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Why do dead alkaline batteries bounce?

Andrea on September 13, 2014 - 4:37 pm in Gadgets
Batteries
You might have heard that the difference between a dead and good alkaline battery can be determined by seeing which one bounces when dropped. A battery with usable capacity doesn’t bounce, but a dead once bounces quite a bit when dropped. If you are curious why that is, a genial older gentleman has a YouTube video that explains everything.

The discussion that spawned the video below resulted in two theories for why a dead battery is so much more bouncy. One notion is that hydrogen out-gassing from the electrolyte increases the internal pressure of the battery resulting in an increase in spring constant–similar to the measurements of elasticity governed by Hooke’s Law. You can think of this as the battery getting more “springy.” The other possibility is that hardening of the gel-like electrolyte reduces the anti-bounce effect. Being scientifically minded gents, they decided to find out.

To test the out-gassing hypothesis, a brass weight was dropped on top of the batteries to see if there was a difference in the bounce height of the weight. As you can see, there is no appreciable difference. Even drilling a hole in the batteries to make sure there was no pressure buildup resulted in no difference.

So if it’s not out-gassing, is it the anti-bounce effect? A solid object will bounce when it strikes a surface, but the anti-bounce effect can be harnessed to dampen that. The video uses an anti-bounce hammer to demonstrate the effect. The hammer is filled with buckshot that lags behind a bit as you swing it, but catches up just in time to keep the hammer from bouncing up after hitting a surface.

The electrolyte in an alkaline battery is a gel composed of potassium hydroxide and zinc powder–it’s like the buckshot in the hammer. As the battery becomes depleted, this gel-like material becomes more dense and doesn’t shift around, thus it can’t dampen the bounce when you drop the battery. Cutting a few batteries in half confirms there is a clear difference between a good and dead battery. Note: don’t cut batteries in half at home.

Source: Science – Geek.com

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