Shuts off unless it's plugged in but says the battery is fully charged

I bought a new battery for this a couple months ago and it worked fine until a few weeks ago. Now it can only stay powered on if it's plugged in despite the battery saying it's fully charged. I never had any issues with this until now, I didn't leave it on the charger all the time, and did nothing different from the months prior.

I tried taking the battery out and restarting it, I dusted the ports (but can't get the case off due to 3 stubborn and now stripped screws...) I don't know what else to do or how this even happened. Any other ideas?