The iPhone 6s could suddenly turn off even if there is a significant amount of battery left in the phone. This might happen due to a lot of reason including liquid damage or fallen from height. Furthermore, there are a few common reasons behind iPhone 6s switching off and that could be a firmware restore issue, battery replacement requirement or other. Another common reason is that the software of the phone doesn’t connect to the battery properly and shows a wrong percentage left in the battery life.
However, firmware restore issue is the most common issue that we have experienced and that is related to the low level code on the iPhone 6s. This problem is technical and requires assistance of professionals and we recommend our clients to not reload the iPhone firmware by your own and let MyCelCare take care of it professionally.