Bouncing Batteries

27 May 2016

An ordinary family can easily have dozens of alkaline batteries operating in various devices around the house. They'll be in smoke detectors and remote controls, thermostat controls and wall clocks, children's toys, flashlights and the like.

So I was very interested when I saw a viral YouTube video, showing how you can test the 'state of charge' of a battery by dropping it and seeing how high it bounces. I was sceptical, so I tried it out at home with half-a-dozen Double A batteries. I measured their state of charge with a multimeter set to the 10Amp scale, and I measured their bounciness by dropping them all from the same height, and seeing how high they jumped up.

I found no link between the 'bounciness of a battery' and its 'state of charge'. The reason that I found no link was because of my woeful and totally inadequate experimental technique. The batteries came from different manufacturers. I neglected to ensure that they landed truly vertically (for example, by running them through a skinny 25-cm-long hollow tube). And of course, my sample size was too small.

Luckily, I found a paper in the Journal of Materials Chemistry A, authored by some authentic engineers and real scientists. It turns out that under certain circumstances, there is a link between 'how flat a battery is' and 'how far the battery bounces back up when dropped'. This link happens NOT when the battery is either full or flat - but only in the middle of its state of charge.

When the battery is between 80-100% full, it will bounce back upwards about 20% of the distance you dropped it. And then, as you gradually flatten the battery from 80% down to 50% full, it will bounce higher and higher, getting up to about 60% of the distance you dropped it. But once the battery is down to half full, and then you discharge it down to totally empty, you don't get a significant increase in bounce - perhaps just another 5% or so.

But, in that range between 50-80% full, the flatter the battery - the higher it bounces.

How come?

Well, you need to know what's inside a typical alkaline Double A battery. Beneath the plastic skin lies a cylindrical steel case. Inside that are three concentric layers.

The outermost layer is the cathode made from manganese oxide. It's connected directly to the positive end of the battery, at the top.

Inside that is a thin-walled cylinder called the separator, which keeps the outer manganese oxide cathode separated from the gelatinous zinc anode inside it. This separator keeps the manganese oxide and zinc gel apart, so they are not touching each other - but it still lets electrons pass through.

And right in the centre is the zinc gel anode. The gel carries about 2.5 grams of tiny zinc particles. The zinc anode is connected directly to the negative end of the battery, at the bottom.

When you connect the positive and negative ends of the battery to a flashlight, chemical reactions start happening. These reactions liberate electrons. But let's put aside the little electrical miracle - and look at the physical side.

The zinc atoms get converted into individual molecules of zinc hydroxide, which gradually saturates the gel. By the time the battery gets down to 80% fully charged, the gel is fully saturated with zinc hydroxide. At this stage, the gel is still very gelatinous. In a battery that's pretty full, the soft gloopy zinc gel just absorbs the energy of the impact when dropped, leaving hardly any energy available to push the battery up off the table.

But, as you gradually flatten the battery down from 80-50% full, the zinc gel converts into a more solid, and far stiffer, chemical called zinc oxide. The energy of the impact passes through the stiff zinc oxide without being absorbed. The impact energy then hits the inside of the top of the battery, and bounces back down to the bottom - and propels the whole battery into the air.

The bounciness of a battery can roughly tell you how much charge it carries - but only in the relatively narrow range between 50-80% full.

Above that state of charge, between 80-100%, all you get is a pretty constant 20% bounce.

From 50% fully charged down to dead flat, again you get a pretty constant bounce - but a lot higher, around 60%. Again, the bounce test doesn't tell you the difference. A battery that's half-full is usable, but a flat battery is not. So in this lower range, the bounce test is useless.

Nevertheless, batteries should never be taken for granted. They're a little power cell of wonder that can give bounce to your life.


'The Relationship between Coefficient of Restitution and State of Charge of Zinc Alkaline Primary LR6 Batteries', by Shoham Bhadra et al, Journal of Materials Chemistry A, 2015 DOI: 10.1039/c5ta01576f

© Karl S. Kruszelnicki Pty Ltd 2015