Why my phone no charge up to 80%

Baba, as lithium ion batteries age they lose the ability to hold the amount of charge they were originally capable of. That 80% means the battery can only hold 80% as much charge as it did when it was new and that percentage will continue to drop as the battery ages.

The only thing you can do about this is to replace the battery with a new one, which will restore the phone's ability to charge the battery to 100%. If you want to do the repair yourself, iFixit has a guide giving you step by step directions on the repair.

iPhone 11 Pro Battery Replacement - iFixit Repair Guide

Note that Apple has continued its policy of making repairs difficult or impossible for independent shops and DIY-ers like us. With respect to your battery, if you replace it you will get a pop-up warning to the effect that the battery may not be genuine - whether it is or not. After a couple of weeks that warning will go away, but you'll be stuck with a permanent badge on your Settings icon and a warning to that effect under the Battery settings.

Also of note is that you will lose the ability to monitor your battery's health information, making it more difficult to tell when the battery is in need of replacement.

There are ways around those issues, but they're a lot more complicated than just swapping out the battery like you used to be able to do. To eliminate the genuine battery warning message you have to cut the BMS, or Battery Management System, off of the old battery cell and spot weld or solder it onto a replacement cell.

That will get you a brand new battery without the bogus warning, but unfortunately will leave your battery health reading the same as it was before you put the new cell in. In order to reset the health info, you have to use something called a tag-on flex cable that plugs in between the battery and the motherboard. A device programmer like the QianLi iCopy has to be used to reset the health information using the tag-on flex connector, then the battery and the tag-on flex are both permanently installed in the phone.

All that trouble when you used to be able to just swap in a new battery. Thanks, Apple, for nothing.