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Science
by Jerry Oltion

Portable Power

 

Science fiction stories are full of amazing portable gadgets. One of my favorites is Larry Niven's flashlight-laser. As its name implies, it's a super-bright light that can function as either a flashlight or a weapon. At its latter setting, it's powerful enough to chop a tree into logs (setting them aflame in the process—an easy way to build a campfire!) And it can do that over and over again.

Ever wonder what kind of battery that thing must have?

In our increasingly mobile society, we rely on batteries to power everything from our phones to our cars. Batteries keep us ticking, quite literally: We have batteries in our watches and in our pacemakers. Hearing aids run off batteries, as do our book readers, our toys, and even our socks. And yes, batteries power our flashlights and our lasers.

So how do batteries work, anyway?

 

The First Batteries

 

The earlieast known battery dates to about 250 bc. The "Baghdad battery," as it's called, is a pottery jar containing a copper cylinder that surrounds an iron rod. If an acidic solution is poured into the jar, an electrical potential develops between the copper and the iron. That offers a clue: If you bathe two dissimilar metals in a fluid that reacts chemically with them, you'll generate electricity.

What's happening here? Remember those drawings you saw in grade school of atoms with the protons and neutrons down in the nucleus and the electrons zipping in orbit around them? Electrons, not surprisingly, are the carriers of electrical charge. And different metals hold onto their electrons with differing degrees of affinity. Some metals cling tightly, while others let them go relatively easily. When you bathe two metals in an "electrolyte" (a substance that allows electrons to flow through it), the difference in their affinity for electrons sets up an imbalance. If you don't let them actually touch, nothing much happens, but once they do, electrons flow from one to the other. That creates a chemical reaction between the metals and the electrolyte, which replenishes the electrons that traveled between metals.

The trick is to put your device in between the two metals, so those flowing electrons go through your device on the way from metal A to metal B. That "completes the circuit" and allows the battery to operate. Eventually you run out of metal or electrolyte, and the battery goes dead.

We don't really know if the Baghdad battery was actually used to generate electricity or not. It's hard to imagine what its inventor would have used it for 2,250 years before the invention of the cell phone. Maybe he just stuck his tongue against the terminals and got a cheap tingle.

As far as we know, there were no significant developments in battery technology for another two millennia. The first for-sure chemical battery was built in 1800 by an Italian physicist named Alessandro Volta, for whom we have named the basic unit of electrical potential: the volt. Volta used a stack of copper and zinc disks separated by brine-soaked cloth. Volta didn't realize that it was a chemical reaction that produced electricity, but the battery worked nonetheless.

Each copper-cloth-zinc element constituted a "cell," and each cell produced 0.76 volts. Why isn't the amount of potential developed by Volta's first cell considered to be 1 volt? Because by the time scientists decided to honor Volta by naming electrical potential after him, there were already amperes and ohms and watts and joules and coulombs and scads of other units named after other people, and for volts to fit in mathematically with these other already-defined units, Volta's copper-zinc cells had to produce 0.76 of them.

Volta didn't call his stack of cells a battery, by the way. He called it a "pile." (Which is what a battery is still called in Italy.) Benjamin Franklin coined the term "battery" in 1749, but he was using it to describe a bank of linked capacitors, devices that store an electric charge in a completely different way. "Battery" referred to the concept of linking several devices together to increase their power output. So technically, a single cell is just that: a cell. Multiple cells connected together form a battery. But very few people make that distinction anymore. Nowadays, anything that produces electricity through chemical reactions is a "battery."

 

Primary Versus Secondary

 

Batteries powered practically everything that needed electricity for at least 30 years, until the invention of the dynamo. Dynamos generate electricity by moving a coil of wires through a magnetic field, essentially using magnetism to push electrons down the length of the wires, and they can be powered by pretty much anything that can turn a crank. Dynamos, and later refinements of that invention that are now all lumped into the generic term "generator," are much more efficient than batteries, so the vast majority of electricity used in the world today is produced by generators. Batteries are reserved for situations in which their portability or storage capability outweigh their relative inefficiency.

How inefficient are batteries? It's estimated that it takes about 50 times as much energy to produce a battery as you'll get out of one. And most batteries are used only once, which means that the United States disposes of over 3 billion batteries a year.

It's not just whimsical wastefulness that prompts us to throw batteries away when they're used up. It's the nature of the chemical reactions inside them. Most batteries are called "primary cells," and the chemicals that make them work only function one way. You can't stuff electricity back into them and re-use them, because once the electrodes have combined with the electrolyte, they're more or less inert.

In 1859, French physician Gaston Planté invented a new type of battery. It used lead and lead dioxide plates for electrodes and sulfuric acid for the electrolyte. It produced a healthy 1.5 volts per cell, and had an amazing capability: It could be recharged. You actually could stuff electricity back into it and get that electricity back out later. The sulfuric acid combined with the lead to produce lead sulfate and water while the battery was being discharged, and when you shoved electricity back into the system the lead sulfate and water recombined to make sulfuric acid again. Essentially, sulfuric acid became the storage medium for electricity in these "secondary cells."

