Is my iPhone battery replacement under warranty?

Take it to Apple and they will determine that.

I presume this is for an iPhone. You do not say.

Are you sure you need a new battery?

Referring to the "Your battery's maximum capacity" section of the support article: iPhone Battery and Performance - Apple Support

"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions." Even rechargeable batteries get worn down and 1-2% health loss per month is perfectly normal. According to iPhone Battery and Performance - Apple Support , Apple apparently only considers it unusual (and warrant-able) if a battery drops below 80% maximum capacity in less than a year after you receive it. Ergo, some storage loss as you use it is anticipated and acceptable. Prorating that possible 20% drop over the first 12 months equates to 1-2% drop per month being within the range of what Apple finds acceptable. "The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge." In other words, if maximum capacity is below 80%, the battery is below normal performance and worth replacing. You can still use a device with a lower health percentage than 80% but for optimal usage you may wish to consider having it changed when it reaches 80% battery health.

- If battery health drops a few percentage per month that is normal.

- If battery health drops below 80% in the first year, contact Apple about a warranty replacement.

- If battery health drops below 80% after the first year, consider getting the battery replaced but anticipate paying for it.

Go to this web site for information about how to request battery service for your iPhone: iPhone Battery Repair & Replacement - Apple Support

For cost and availability, enter your model information in the "Get an estimate" box at the bottom. (I will let you do this since the answer will depend upon the country where you are located.)

For instructions on how to get your iPhone, iPad, or iPod touch ready for service click here --> Get your iPhone, iPad, or iPod touch ready for service - Apple Support

If the battery is defective and has run down to less than 80% Battery Health / Maximum Capacity during the 1 year AppleCare warranty period, Apple or an authorized Apple Service shop will replace the battery at no charge.