Battery health

"Decreasing" is normal, as per this support article:--> iPhone Battery and Performance - Apple Support .

It depends on how much it's decreasing.

See this useful user tip:--> When to charge your iPhone or iPad - Apple Community

If you still think it is decreasing too much, since your iPhone is still covered by warranty (and for your peace of mind), I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

Well, 99% ??? Absolutely normal, bearing in mind that it is an approximate value AND it is related to when your battery was new (most likely 9-10 months ago).

Start worrying when its maximum capacity goes down below 82-80%.

Regards

Giulio