I went to a Samsung service store today with my S9 phone to ask about replacing my battery.
The people there plugged the phone into a USB port (I don't know where it was connected) and after a minute or so the program in their PC finished reading my battery capacity. They told me it was 70% (after 3 years.)
Now I don't think my phone has any built-in spectrometer for the battery (although with all the €€ it cost it should.)
From reading through the literature like here, I've seen the typical way to do this is via cycling the battery fully and Coulomb counting.
How did these people diagnose the battery so quickly?