yesterday I realized that when I charge my Iphone 11 pro, my battery did not increase from 80%. I didnt worry too much. This morning, I connected the phone to my charger and its reach 80% and didn't complete

If you fast charge an iPhone it will “pause” at 80% to cool off. It will resume later and complete the charge to 100%, but much slower to avoid overcharging. The time for the last 20% can be an hour or two.

There’s also another possibility: Go to Settings/Battery/Battery Health. If Optimized Battery Charging is on it will also stop at 80%. This switch should only be on if you routinely charge overnight (and you should).

The absolute best way to improve the life of your battery long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the night the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern.

Overnight charging also has the advantage of automatic nightly backups. Go to Settings/[your name]/iCloud and turn on iCloud Backups to enable this feature, then when the phone is plugged in, locked and connected to Wi-Fi it will charge overnight. You will then have a fresh backup every morning.