Apple has released an update through its public beta program that enables users to check the health of an iPhone's ($1,199.99 at Amazon.com) battery. Currently, the only way to install iOS 11.3 is to sign up for the beta program.
Eventually, this same feature will be released as an official update, once the beta period has ended sometime this spring.
If you want to sign up and test Apple's latest software update, keep in mind because it is a beta it's a little rough around the edges. You may experience issues with battery life or apps randomly crashing -- that's just part of helping Apple test. Follow the instructionsif you're OK with that.
With iOS 11.3 installed on your iPhone, there's a new section on the Battery Settings page that details your battery's health. View it by going to Settings > Battery > Battery Health (Beta). There you will see your battery's current health, as well as a description of how its health impacts performance.
If your iPhone's performance is being throttled, you will see a statement similar to the one above, where you can disable the feature if you want. However, if your iPhone does randomly shut down due to battery health and performance issues, this feature will automatically enable itself once again to prevent future shutdowns.
Also worth pointing out is that if iOS is unable to determine the health level of your battery, or if the level is low enough, you will be directed to take your iPhone to an Apple Store or an authorized service center to have it tested and possibly replaced.
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