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Comment by silisili | Hacker News Reader

On the subject of AA alkaline batteries - one thing I learned a long time ago somehow I'll share that people seem amazed by is that you can approximate their charge by bouncing them on a hard surface. Drop an AA battery negative/flat side down an inch or two onto a hard surface like a counter or table. A new battery will just land and thud. A dead battery will bounce repeatedly(you'll hear it more than see it).

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I didn't know that, thanks for sharing! It seems to be a lot more civilised version of what I was doing for years - test by licking it... Can do it almost in one gesture: hold the battery between thumb and middle finger, lick index finger and touch it on one end of the battery, then lick the other. With some practice can get pretty good accuracy in 0.5-1.5V range. Bit barbaric though :)

I wonder what phenomenon is driving this. A spent battery weighs more?

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...

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