Is my phone actually heavier when it’s 100% charged compared to 0%?
This is where you can actually use E=MC2 to figure out the answer. It’s not even difficult math, as such things go. In fact, it’s dead simple.
I’ll spare you the full math (though it just involves converting the Wh into joules, then figuring out the mass added by energy increase with E=MC2) It is just a simple identity, essentially. Just how the universe works.
So, charging an iPhone from 0% to 100% stores about 49,320 joules of energy (for a 13.7Wh battery). By mass–energy equivalence, that stored energy gives us an added mass of about 550 picograms. Or if you want to be exact that is around 0.000000000548759 grams.
These questions have pretty exact and easy answers but Reddit types never seem to realize that.