So why this post?
Because Reddit and other sources, us included have been advising 5C owners to replace their battery with a 5S battery if they feel like their original battery is no longer performing up to standard and the phone is out of warranty. Click the link below for info on iPhone battery swapping on Reddit.
Why is this now a problem? We found out today that with iOS 8.1.2, Apple has made it so that you cannot interchange the batteries on these two phones. If you have a 5S battery in your 5C (or even the other way around for some reason), you may notice your phone "boot-loops" for an unknown reason. If so, and you know you had a battery swap - swap back and retry.
How do I know if I have the right battery?
Underneath the text, "Li-ion Polymer Battery" is an APN number. Below are the APN's for the two devices in question. If your 5C has a battery with a 5S APN#, you may experience problems on iOS 8.1.2
5S Battery APN# 616-0721, 616-0728, 616-0719, 616-0718, 616-0722, 616-0728, 616-0720
5C Battery APN# 616-0669, 616-0667, 661-8300