One of the most power-hungry components on an iPhone is the screen. Battery life on the iPhone is naturally extended by turning off the screen whenever possible. This is why the device is normally set to auto-lock after a minute or two. Many iPhone owners don't bother to manually lock, so this feature keeps our devices secure as well.
iPhone batteries have increased in size and capacity over the years. While the original iPhone had a 1400 mAh battery, the iPhone 13 Pro Max power pack is over 310% larger. Processors and displays are engineered to be more efficient, however the demands placed on the latest mobile devices have also skyrocketed.
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.