My 6 is having this issue. Yesterday my phone went from 30% to 10%, to 1%, then promptly died. Plugged it into an outlet and immediately jumped back to 30%. When this happened with my iPhone 5, I just brought it in to Apple and they ran diagnostics on the battery to test that it was failing. That was under the replacement program though so hopefully they recognize that this is a problem on the 6 as well.