The iPhone’s lithium-ion battery tends to have a longer lifespan when it isn’t fully charged. Apple’s optimized battery charging feature—releasing with iOS 13—keeps the battery below 80 percent until a full charge is needed, reducing unnecessary battery aging. Here’s how to enable it.
To begin, open the “Settings” app on your iPhone. If you can’t find the icon, swipe down on your home screen and use Apple’s Spotlight search to find the app.
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Scroll down and choose the “Battery” option.
Select “Battery health.”
Tap on the toggle next to “Optimized battery charging.”
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Source : How to Enable Optimized Battery Charging on Your iPhone