Always Keep Your iPhone’s Battery Between 20% and 80%
One of the golden rules to keep your battery as healthy as possible is to never go to extremes. That means you don’t let your battery go below the 20% mark, and you don’t charge your iPhone above the 80% mark — and you should especially try to avoid leaving it at either extreme for extended periods of time.
Granted, thanks to Optimized Battery Charging, you don’t necessarily need to avoid the 80% mark, but if you really want to be careful, you can and probably should avoid it. 80% should be more than enough to last you through the day, especially if you use Low Power Mode.