How to diagnostic, if PMIC or U2 IC or battery is defektive?


i need an advice or some theory

im a beginner with the repairs.

how can i firstly and quickly check, what of these is defective?

ipad or iphone is not charging (i use an electrometer) and i see …
some of the ipads or iphones are charging with about 1W constantly (normally its on 10-11W), but no change at the end and some of the devices are standing zero Watts.

which is of them could be typically for the u2 ic or the power managent.

how can i also check, if the battery is the defective…

thanks a lot for you answers