Apple are replacing certain iPhone 6s batteries due to them been faulty and just shutting down,
You can click the link at the bottom of this page to go to the Apple page and insert your serial number to see if your device is eligible.
The iPhone 6S battery cannot be replaced through your wireless carrier. And Apple said that the iPhone 6S battery replacement program does not extend the standard warranty coverage. It’s also worth mentioning that the battery replacement program covers affected iPhone 6S batteries for 3 years after the first retail sale of the unit.
If your iPhone 6S has a cracked screen that impairs the replacement of the battery, then that issue will have to be resolved before the battery can be replaced. And there might a cost associated with the repair. Apple is offering the iPhone 6S battery replacement program worldwide, but the repairs are restricted to the country or region where the device was purchased.