For a full test of your battery health, and for your peace of mind, I suggest that you contact Apple Support directly (they have tools to run remote diagnostics, battery included) or, even better, 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.