For iPhone 6 and later, iOS 11.3 and later add new features to show battery health and recommend if a battery needs to be replaced. These can be found in Settings > Battery > Battery Health.
Additionally, users can see if the performance management feature that dynamically manages maximum performance to prevent unexpected shutdowns is on and can choose to turn it off. This feature is enabled only after an unexpected shutdown first occurs on a device with a battery that has diminished ability to deliver maximum instantaneous power. This feature applies to iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone SE, iPhone 7, and iPhone 7 Plus. Starting with iOS 12.1, iPhone 8, iPhone 8 Plus, and iPhone X include this feature; iPhone XS, iPhone XS Max, and iPhone XR include this feature starting with iOS 13.1. The effects of performance management on these newer models may be less noticeable due to their more advanced hardware and software design.
(Note: Devices updating from iOS 11.2.6 or earlier will initially have performance management disabled; it will be reenabled if the device subsequently experiences an unexpected shutdown.)
All iPhone models include fundamental performance management to ensure that the battery and overall system operates as designed and internal components are protected. This includes behavior in hot or cold temperatures, as well as internal voltage management. This type of performance management is required for safety and expected function, and cannot be turned off.
Your battery's maximum capacity
The Battery Health screen includes information on maximum battery capacity and peak performance capability.
Maximum battery capacity measures the device battery capacity relative to when it was new. A battery will have lower capacity as the battery chemically ages which may result in fewer hours of usage between charges. Depending upon the length of time between when the iPhone was made and when it is activated, your battery capacity may show as slightly less than 100%.
A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions.