The Intriguing Science Behind Bouncing Dead Batteries.

Everyone can relate to the frustration of having a drawer full of batteries that are in an unknown state of charge. For most people the only method they have to deduce whether they’re good or not is to try them out in a device, something that inevitably leads to frustration when your spares show up dead as well. The inclusion of battery testers on the batteries themselves (or in the packaging) seemed like a great idea however it never seemed to catch on presumably due to cost factors. Whilst geeks like myself might have a voltmeter handy to get accurate readings in an instant they’re not a ubiquitous device and an effective way of testing batteries still eludes most.

That is until they see this video:

Honestly when I saw this video I was pretty sceptical as the video, whilst highly informative, is anything but scientific. Instead of having 2 batteries from the same brand (and preferably from the same batch) for comparison the effect could be explained by differences in manufacturing between the two. I didn’t take the opportunity to test it myself however, even though I do have a drawer full of batteries that are all in unknown states, but after seeing this video parroted around various life hacking sites I figured that if it was total bunk someone would’ve called shenanigans. It seems that the video is accurate and the science behind why empty batteries bounce is very interesting.

It’s not, as many have speculated, related to a reduction in weight between a full battery and a discharged one. Batteries like this are a closed system, chemically speaking, so save for a few milligrams here and there due to handling or (more catastrophically) a breach in them batteries don’t change their weight. Instead it’s a quirk of the manufacturing process and the change in densities of the various materials inside the battery, all of which result in it becoming bouncy.

In a typical alkaline battery the chemical reaction that takes place to produce charge also results in the materials shrinking. The reason for this is that as the battery discharges oxygen molecules from the cathode (negative ) manganese oxide terminal migrate to the anode (positive) zinc anode, producing zinc oxide. When this occurs the total volume decreases as the oxygen atoms are able to pack themselves much tighter on the zinc oxide terminal than they can on the manganese oxide. This results in the internals shrinking somewhat and, as a consequence, tugs on the side of the pressure seal on the bottom of the battery. This causes it to bow outwards providing a spring like structure which results in the bounce when dropped.

Now I haven’t looked at a lot of batteries recently but I can image that some other designs might make this trick fail due to the design of the cathode terminal. This also means that the trick is probably unique to the cylinder style batteries (A, C, D, etc.) as whilst other types of batteries have similar chemical reactions their construction is vastly different. So I wouldn’t recommend dropping your car or latern batteries to try and test them out, lest you want to spend some time in the chemical burn ward and paying for a large chemical spill.