Apple has released iOS 11.3 beta 2 and it has a new Battery Health feature. Available by going to Settings —> Battery —> Battery Health, the section provides a look at the current maximum capacity of your battery relative to when it was new. It also will let you know if the battery is currently supporting peak performance capability.
Apple also links to a support document providing more information about the iPhone battery and performance.
Anyone with an iPhone 6, iPhone 6 Plus, iPhone SE, iPhone 6s, iPhone 6s Plus, iPhone 7 and iPhone 7 Plus can disable the power management option designed to prevent unexpected shutdowns as a battery ages.
The final version of iOS 11.3 will arrive this spring. The Battery Health feature is listed as being in beta as well and may change before the software is officially released.