Is iPad battery under Apple warranty coverage?

All rechargeable batteries are consumable items that have a limited lifespan. An iPad battery is rated to maintain at least 80% of its initial capacity after 1000 full charge/discharge cycles.

Should the iPad battery fail, or have less than 80% of its original capacity within its one year limited warranty, Apple generally service the battery within the warranty period. If you have benefit of an active AppleCare+ plan, a failed or failing battery will be covered for replacement.

The cost of an out of warranty battery replacement can be found here:

iPad Repair & Service - Apple Support

If your iPad requires service, you would be best advised to 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

For practical purposes, in lieu of replacing the battery and returning your own iPad, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for the battery replacement.

This would be something that you're going to have to get in touch with Apple about. It may be covered under certain situations but there is no way that we as end users like yourself can tell you what Apple will or will not do. You can always contact Apple Support and explain to them what you have going on and they will be able to answer this for you. It's really going to depend on things and if they determine the battery is indeed at the end of its life and if the iPad is still under warranty.