Iphone Battery health

If it is a brand new phone from Apple, take it back (although officially the warranty kicks in at 80% inside 12 months)

If you bought the phone used or "seller refurb" then ask them if they fitted a genuine battery.

Hello,

Hmm, where did you buy the phone?

In general, Apple will replace the battery under warranty if the battery health falls below 80% durning the first year.

Start here:

1) Have you tried a restart?  If not, do it now.

2) Go to Settings > Battery > BATTERY USAGE BY APP to see the apps that are consuming your battery.

3) See this for some charging tips: When to charge your iPhone or iPad

Good luck.