seems I can't attach a 8kb .zip file. Maybe because I only have 11 posts.

I took out the battery and put it back in and everything is now fine, although the battery started charging from 15%, which is weird because it was full. And I just ran coconut battery and my battery's current capacity (maximum battery charge) is still 5071 mAH

Taking it to a AppleCare provider is your best bet. (possible hardware problem)