Battery
Can I get Battery Replacement Option on my Iphone6s?
iPhone 6, iOS 12
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Top-ranking reply
Yes.
If you live near an Apple Store, make a Genius Bar appointment to have the device/computer tested. Supposedly there is no charge for testing. Use 2nd link if not near an Apple Store or aren’t in the US. Take the original purchase receipt with you.
Genius Bar Reservation - US.