Though Apple is offering $29 battery replacements to any customer with an iPhone 6 or newer regardless of diagnostic test results, this policy can only be taken advantage of once, according to new fine print on its iPhone service pricing page — as noted by MacRumors.
After you’ve had your iPhone’s battery replaced once this year, the device must explicitly fail the diagnostic test to qualify for any additional battery replacements for $29. If the test passes, a customer can still choose to have the battery replaced, but Apple’s standard $79 fee applies.