Whether or not the battery retains less than 80 percent of its original capacity
https://www.theverge.com/2018/1/2/16...bar-diagnostic
Apple has started offering battery swaps for $29 (a month earlier than promised), after apologizing to customers for slowing older iPhones down as the batteries aged. However, the company did not clarify how it qualified batteries as eligible for the discounted replacement, as the Apple Genius Bar uses a diagnostic test to check whether a battery can retain 80 percent of its original capacity at 500 complete charge cycles. This led to some confusion over the weekend, with some consumers claiming that Apple denied them a battery swap since their devices passed the diagnostic test.
Apple has now confirmed to MacRumors that it would replace batteries on an iPhone 6 or later, regardless of whether or not a diagnostic test shows that the battery retains less than 80 percent of its original capacity. The clarification came after the company reportedly passed along an internal memo regarding the new replacement policy, adding that customers who paid $79 for a battery swap prior to the offer are eligible for a refund in price difference. Though Apple doesn’t consider battery condition in the replacement offer, it’s important to note that it might not replace your battery if it finds other damage to the phone, or if there are third-party components in your device.
