https://www.theverge.com/2018/5/23/17384702/apple-battery-replacement-refund-credit-iphone

Apple said today it’ll refund customers who paid for out-of-warranty battery replacements on their iPhone 6 or later devices between January 1st, 2017 and December 28th, 2017. They’ll receive a $50 credit so long as the battery swap was done at an Apple-authorized service location. It’s not a total refund, but it brings the price down to the discounted one that Apple is currently offering for battery changes.

The credit will come as an electronic funds transfer or a credit on the credit card used to pay for the battery. Customers might have to provide a proof of purchase. Apple says it’ll reach out to eligible customers through email between May 23rd and July 27th with instructions on how to proceed.