Certain models of the iPhone 6 and iPhone 6s had issues with the battery. Apple started a US$29 battery replacement program last year. And that program ends in December 2018.
Battery Replacement Program
Some Apple customers with iPhone 6 and iPhone 6s models were having battery issues. To address this, Apple reduced the price of out-of-warranty battery placements to US$29. That program ends in December, so if you haven’t replaced your battery yet, do so soon.
To see if your iPhone qualifies, visit Apple’s support page and enter your iPhone’s serial number. You can find the serial number in Settings > About. Then, set up an appointment at an Apple retail store or an Apple Authorized Service Provider.