Wow! I was asked by a friend if dead/flat batteries were weighing less, since I was working a little bit with batteries. Back then I dismissed it as utter nonsense. But I still googled the question because you never know right? and I stumbled upon either this article or a similar one: https://www.quora.com/Is-a-charged-battery-heavier-than-a-de...

After finding and reading an answer that agreed with my initial assessment I never continued to investigate why my friend would make such a claim, but now I guess it was related to this phenomenon. Guess I owe somebody an apology. Crazy how somethings seem so outlandish that I did not even ask myself if this was possible or what fundamental differences there are between batteries of different states of charge. Also it would have been quite easy to test this in retrospective... Note to myself: I should be more careful and curious in the future!