Battery loses power even when plugged in

These support pages should help with troubleshooting charging and battery issues:

Charge and monitor the iPad battery - Apple Support

If your iPad won't charge – Apple Support

Troubleshooting generally requires substitution of the Power Adapter and cable for another known-good item.

If you continue to experience difficulties, you might be best advised to first seek assistance directly from from Apple Support - and request a remote diagnostic test. You can contact the Support Team using the Get Support link at top-right 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, Power Adapter and cable(s):

Genius Bar Reservation and Apple Support Options - Apple

You can contact Apple Support and have them do a remote diagnostic of your battery. Use the Get Support link at the top of the page to contact Apple. Alternatively, if you have an Apple Store close by, you can make a Genius Bar appointment and have it diagnosed there and they can explain your options.