Apple added a new Battery Health feature to iOS 11.3, which was released yesterday. This gives you information about how good (or not) your iPhone’s battery is. This is in response to issues around iPhones being throttled if their batteries are old.
To access this setting, go to Settings > Battery > Battery Health. You’ll see the battery’s maximum capacity – the amount of power it can hold when fully charged – and its peak performance capability; this latter will be reduced if the battery is old.
This information shows up on my iPhone 8+, but not on my 10″ iPad Pro, or my iPad mini 4. My guess is that it only displays on those iPhones whose processors can be throttled if their battery is below nominal capacity. (iPhone 6 or later, and iPhone SE.) It would be useful, however, if it displayed on all iOS devices; I think users of old iPads might like to know what the maximum capacity of their batteries is, and potentially replace the battery when it gets low.