I have had this problem for a while now, my iPhone 5 has been dying with at least 20% battery, sometimes even nearer 30% battery remaining and I had learnt to deal with it. I got a replacement iPhone 5 in December 2012 after the lock button broke and it has only been happening since I got that one. Like I said, I had learnt to deal with it until now. It has recently been dying at 40-50% charge and wont turn back on for another half an hour to an hour. I decided to research and found out it happens to a lot of people. I have re-calibrated the battery and that didn't work. Just wondering what I can do? Or what you've done if its happened to you. I live in New Zealand and we have no Apple Retail Stores, so that option is out of the picture.