Hello raisa183,
We see you have questions regarding your iPhone's Battery Health, and we can help out with that.
We noted you mentioned it has dropped 20%. If the Battery Health (Settings > Battery > Battery Health) now shows as 80% or lower, then this would indicate that the battery needs servicing/replacement. A normal battery is designed to retain up to 80% of its original capacity.
There is a good deal of information on the topic here: iPhone Battery and Performance
You may also find tips on how to maximize battery life, as well as the lifespan of the battery, on this page here:
Maximizing Battery Life and Lifespan
To help explore servicing options in your region, you can reach out directly to Apple Support here: Get Support
Take care.