Why does my iPad shut down at 50%-80% battery and show a low battery icon?

As described, your iPad battery may have a fault, or may be reaching the end of its usable life. If the battery has failed or is failing, the iPad may unexpectedly turn-off - or may not successfully boot to a running state even when connected to its Power Adapter.

Are you able to use the iPad while remaining connected to your Power Adapter? If yes, while monitoring the indicated battery state, disconnect the Power Adapter; if the indicated battery state suddenly falls, or the iPad shuts-off, this test demonstrates the iPad battery having reached the end of its usable life:

iPad Repair & Service - Apple Support

If you continue to experience difficulties, you will might seek assistance directly from Apple Support and request a remote battery test. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) where the technicians will be able to test and assess your iPad - and you can explore your available options:

Genius Bar Reservation and Apple Support Options - Apple

Two reasons:

• A diagnostic test performed by Apple is free.
• Discharging a Lithium battery to 0% is not recommended. This was common practice and recommended with the older NI-Cd batteries that had a Memory Effect, but not the batteries that are used nowadays in phones and mobile devices. You can still find this maintenance step for those batteries and even in some AI responses because it was so common, but for Lithium batteries this can be harmful. For a battery that is already damaged, this can lead to excessive heat, swelling, and in the worst case a fire. Not something you would expect to see a recommendation for here.

I still recommend taking it to Apple for a diagnostics to have your battery checked out. It has already given you a warning that it is not working correctly and ignoring that may lead to safety concerns.

The full text of my reply suggested that you visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) where the technicians will be able to test and assess your iPad. This provides opportunity to explore your available options based upon diagnosis - if a problem is identified.

Genius Bar Reservation and Apple Support Options - Apple

Sounds like your battery is failing. Schedule an appointment at Apple for diagnostics.

Genius Bar Reservation and Apple Support Options - Apple