Q: IPhone 6 constantly shutting down

For several months now my iphone 6 continues to shutdown. It shuts completely off then shows the battery image like the battery is dead even though it has a 80-100% charge on it! I've taking it in the Apple Store twice and had the online apple engineers look at it and no one seems to have a clue what's happening. They all agree that my battery is perfectly fine. Phone is less than 2 yrs old!! I have spoken with a few others who are experiencing the same issue with no resolution. In order for my phone to not shut down I have to constantly keep it hooked to the charger. Is anyone else having this problem?