With the outcry over Apple throttling performance due to bad battery life Apple has introduced a new Battery Health feature that will be available in a new iOS 11.3 update. iOS 11.3 will be available to download on all devices that qualify for iOS 11.
When the update becomes and it’s installed on your phone, go to Settings>Battery>Battery Health. This menu will provide a percentage of the maximum capacity of the battery. Over time as you use your iOS device and recharge your battery the health will decrease.
After your first unexpected shutdown occurs your performance will start being throttled Apple calls this feature performance management. To disable performance management tap “Disable…” in the “Peak Performance Capability” description. If you choose to disable this feature you will have to re-disable it every time your iPhone “experiences and unexpected shutdown”.
To get this problem fixed for good Apple recommends that you contact Apple Support to schedule an appointment to get the battery replaced. The first time Apple replaces your battery the price will be $29, the next time you need to get your battery replaced the price from Apple will be $79. Apple also recommended that you get this procedure done at an official Apple Certified Repair shop. Getting the battery replaced by a 3rd-party could result in wrong battery capacity percentages or no percentage at all. So be safe and go to Apple.
To learn more about your iPhone and it’s battery read this article from Apple regarding “iPhone Battery and Performance” which also goes more in-depth into the new battery health features.