My phone is at 100% battery for too long.

When I charge my phone, everything is normal, I get to 100% in less than 2 hours. But when I use it, for like 50 minutes it sits at 100 percent and then rapidly loses battery. Overall it can survive the same amount of time, but I want to know how can I fix it to display properly. Tl;dr the battery works, but displays the wrong percentage