The iOS 11.3 beta 2 update, seeded to developers this morning, introduces a new "Battery Health" feature that's designed to provide iOS users with more information about their batteries.
Located in the Battery section of the Settings app, the new Battery Health feature tells you the maximum capacity of your battery and whether or not it is operating at peak capacity.
On the iPhone X, there is no promised toggle to turn off the throttling feature that causes iPhones with degraded batteries to run more slowly, so it's possible this feature is designed to only show up on devices that are impacted by throttling. We're installing the beta on older devices and will update if we see the power management toggle.
Apple promised it would introduce much more detailed battery health features in iOS 11.3, and Apple will likely continue to refine and update its Battery Health feature over the course of the beta testing period.
Related Roundup:iOS 11