So this started happening a few months ago. I've got an 18 month old 6. Latest software. At 20% it will prompt for low power mode. Whether I switch it on or not the phone will die long before 1%. Usually at 13-17%. It's worse if the phone is under heavy use. I was out yesterday taking some photos, sending a few messages, on FB etc. I was in an unfamiliar area and needed Google maps as a backup to get me back to my car. It died at 34%. Funny thing is if you leave for 5 mins it will boot back up and get as far as the passcode screen... I've done all the usual troubleshooting. Full discharge several times. Hard reset to recalibrate. I can swap it out at the Apple Store as we get a 2 year guarantee in the UK but just wondering if anyone else has this problem. Was hoping 9.3 would fix it but starting to suspect it's hardware related. Either way it's pretty poor from Apple. My last phone (4s) had to be swapped out (again after about 18m) as the wifi stopped working.