AppleCare+ for iPhone, iPad, iPod, and Apple Watch has had its terms updated by Apple. The protection plan now covers battery replacements for devices that retain less than 80% of their original charge capacity. The plan used to cover battery replacements at 50% retention.
From Apple's updated terms:
If during the Plan Term, you submit a valid claim by notifying Apple that (i) a defect in materials and workmanship has arisen in the Covered Equipment, or (ii) the capacity of the Covered Equipment's battery to hold an electrical charge is less than eighty percent (80%) of its original specifications, Apple will either (a) repair the defect at no charge, using new parts or parts that are equivalent to new in performance and reliability, or (b) exchange the Covered Equipment, with a replacement product that is new or equivalent to new in performance and reliability.
AppleCare+ provides protection coverage for your various devices, though each one needs its own coverage. For the iPhone, iPad, and two Apple Watch models (Sport and stainless steel), you'll have your one-year hardware warranty extended to two years, and does the same for technical support. It costs $99 with a new iPhone or iPad, and $49 and $59 respectively for the Apple Watch Sport and stainless steel Apple Watch.