Self proclaimed iPhone battery expert, I've had every battery issue in the book. The quickest thing you can do is restore, preferably start fresh, but one time I was having battery issues and I restored from an iCloud backup and my battery life still increased by 100%
If the restoration doesn't work out and you're using a legitamite charger, I might suggest battery diagnostic software. I had the pleasure of seeing just how messed up my iPhone 5 battery was thanks to ibackup bot which is a free OS X app that I downloaded online not off the App Store (not sure if it's on App Store.) It is questionable looking but I have been using it for over a year and had no issues so I think it's legit. Anyway it has an information section that provides battery health information (I am including a picture of what it will tell you.)
If you want to spend 4 dollars for convenience, download the iOS battery health app in the App Store. It will show you cycle count, max charge capacity, current system reported battery charge and actual battery charge (actual and system can vary A LOT) and even detect whether or not a charger is Apple certified.
Don't hesitate to ask me if you have any questions because I know way more about this than I ever would have liked to learn.