How to check how good the battery is on an iPhone
Now a days, we're relying more and more on our iPhones. Because of this, its battery has become an important part of the hardware.
Up until iOS 11, their wasn't a way to check the health of the battery to see if you needed to take it to an authorized Apple repair place to have it replaced.
Here's how to check it:
1. Go to SETTINGS > BATTERY.
2. Tap BATTERY HEATH (BETA).
If the section says that the battery is at "Maximum Capacity" and "Peak Performance Capability" then it is at peak performance. But as the device gets older, this information may change.
Note: If you don’t see the “Battery Health” section in iOS Settings, it means you either do not have iOS 11.3 or newer on the iPhone, or it’s not an iPhone. As mentioned before, iPad does not currently support the Battery Health feature.