only charges to 40%

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If your iPad won't charge – Apple Support

Charge and monitor the iPad battery - Apple Support

As described your battery may require replacement. Assuming that the iPad remains functional, you would be well advised to directly contact Apple Support - and request a remote battery test. The result of the test may better inform your next steps.

You will find the Get Support link at top-right of this page. Alternatively, you can open a support case 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

As a further alternative, you might visit the Genius Bar at your local Apple Store - where the technicians will be able to test and assess you iPad.

You most likely cannot ‘fix’ it. You probably need a new battery.

iPad Service and Repair