Awhile back, there were news stories about how Apple’s newer Operating System was causing older iPhones to slow down and the batteries to overheat. As a result of all the bruh-ha-ha, Apple made the following offer:
Make an appointment at your local Apple Store, bring in your old phone (iPhone SE, 6, 6 Plus, 6s, 6s Plus, 7, 7 Plus, 8, 8 Plus, and X), and they will replace the out-of-warranty battery with a new one for only $29 (normally $99).
The deal ends on December 31, 2018. The price will go up to $49, which is better than $99.
Instead of sitting around for hours at the Apple Store, start the process to get your battery replaced.
You can read additional information here.