If you are not using Apple USB power sources or USB power sources certified as Made for iPhone the power source can damage the circuit in the phone that manages battery charging. With older phones (pre-5S) sometimes the battery gauge could get out of calibration; it could be recalibrated by running the phone until it shut off, then charging for 4 hours with a known good USB power source. That is generally not necessary with the 5S and later, which has a more sophisticated battery management chip. But one that may be more likely to be damaged by off-brand power sources.
That's just for background; what you can do is call Apple, or have them call you (click on the Contact Support link at the top of this page) and they can test the phone remotely. Or make a genius bar appointment. You can also download the app Battery Life, which does a pretty good job of evaluating the status of the battery (Apple's test is better, however).
As to why the forum is here, it may not have occurred to you, but there are many different issues that people can have with an Apple product, not just battery problems. And many of those issues can be easily corrected with just an explanation of how to use a feature of the phone.