My iPad Air2 loses battery life when turned off and left unplugged. Why?

Your iPad internal, rechargeable battery maybe failing.

You need to search online for your nearest, “local” Apple store location and make an appointment to have your Apple iPad diagnosed and options presented you.

Failing that,

You need to contact Apple support via the contact support link located at the upper right hand corner of every Webpage of these Apple support communities.

Use the Hardware Repair & Battery usage & Charging support categories.

OR

by installing and using the Apple Support app found in the iOS App Store.

Tap the iOS App Store link below.

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

OR

Call contact Apple via the support numbers found in the link bekow.

Contact - How to Contact Us - Apple

Tap the link below.

https://www.apple.com/contact/

Best of Luck to You!