Let me give you some free advice...if your taking your phone in to an Apple Store don't bring your phone in with a full charged battery. If it's shutting down at say 30% bring it in at 35 or 40%.
Apple will hook up your phone to their diagnostic tool and if your phone has got a full charge it's going to throw a false code that there is no issue. I know because I had this same thing on my iPhone 5, it would shut down at 20 or 25% and I ended up having to pay 89.00 for a new battery then Apple issued the recall on the iPhone 5 and I got a refund check in the mail.
Apple will hook up your phone to their diagnostic tool and if your phone has got a full charge it's going to throw a false code that there is no issue. I know because I had this same thing on my iPhone 5, it would shut down at 20 or 25% and I ended up having to pay 89.00 for a new battery then Apple issued the recall on the iPhone 5 and I got a refund check in the mail.