The physical chemistry changes as a battery is spent. The internal chemicals are being converted from one molecule into another.

Its not surprising to me that the two different chemicals have different physical properties. (Elasticity, density, who knows? They're a fully different chemical once the electrons move around)

Almost all the mass of a molecule is in atom nuclei, and these masses do not change under any battery chemistry I've ever seen.

The only way to change mass appreciably is to add or remove atoms from the interior of the battery.

IIRC, new batteries contain a semi-liquid electrolyte, which damps the bounce. When discharged, it becomes much more rigid, so the battery bounces. Compare dropping a gallon jug with ~a cup of water in it vs a cup frozen into ice to see what i mean.

Previously discussed on HN: https://news.ycombinator.com/item?id=21693423

IIRC alkaline batteries produce some kind of gas while being discharged. When the battery is dead it is probably inflated like an unopened soda can, assuming it hasn't leaked.

Isn't that what the vents holes are for?

I'm not sure about alkaline cells, but NiMH and lithium cells have burst discs or some other type of pressure relief valve rather than just a hole. My assumption is that when alkaline cells leak they have over pressurized and blown the pressure relief valve.

Here's a good reference https://pubs.rsc.org/en/content/articlelanding/2015/ta/c5ta0...