Apple has confirmed suspicions that they are throttling older iPhones with older batteries. CNET reports that Apple has admitted that they have been purposefully throttling iPhones with older batteries since iOS 10.2.1. According to Apple, the processors running at full strength were drawing power from the older batteries too quickly causing unexpected shutdowns. By throttling the processors, performance takes a hit, but the phone is more stable and usable for a longer period. This feature will continue in future iPhones and versions of iOS.