Whose to say that the 11 will have longer battery life than the XS series?
Except for the outlier, the XR, Apple usually designs the battery size so the phone can run a set number of hours based on their desired criteria. For example, if their goalpost is 12 hours (based on some proprietary test) then they'll design the battery size to hit it.... but not more.
It is not without precedent for them to shrink the battery on future generations compared to the previous one if it can hit their goalpost. The iPhone 8 did exactly that.
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