[UVB-76]

iPhone 6 here. Have experienced exactly this issue since upgrading to iOS 10 (and 10.1)

The battery will drain extremely fast (typically lasts about half a day, with light usage), and will randomly and suddenly die when reporting 20, 30, or even 50 percent remaining.

The unpredictability of when the phone will completely die makes this problem particularly frustrating.

Connecting a charger cable results in it immediately reporting 20/30/50 percent battery again.

Have already had a Genius bar appointment to resolve. The 'Geniuses' ran the usual battery diagnostics, and said the battery was close to their 80% threshold, but were adamant a full restore would resolve the issue. They were rude and dismissive, and point-blank refused to replace the battery, or handset.

I did a software restore, and lo and behold, still have the issue.