When you have iOS 11.3 or later installed on your iPhone, you can find the battery capacity under Settings > Battery > Battery Health. This feature is only available for iPhone 6 and later models.

Additionally, you'll see a secondary metric in your Battery Health screen: Peak Performance Capability. Due to the above-mentioned nuance, Peak Performance Capability might actually be more important than capacity.

When your battery performs optimally, you will see a simple message saying that your phone is operating at peak performance. When your battery is worn and potentially degraded, you will see a warning that your phone should be replaced.

Whenever you see the warning message, your phone will still work, but the time between charge cycles will be significantly shorter. The warning message only appears when your battery capacity is 79% or less.

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