If you can’t plug it in but need it powered, you better have a battery handy. Often overlooked, they keep our world in working order. But what are they? How do they work? And how come you can use a lemon to power a light bulb? Let’s find out.
Technology lets us do some pretty amazing stuff. Talking on the phone in the middle of nowhere, for example. Or reading these words even though I’ve just finished typing them, halfway across the world, a few seconds ago. You can whoosh through the sky in a chair, which probably drives birds green with envy. We’ve sent people and robots to outer space, a place that will freeze you, choke you, and give you the mother of all sunburns while you’re drifting along lazily because your legs aren’t any good there — all in complete safety. Doing all this eats up energy, however. A lot of energy.
Herein lies the issue. Because nature is sometimes annoying, you can’t carry around electricity like you can do with water — electricity either flows or doesn’t, you can’t fill up a bucket with it and use it later. Generators, on the other hand, don’t fit inside an iPhone, and strapping power-lines to a space shuttle kinda defeats its purpose. Thankfully, there’s a way you can carry power around with you.
Annoying when they run out in your mouse, thrown away more often than you can remember but making the world go round, let’s take a look at the unsung heroes of the modern day: batteries.
What batteries are made of
Electricity can’t be stored as such, but what you can do is transform it into another kind of energy and store that — which is exactly what batteries do. They store energy in chemical bonds and release it when needed as a flow of electricity.
It sounds complicated, but they’re actually surprisingly simple devices — you probably have the materials to build a (fairly weak) battery lying around your house. Batteries are formed from voltaic cells. Each is built using a positive negative electrode (cathode and anode), something to isolate the two (a separator), and something to bridge them — a conductive medium known as an electrolyte which gives ions a way to flow between the electrodes.
The cathode is submerged in the electrolyte, forming what’s called a positive half-cell. The anode and electrolyte combo forms a negative half-cell. These half-cells don’t do much on their own, but their atoms very much want to mingle — which is exactly why you need a separator (we’ll get to that in a moment).
Today, most batteries you’re likely to see (with the exception of car batteries) are known as “dry”, meaning they use a paste electrolyte which is less likely to leak. While they maintain a more or less constant structure across the board, materials vary widely. Zinc/Carbon batteries are ubiquitous non-rechargeable batteries as they’re laughably cheap to manufacture. Zinc/Manganese Dioxide with a Potassium Hydroxide electrolyte is also commonly used mix as it offers a good price-weight-output ratio.
Nickel variants (with Cadmium or metal hydride cathodes) are common for rechargeable batteries as they can sustain a large number of cycles even if they’re limited in regards to energy density — metal hydride cathodes offer better output at the expense of cycle life. And of course, there’s the Lithium Ion variety used in your phone — these are more expensive to produce but they have solid output and low weight.
How batteries work
The electrolyte allows the two ends of the battery (the electrodes) to trade ions (charged atoms) in a Redox reaction. Cations ( + charged ions) migrate to the anode where they dump excess electrons, leading to an electron build-up at the anode over time.
Since all electrons hold the same electrical charge they generally don’t really like one another. So they do all they can to get as far away from their kind as possible. In a battery, the only place they can go and do that is the cathode. This buildup — electrons wanting to move from one point to another — creates an electrical potential in the battery.
They can’t, however, move freely about through the electrolyte. Electrons need either a carrier in the form of ions to shuttle them to the other end of the battery, or for the cathode and anode to touch to disperse in a short-circuit. Since no ion in its right mind wants to move towards a point of the same electrical charge and the separator keeps the two electrodes well, separated, all those electrons are just dying for a way to flow to the cathode.
Then you come in the picture to deliver them from anguish. When you socket the battery in a computer mouse, for example, you complete its electrical circuit by bridging the anode and cathode together — giving the electrons a way to flow.
This is electricity.
What batteries can’t do
The hook with chemically storing energy is that over time the cathode becomes depleted of ions, which are now all snugly bound on the outside of the anode. This is why batteries run out.
For some of them (called secondary charge or rechargeable batteries) this process can be reversed by pumping electricity back into the battery. These are built from materials that can store an electrical charge, and need to be charged before their first use. After use, the influx of electricity pushes cations ( + charged ions) towards the anode where they dump excess electrons, refreshing the electrodes.
Over time, they lose their ability to hold a charge. One thing to look out for in secondaries is bloating — exposing the battery to extreme temperatures, extreme temperature shifts, as well as overcharging (which overheats the battery), can make them bloat. If the seals break they start leaking acid. Not good. The lead-acid battery that runs your car is a rechargeable battery, for example.
Primary charge batteries, on the other hand, use materials that can generate a change. They can be used right after assembly and don’t require charging. In theory, you could re-charge them. However, the chemical reactions that power them are very hard to reverse so it’s usually not economically viable, however. It’s also r-e-a-l-l-y unsafe, since the casings aren’t able to take the much higher thermal strain. Manufacturers strongly recommend against trying to do so for good reason. Results vary from “a bit of charge” to “fire”, “acid leaks everywhere and fire”, to “fireball-belching, acid-spewing, boom“.
So don’t do that. Don’t.
There’s actually a lot of wiggle room in what materials can form a battery. A Magnesium/Copper/Lemon cell can reach a better output than run-of-the-mill batteries (1.6 volts compared to 1.5 volts). Potato batteries have even been suggested as a viable source of power for people living off-grid. As long as you have a type of acid and two different metals available, you can make a battery — though the output will vary quite wildly from system to system.
So grab an LED, some fruit, and go experiment. Maybe you’ll hit on the battery of the futureClick here for reuse options!
Copyright 2017 ZME Science
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