Apple recognized, through a support article, that the battery for iPhones 6s / 6s Plus it may not work properly in certain cases.
According to the company, the bug reported by numerous users manifests itself when users manually change the device time or switch from one time zone to another. In such cases, the battery percentage may no longer be updated.
To correct the problem, Apple recommends that users restart the iPhone and leave the date / time set automatically within General Settings. She claims to be investigating the bug and plans to fix it soon.
(via MacRumors)
