[sergj]

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!

[lightedman]

Energy equals mass times the speed of light squared.

That alone tells me a dead battery of one composition will weigh some fraction less than a fully-charged one of the same composition, assuming all other things are truly equal (number of atoms in the containing can, etc.) The fractional difference might be very, very small (billionths of a gram difference?) but should still be present.

[dcminter]

A colleague at work tested two batteries by bouncing them in front of me. Had he merely told me about the bounce effect I would have been extremely skeptical, so I'm glad he didn't give me the opportunity to get egg on my face!

I found and submitted this article when looking for the physical basis of the effect afterwards.

I guess I was one of that day's lucky 10,000! https://xkcd.com/1053/