“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.
Replacement Process –
Choose one of the service options below to have your battery replaced. Your iPhone will be examined prior to any service to verify that it is eligible for this program and in working order.”
This is interesting to note as my wife’s iPhone 6S has this exact problem. The phone just turns off, even though when plugged into the mains, it turns on at around 20-30% charge. Seems she will need a replacement battery now.