Surely if your iPad when purchased is defective from new, it will be replaced by the retailer?
You can establish eligibility and cost of a battery replacement by Apple, in your country/region, here:
iPad Repair & Service - Apple Support
If the retailer will replace your iPad, a to visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) would be advisable. The technicians will be able to test and assess your iPad - prior to committing to a battery replacement or other repair:
Genius Bar Reservation and Apple Support Options - Apple
For practical purposes, in lieu of servicing 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 battery replacement.
If you have an active AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.