If you want your iPhone’s battery to hold its charge over time, you’ll enable this feature.

If you have several-years-old iPhone, or know someone who does, you’re probably aware of the fact that the battery life is a lot shorter than when it was new.

The rechargeable batteries in today’s electronics degrade over time and become less capable of holding a charge or delivering the same peak power.
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https://www.macworld.com/article/3408336/ios-13s-new-optimized-battery-charging-feature-explained.html