iPhone 6s not displaying battery percentage correctly
I recently got an iPhone 6s and was playing around on it for a couple of hours but noticed that the battery percentage didn't go down at all. When I plugged it in for the night, it finally showed that instead of being 89%, I was at ~40%. My boyfriend also has an iPhone 6s which was at 4% when he plugged it in at night and was still at 4% when he woke up, but after a restart was at 100%.
Is there a way to fix the battery percentage? I tried turning the "show percentage" toggle off and on in the Settings but it wouldn't even toggle off the percentage icon when I left it for a few minutes. I'd rather not have to restart my phone whenever it's suspiciously high for my usage or when I really need to know how much battery life I have left.
iPhone 6s, iOS 9