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Olivia: If you’ve got a bunch of batteries in your cupboard, and you’re trying to figure out which ones still work... you might want to try dumping them all on the floor.

There’s this idea going around that dead batteries will bounce when you drop them, while brand-new ones won’t bounce. And that’s not too far from the truth – except it’s not exactly foolproof. It turns out that any household battery will be at least a little bit bouncy if it’s been used before. So if a battery bounces, it’s not brand new, but that doesn’t necessarily mean it’s dead.

It all has to do with the substances inside the battery. In a brand new battery, the outer layer is normally made of zinc. Inside this material, zinc atoms are able to slip and slide next to each other... so if you drop a brand new battery on the floor, it will just kind of fall with a thud. Essentially, the kinetic energy from the fall gets transferred into those individual atoms, causing them to shift around, and even get a bit hotter. But in bouncy objects, this energy is transferred to the bonds between molecules, which hold the energy for just a moment, then release it like a spring.

So, a brand new battery just has a layer of zinc atoms, not a network of molecules. But all of that changes when it’s connected to a circuit, and the zinc undergoes a reaction known as oxidation. In oxidation, two important things happen. One: electrons are released, which end up going around the circuit and creating electricity. And two: the zinc bonds with oxygen to create molecules of zinc oxide.

In the used-up part of the battery, the zinc oxide molecules form a network that’s almost like a trampoline where the kinetic energy can be momentarily stored, then released. In fact, zinc oxide is sometimes even used in golf balls to make them more bouncy! The more you use a battery, the more zinc oxide forms, and the higher the restitution – its ability to preserve kinetic energy.

So if you drop a used battery, it’ll bounce. But about halfway through a battery’s use-life, the drop-test isn’t as useful to figure out whether that battery in your junk drawer will actually work. That’s because the battery oxidizes the zinc that’s closer to the edge first, and eventually, there are zinc oxide molecules throughout the entire layer.

When you drop a battery, the kinetic energy is mostly transferred to the parts of the molecule network that are closest to the ground. So when this network is big enough throughout the zinc layer, the battery has hit a maximum bounciness, and more oxidation doesn’t really have an effect. As a result, it’s pretty tricky to tell the charge of a battery just by dropping it. So as always, don’t believe everything the Internet tells you.

Thanks to Patreon patron Diana Gibbs for asking, and thanks to all of our patrons who keep these answers coming. If you’d like to submit a question to be answered, or get some videos a few days early, go to Patreon.com/SciShow, and don’t forget to go to YouTube.com/SciShow and subscribe!