Apple offers $50 discount for iPhone battery replacement as a damage control measure after revelation late last year that Apple purposely slowed down iPhone with aging battery. Unfortunately this offer will end December 31, 2018.
Apple is offering iPhones that are out of warranty and not in AppleCare+ a price of $29 to replace the battery. Eligible iPhone models are iPhone 6, 6 Pus, 6s, 6s Plus, SE, 7, 7 Plus, 8, 8 Plus, and X. This deal only applies once per device. And the price will go back to $79 from next year onwards.
Even if you do not encounter issues with battery yet, it is advisable to take advantage of the deal especially if you have an iPhone with battery that are two years or older. You do not want to wait till year end when Apple Store is the busiest.