This is a Data Privacy respecting app without any trackers, ads or unneccessary permissions:https://reports.exodus-privacy.eu.org/en/reports/search/eu.roggstar.batterycalibrate/

How to use it:
1. Charge your phone to 100%
2. Click on the calibrate button
3. Disconnect your charger
4. Profit!

What does it do:
This app deletes the battery configuration file. And your new ROM will create a new one (a better one).

Note that:
Root is needed in order for this app to work.
There are many apps out there just pretending to do something...
There are many apps with annoying advertisements out there...
This app does really it's thing without any ads. Long live FOSS.

Disclaimer:
I'm not responsible for any damage that may or may not happen to your device.