Apple released an advisory for certain i6S handsets with battery issues. Apparently, some iPhone customers are concerned with their handsets shutting off for no good reason. It is a battery issue, but Apple states that is not a man-safety issue.
Apple has determined that a very small number of iPhone 6s devices may unexpectedly shut down. This is not a safety issue and only affects devices within a limited serial number range that were manufactured between September and October 2015.
If you have experienced this issue, please visit an Apple Retail Store or an Apple Authorized Service Provider and have your device’s serial number checked to confirm eligibility for a battery replacement, free of charge.
The replacement process kick-off is simple enough. Contact one of three options on the i6S battery replacement support page here.