Potentially one of the best features to hit the iPhone in the past few years was Optimized Battery Charging.
First introduced a year ago with iOS 13, Optimized Battery Charging was designed to reduce battery wear, and therefore increase its lifespan, by limiting how long your iPhone remained fully charged by pausing the charging process at 80 percent, and then using on-device machine learning to learn your daily charging routine to determine when to add the past 20 percent of charge so your iPhone is ready for you when you wake up.
Apple then went on to add the same to macOS Catalina 10.15.5.
And based on testing I've carried out, this feature does indeed reduce battery wear, and the machine learning is quick to pick up on your habits and make this feature something that works in the background and which doesn't inconvenience you when your schedules change.
And watchOS 7, released last month, has now brought this battery saving feature to the Apple Watch.
You can check that you have it and that it is switched on by going to following on your Apple Watch:
Settings > Battery > Battery Health
Optimized charging is designed to only turn on when you are charging your Apple Watch in places where you spend the majority of your time -- usually your home and office, for example. The feature is smart enough to turn itself off when you shift away from those regular habits, for example when you are traveling.
To allow Optimized Battery Charging to work properly the following have to be enabled on your Apple Watch (by default, they will be on):
Settings > Privacy > Location Services > Location Services
Settings > Privacy > Location Services > System Services > System Customization
Settings > Privacy > Location Services > System Services > Significant Locations > Significant Locations
Note that all this data is stored and processed on your devices, and none is sent to Apple.
If you feel moved to disable Optimized Battery Charging on your Apple Watch, go to Settings > Battery > Battery Health and then tap the toggle next to Optimized Battery Charging. You can then choose to Turn Off Until Tomorrow or Turn Off.
OK, but how does any of this help you tell if your Apple Watch battery is worn? Go back to Settings > Battery > Battery Health. Notice there you get a MAXIMUM CAPACITY measurement. If that is over 80 percent, you should be good. Below that, then your battery is getting suspect and you're probably noticing that your Apple Watch no longer holds its charge like it once did.
You can do this without taking it off your wrist!