A Simple Way To Separate Fresh Batteries From Used Ones

A Simple Way To Separate Fresh Batteries From Used Ones
Image: Shoham Bhadra/Daniel Steingart/Princeton University

Ever seen a AA or AAA alkaline battery lying around and not been sure whether it was used or not? Don’t worry, there’s a simple trick you can use to find out.

The trick, as shown by Science Purely on Instagram, involves bouncing the battery on its end. It’s also known as the prison drop test, and it’s a quick method to tell fresh batteries apart from used ones.

Researchers at Princeton University investigated the phenomenon and found that the bounce is related to the way that batteries produce power. As a battery discharges power, a layer of zinc forms around a brass core in the battery.

“The zinc oxide begins to form on the outside and it pushes its way to the core,” Daniel Steingart, an assistant professor of mechanical and aerospace engineering, explained. “As you get more and more zinc oxide, and the zinc oxide begins to appear everywhere in the zinc layer, the battery gets bouncier and bouncier.”

And while the researchers found that you couldn’t accurately determine the charge of a battery with the test – a battery with half its charge remaining bounces just as much as a dead one – the test below shows that it’s a pretty effective method for telling fresh batteries apart.