All reachable batteries have a finite life. The battery in your iPad mini2 may be approaching the end of its usable lifespan.
These support pages may be helpful:
If your iPad won't charge – Apple Support
Charge and monitor the iPad battery - Apple Support
If the iPad is still functional, you might consider making direct contact with Apple Support - and request a remote battery diagnostic test. You’ll find the Contact Support link at top-right of this page. You can also initiate a support case using the excellent Apple Support App. If not already installed on your iPad, it can be obtained from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044
Alternatively, the technicians at the Genius Bar of your local Apple Store will be able run full diagnostics - and provide appropriate advice and options on the basis of test results.