Battery management features on iOS have improved greatly over the past few years. iPhones manage their own charging to optimize battery life, sometimes delaying a full charge based on usage patterns. The iPhone may display a battery percentage while charging, however there's no audible alert when charging has completed. For anyone interested in knowing exactly when their device has finished charging, Apple offers the Shortcuts app.
Low Power Mode is a great way to save battery life when you know you aren't going to be able to charge for a while. It stops automatic downloads and background app activity like iCloud Photos, reduces some visual effects, stops email fetch, applies auto-lock faster and does a bunch of other things all aimed at reducing battery usage. iOS 14 allows you to customize your Low Power Mode (and charging mode) by adding more settings through the Shortcuts app.
Imagine a scenario where your iPhone battery is running low, and there's not a charger in sight. But in your back pocket is an extra battery. With a quick pull, the empty battery pops off the iPhone, and the fully charged backup snaps onto the device. Boot up iOS and you're back in business. Not long ago, most cellular phones came with fully removable batteries.
Hidden among the many changes and improvements to iOS 13 is a feature Apple calls Optimize Battery Charging. All rechargeable batteries have a limited lifespan, which means over time their capacity to hold power declines. Apple has added automatic charging optimization to the Battery Health feature of iOS. From the time your iPhone is updated to iOS 13, this feature is switched on by default.
Even with all Apple's battery health tools the battery on your older iPhone 7 or iPhone 8 could probably still use a little more juice. There's no such thing as too much battery life, and a battery case can provide up to 20 hours of extra browsing, video watching and texting.