Here’s Apple’s explanation of what you can expect if you enable the setting:
When you choose the 80% limit, your iPhone will charge up to about 80 percent and then stop charging. If the battery charge level drops to 75 percent, charging will resume until the battery charge level reaches 80 percent again.
While testing the settings on our review devices, we noticed that our phones have been charging over 80 percent anyway. Apparently, this is something you can expect to happen occasionally, according to Apple.
With the 80% limit enabled, your iPhone will occasionally charge to 100 percent to maintain accurate estimates of the battery’s state of charge.
This new option joins the existing optimized battery charging setup that has been on Apple devices for a few years. That setting will keep the battery’s charge at 80 percent before taking it to the maximum before you think you’ll need to use the phone, depending on your usage patterns. Apple also just released iOS 17.0.2 for the iPhone 15 series and we’re installing it now to see if charging actually starts to hover around 80 percent with the new setting enabled.