I tested, I put the phone to play videos until it turned off. In 1% I cleared battery stats.
Then plugged the charger and entered the TWRP and unplugged. Put the backlight to maximum, and the phone stayed on more than an hour!
Then I plugged the charger 1.5A and let it charge to max.
It worked, now the phone has a steady discharge, not discharge 100% to 80 in an hour.
I will do this procedure from time to time. not the best for the battery, but is needed in mi9...
It's an absurd that this problem exists in 2020, my galaxy S2 don't have such harsh problems with something so simple and basic like battery management!
I have Just reset the battwry stats a d ir looks likebgixed thebpriblem with battery capacity. After the procedure the charge counter got to 2998 as in the picture
I'll be commenting in SOT as soon as I reach zero
Thank you very much