I've noticed that before too. Do not top off the battery frequently. Let it drain to at least 40% before you put it back on the charger, if possible. However, if it's still on the original battery, has the phone also been shutting off or the screen flickering? Has it shown you still had ~50% battery life and then it drained really fast? Those are signs the battery is old and needs replacing, or it's defective, if you have replaced the battery before. That being said, the Li-ion battery in my iPhone 5s just expanded this morning and popped my screen on the left side. That in turn broke my Otter Box Defender's inner plastic case too. I posted here just after you about that. After doing some more reading, I think I need to go to the Apple store immediately and see what they can do for me. Pretty sure I bought the phone from a different carrier than I'm using now, so not sure they'd be of any help.
I chatted with Apple about my iPhone issue just a moment ago, and asked about yours too. The tech said the folks at any Apple store can plug in any iPhone to a handheld device and do a battery check for free. Many independent Mac fix-it stores will do the same thing, and you can probably get the same help at Best Buy, and maybe even Office Depot, if there aren't any Apple stores close to you. If you don't have any of those big-box stores close to you either, try to make sure the indy fix-it shop is Apple certified. You might pay a little more, but in the end, it could be useful if things continue to go south with your iPhone.
As for my issue, I might be S.O.L. at this point. I was considering getting a new phone this year, but I wasn't expecting to have to do it now! 😮 I need to replace my 2011 laptop and I need a car now too. That old saying sure holds true for me... when it rains, it pours! Guess I'll have to sell some Apple stock to pay for new Apple tech. 😟