The lead-acid battery was such a successful invention that we're still using them, essentially unchanged, today. The problem with lead-acid batteries is that they're heavy. And when they're fully charged, they contain sulfuric acid, which is pretty nasty stuff if it spills. People kept looking for better materials, and it wasn't long before they discovered several. Nickel-cadmium was an early combination, followed by nickel metal hydrides, lithium-ion, and several others. Each has its advantages and disadvantages.

 

Energy Density

 

One of the biggest disadvantages of lead-acid batteries is their weight. Lead is heavy stuff, and the thick casing necessary to keep the sulfuric acid from leaking out is also heavy. If you divide the amount of power you can get out of the battery by its weight, you get what's called the "energy density" of the battery, and not surprisingly lead-acid batteries are on the bottom of the...er...pile.

Lithium, on the other hand, is a very light metal, so lithium-ion batteries have a relatively high energy density, about six times that of lead-acid batteries. So where weight is an issue (as in cell phones, ear buds, hearing aids, and electric cars), lithium-ion batteries make a lot more sense. That's why Tesla decided to power its cars with lithium-ion batteries, and those fun little propeller-driven drones that people love to take pictures of their neighbors with all use lithium-ion batteries, too. (Or more properly, lithium-polymer batteries, which are a variant on lithium-ion technology.)

Even lithium-ion batteries aren't powerful enough to really do the job we ask of them, though. Gasoline has about 100 times the energy density of lithium-ion batteries, which is why gasoline engines still power most cars and almost all our airplanes. We haven't designed a hot enough battery yet to compete directly with gasoline.

 

Speaking of Heat...

 

One other unwanted side effect of high-capacity batteries is heat. Most chemical reactions produce heat, and when you charge or discharge a battery, you're driving a chemical reaction. That heats up the battery. If the battery can't dissipate the heat quickly enough, it heats up enough to burst or catch fire, or both. Lithium metal burns quite readily in air, and it burns really hot, as many people who have been unlucky enough to have battery failures can attest. So battery chargers (and some batteries themselves) have protective circuitry built into them to shut down the charging when the battery is full. Likewise, you don't want to discharge a lithium battery (or any battery, really) too quickly. Short-circuiting one is a good way to create an explosion. So most lithium batteries have built-in protective circuitry to prevent rapid discharge, too.

 

In Search of Better Batteries

 

Even with an energy density of about 600 watt-hours per kilogram, lithium-ion batteries aren't really all that powerful. Your cell phone will discharge in a couple of days with moderate use, and a Tesla will only go a few hundred miles on a charge with a battery pack that weighs over half a ton. If we're ever going to get a battery efficient enough to power Larry Niven's flashlight-laser, we're going to need something better than lithium-ion technology. Unfortunately, lithium is the lightest metal, so we're not going to find anything better in terms of weight. That leaves electronegativity, the desire of a material to attract electrons. Lithium is relatively low in that regard. Many other materials are higher. One possibility is iron, with an electronegativity twice that of lithium, but iron is 15 times heavier.

Fuel cells, which generate power with chemicals that can be consumed and replaced continuously, can have energy densities up to ten times better than lithium-ion batteries, but the most efficient ones are fueled with hydrogen, which is difficult to store and expensive to produce.

We need something fundamentally different. How about if we could simply stuff electrons in a box, then let them out when we needed them? Ironically, that's what Benjamin Franklin was doing when he coined the term "battery." He was using Leyden jars, devices that store static charges on two metal plates. Nowadays we call these devices "capacitors," and the technology has come a long way, but even our supercapacitors have lower energy densities than existing batteries. The problem lies in the separation of the two metal plates: If you stuff too many electrons in there, they tunnel across from one plate to the other, essentially short-circuiting the capacitor.

In one of my science fiction stories where I needed an unusually powerful battery, I came up with what I called an "electron plasma battery." It was basically an insulated container that you filled with electrons, no metal plates involved, and you let the electrons out when you needed them by simply turning a valve. I made the box out of handwavium, and I have no idea how you would stuff the electrons in there, but it powered my characters' hyperdrive engine all around the galaxy.

 

Make Your Own Battery

 

You, too, can make your own battery. I mean a real, functional battery. All you need is a copper electrode (heavy gauge wire will do), a zinc electrode (a galvanized nail works fine), and a potato. Stick the copper and zinc electrodes into the potato and connect a voltmeter to them, and you'll see about half a volt. That's not enough to power much of anything, so you need to make a true battery, connecting several potato cells together to increase their voltage. Use wire to connect the copper electrode of one cell to the zinc electrode of the next cell, and so on like that with six cells in a row. That should get you about three volts, which will be enough to power an LED or a digital clock. The free copper electrode on one end of the battery is positive and the free zinc electrode on the other end is negative.

You can use a lemon (or a glass full of lemon juice, but be careful not to let the electrodes touch one another) instead of a potato, and that will generate more voltage. You only need four lemons to get three volts. Vinegar works just as well as lemon juice. And of course, as Alessandro Volta discovered 219 years ago, so will salt water.

Give it a try!
 

__________________________________

Jerry Oltion has been a science nut since he was old enough to spell "curious." He has written science fiction almost as long, and has done astronomy somewhat less. He writes a regular column on amateur telescope making for Sky & Telescope magazine, and spends many, many nights a year out under the stars.
 

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