Not long ago, my less-than-one-year-old iPhone 6 suddenly started to die within several hours after being fully charged. This exact 10-month old iPhone 6 of mine, was actually a kindly-offered replacement from an Apple Genius Bar when the support team wasn't able to relieve the battery problem in my previous less-than-one-year-old iPhone 6.
Concerned about this reoccurring battery breakdown, I headed over to the Apple Store located in Shadyside, Pittsburgh to seek help (and potentially get a new iPhone, but what if it breaks down again, well).
The first technician ran a routine check on my battery hardware -- well, no problems found. He than just kindly advised that I restore my phone. (I went home, did it, battery still kept dying within a few hours.)
I went back hoping to get one more chance to save my young iPhone. A second technician, Gabriel, came to rescue. He told me how the battery would only recover if I restore my phone CONNECTED TO iTUNES instead of just restoring it on my phone like I did before. He said that this is the only way that the battery can be fixed through the problem in its firmware.
He helped me connect to an Air and restored it for me at the Genius Bar. Afterwards, my phone has been performing well in terms of battery.