In January, 2016, I put my two-month-old 6S into the Apple battery case, and it's been there to this day. One of the benefits of the case is that it uses the ca

In January, 2016, I put my two-month-old 6S into the Apple battery case, and it's been there to this day. One of the benefits of the case is that it uses the case's external battery first. During a typical day, I never use the internal battery (when traveling I end up using both). I don't have any empirical evidence to support it, but I've noticed no degradation in performance of the phone. Given this news I'd like to attribute that to the minimal use of the internal battery.