Apple has faced backlash for "the misunderstanding" surrounding how it handles performance for iPhones with older batteries. As a result, Apple began offering discounted battery replacements for $29 (down from $79) to anyone with an iPhone 6 or later whose battery needs to be replaced. Battery monitoring features within iOS is another way Apple said it will appease its customers.

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Apple's performance management feature slows down older iPhones with deteriorating batteries. With iOS 11.3 beta, performance management only enables after an unexpected shutdown occurs on an iPhone with a diminishing battery. It applies to iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone SE, iPhone 7, and iPhone 7 Plus.

Apple explains:

iOS 11.3 improves this performance management feature by periodically assessing the level of performance management necessary to avoid unexpected shutdowns. If the battery health is able to support the observed peak power requirements, the amount of performance management will be lowered. If an unexpected shutdown occurs again, then performance management will increase. This assessment is ongoing, allowing more adaptive performance management.
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After a user has signed up for Apple's beta program, they can download iOS 11.3 public beta. Apple's new battery settings can be found in Settings > Battery > Battery Health (Beta).

For iPhone 6 and later, iOS 11.3 beta adds battery health and recommends if a battery needs to be replaced. It can be found in Settings > Battery > Battery Health (Beta).

If the battery of your iPhone has deteriorated, Apple's new battery health feature will warn you. A message will display:

Your battery's health is significantly degraded. An Apple Authorized Service Provider can replace the battery to restore full performance and capacity.

Apple provides service options to replace the battery. The iPhone's one-year warranty includes service coverage for a defective battery.

Apple will show the iPhone's battery capacity relative to when you bought it new.

Apple explains:

Batteries will start at 100 percent when first activated and will have lower capacity as the battery chemically ages which may result in fewer hours of usage between charges.

Apple says a normal battery will retain up to 80 percent of its original capacity at 500 complete charge cycles under normal conditions.

When your iPhone's battery is operating normal, it will display the message: "Your battery is currently supporting normal peak performance."

When performance management features are enabled on your iPhone, it will display: "This iPhone has experienced an unexpected shutdown because the battery was unable to deliver the necessary peak power. Performance management has been applied to help prevent this from happening again."

Apple gives iOS 11.3 users the ability to turn off performance management. If you disable it, Apple will show the message:

This iPhone has experienced an unexpected shutdown because the battery was unable to deliver the necessary peak power. You have manually disabled performance management protections.

If another unexpected shutdown occurs, performance management features will be re-enabled.

Apple says in a support document newer iPhones, including the iPhone 8, iPhone 8 Plus, and iPhone X may see less noticeable impacts of performance management. Apple wrote that newer models "use a more advanced hardware and software design that provides a more accurate estimation of both power needs and the battery's power capability to maximize overall system performance."