Battery

Actually? Nothing. This would not be considered unusual. Apple does not consider a battery requires replacement unless it cannot hold, at least, an 80% charge after 500 full recharging cycles.

Ref:

• iPhone Battery and Performance - Apple Support
• iPhone Battery & Power Repair - Apple Support

Stop checking your battery health until you've owned the phone for 11 months. If at that time, battery health is at 80% or less, then Apple will replace the battery under warranty. The chances of this are nill to none however. As already mentioned, batteries are consumable parts and every time you charge your battery it's health will diminish just a bit. ALL batteries degrade over time.

Only if the battery drops below 80%.

Batteries are a consumable part of an iPhone and replacing them is at the owners expense.