Article Details
- Title: Why do dead alkaline batteries bounce?
- Author: Ryan Whitwam
- Date: 09/07/2014
- Link: Geek.com
Summary
The article begins by discussing the 'lifehack' where one can bounce an alkaline battery to determine whether or not it still has charge. Two theories were brought up:
- Theory 1: The release of hydrogen gas from the electrolyte gel increases the internal pressure of the alkaline battery, leading to a positive change in the spring constant.
- Theory 2: Once the battery is drained, the electrolyte gel would harden, lessening the anti-bounce effect it would otherwise have.
The article then proceeds to outline the testing experience.
To test the first theory, a small weight was dropped on top of both the depleted and fresh batteries to see if there was a noticeable difference in the bounce height of said weight. As it turns out, there was no significant difference! The experiment even went so far as to drill holes in both ends of the batteries to ensure there was no pressure being built up.
As for the second theory, the aforementioned anti-bounce effect needs to be discussed; what is it? Alkaline batteries are made up of a gel (containing zinc powder and potassium hydroxide). Imagine a scenario where someone drops a fresh battery from a height; the electrolyte gel lags slightly behind the motion of the battery's outer shell, but catches up just in time to stop the battery from bouncing up after hitting a surface.
Since the first hypothesis was disproved, does that mean it's the anti-bounce effect stopping live batteries from bouncing? Yes, it does. As a battery depletes, the electrolyte gel thickens and becomes more dense, stopping it from shifting around, stopping the anti-bounce effect! To prove this fact, the experiment had a fresh and a dead battery cut in half, and as expected, the dead battery was stiff and congealed, whereas the fresh battery was still gelatinous.
Questions
I had a couple of questions when I first started reading into this topic, and I was lucky enough to find a study done by Princeton University covering the same thing! It was able to answer my questions, and went into more detail on how everything works, helping me understand the finer details.
- Does this 'lifehack' only apply to batteries that are completely depleted?
- It actually doesn't! The process that makes batteries bounce is gradual, with batteries beginning to noticeable bounce at around 70% charge.
- Does the bounce height change depending on remaining charge? Does it continually increase the further the battery depletes?
- The height does depend on remaining charge, but it reaches a plateau at a certain level. According to the tests done at Princeton, batteries at 50% charge and lower bounce to the same height
Diagram
As I mentioned, a handful of people over at Princeton University conducted a similar experiment a few years ago, and ended up with the same results. They wanted to know why dead batteries bounced. As documented in the diagram below, batteries begin to bounce increasingly higher, up to a certain point, the lower their charge is. It's interesting to note that past 50% charge remaining, the batteries don't bounce any higher.
How does this relate to what I've learned?
As the experiment demonstrated, the fresh batteries wouldn't bounce due to the anti-bounce effect the electrolyte gel gives, as opposed to experiencing a change in the spring constant. But what if it was the other way around? Theoretically speaking, what would the physics look like? In order to answer this question, we need to look at Hooke's Law.
Hooke's Law states that the amount of force necessary to keep a spring compressed is proportional to how much it's been compressed. The formula Hooke created reads \( F = -k \cdot x \), where
The first theory suggests that the release of hydrogen gas from the electrolyte gel increases the internal pressure of the alkaline battery, leading to a positive change in the spring constant. When the battery falls and collides with a surface, it deforms ever so slightly, and the surface pushes back with an equal and opposite force (Newton's third law), which should stop the battery in its tracks. However, the force of the battery reforming back into its original shape causes it to launch itself back into the air, becoming the bounce we see.
If the spring constant is higher than average, this would make the battery reform faster and with more force, leading to a higher bounce. Fresh batteries wouldn't have experienced the change in the spring constant, leading to a 'normal' bounce.
Hooke's law theoretically gives us the link between depleted, bouncing batteries and the concepts of work, energy, and momentum.
Hooke's Law states that the amount of force necessary to keep a spring compressed is proportional to how much it's been compressed. The formula Hooke created reads \( F = -k \cdot x \), where
- \( F \) is the restoring force,
- \( x \) is the displacement of the spring from its equilibrium position,
- and \( k \) is the spring constant.
The first theory suggests that the release of hydrogen gas from the electrolyte gel increases the internal pressure of the alkaline battery, leading to a positive change in the spring constant. When the battery falls and collides with a surface, it deforms ever so slightly, and the surface pushes back with an equal and opposite force (Newton's third law), which should stop the battery in its tracks. However, the force of the battery reforming back into its original shape causes it to launch itself back into the air, becoming the bounce we see.
If the spring constant is higher than average, this would make the battery reform faster and with more force, leading to a higher bounce. Fresh batteries wouldn't have experienced the change in the spring constant, leading to a 'normal' bounce.
Hooke's law theoretically gives us the link between depleted, bouncing batteries and the concepts of work, energy, and momentum.
Works Cited
Sullivan, John. "Battery Bounce Test Often Bounces off Target." Princeton University. The Trustees of Princeton University, 30 Mar. 2015. Web. 15 Mar. 2017.
<http://www.princeton.edu/main/news/archive/S42/72/95S25/index.xml?section=topstories>.
Whitwam, Ryan. "Why Do Dead Alkaline Batteries Bounce?" Geek.com. Ziff Davis, 9 July 2014. Web. 15 Mar. 2017.
<https://www.geek.com/science/why-do-dead-alkaline-batteries-bounce-1603759/>.
<http://www.princeton.edu/main/news/archive/S42/72/95S25/index.xml?section=topstories>.
Whitwam, Ryan. "Why Do Dead Alkaline Batteries Bounce?" Geek.com. Ziff Davis, 9 July 2014. Web. 15 Mar. 2017.
<https://www.geek.com/science/why-do-dead-alkaline-batteries-bounce-1603759/>.