iPhone 6 battery problems

Hello everyone, so i have an iPhone 6 and a few weeks ago, i noticed battery issues, like sometimes it would turn off at 16 or 15%, or it would go from 40% to 20% directly, and then stop at 20% for hours. I don't really understand the problem. my iphone is still under warranty, i got an appointment at the apple store next friday, do you have any idea how to solve this ? or if Apple is gonna help? Did any of you guys had a similar problem?

Thank you!