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Batteries

Last updated: September 3, 2009.

We can't always generate electricity where and when it is needed so batteries, devices that store electrical energy in chemical form, are very important. Many different types of batteries are produced for a wide variety of applications, from storing solar power for satellites in space to powering heart pacemakers fitted inside people's chests.

You might think a battery looks just about as dull as anything you've ever seen. But the minute you hook it up to something, it starts buzzing with electricity. That dull little cylinder turns into your very own micro power plant! Let's see what's going on in there...

Photo: Disposable batteries like this one are really convenient, but they can be expensive in the long haul and they're bad for the environment. A better option is to use rechargeable batteries. They cost more to begin with, but you can charge them hundreds of times—so they save an absolute fortune and help save the planet!

What are the main parts of a battery?

All batteries contain one or more cells, but people often use the terms battery and cell interchangeably. A cell is just the working chemical unit inside a battery; one battery can contain any number of cells. A cell has three main parts: a positive electrode (terminal), a negative electrode, and a liquid or solid separating them called the electrolyte. When a battery is connected to an electric circuit, a chemical reaction takes place in the electrolyte causing ions (in this case, atoms with a positive electrical charge) to flow through it one way, with electrons (particles with a negative charge) flowing through the outer circuit in the other direction. This movement of electric charge makes an electric current flow through the cell and through the circuit it is connected to. That's the theory anyway. Now let's look at it in practice.

How does a battery really work?

Putting a battery into a simple circuit with a flashlight bulb

Where does the power in a battery actually come from? Let's take a closer look!

Here's my battery hooked up to a flashlight bulb to make a simple circuit. I've unwrapped a paperclip to make a piece of connecting wire and I'm holding that between the bottom of the battery and the side of the bulb. If you look closely, you can see the bulb is shining. That's because electrons are marching through it!

Anode and cathode?

Now here's what's going on inside. The battery's positive terminal (shown just above my left thumb in the photo and colored red in the artwork below) is connected to a positive electrode that's mostly hidden inside the battery. We call this the cathode. The outer case and the bottom of the battery make up the negative terminal, or negative electrode, which is also called the anode and colored green in the artwork. The paperclip wire is represented in the art by the blue line.

Let's quickly clear up one point of confusion. At school, you may have learned that the cathode is the negative electrode and the anode the positive electrode? However, that applies only to electrolysis (passing electricity through a chemical to split it up). Batteries are like electrolysis going backwards (they split up chemicals to make electricity) so the terms anode and cathode are switched around. Okay? To avoid confusion, I suggest it's best not to use the terms anode and cathode at all. It's better to say positive terminal and negative terminal and then it's always clear what you mean, whether you're talking about batteries or electrolysis.

Diagram showing the chemistry of how batteries work

Chemical reactions

Now back to our battery. The positive and negative electrodes are separated by the chemical electrolyte. It can be a liquid, but in an ordinary battery it is more likely to be a dry powder.

When you connect the battery to a lamp and switch on, chemical reactions start happening. One of the reactions generates positive ions (shown here as big yellow blobs) and electrons (smaller brown blobs) at the negative electrode. The positive ions flow through the electrolyte to the positive electrode (from the green line to the red one). Meanwhile, the electrons (smaller brown blobs) flow around the outside circuit (blue line) to the positive electrode and make the lamp light up on the way.

The electrons and ions flow because of the chemical reactions happening inside the battery—usually two or three of them going on simultaneously. The exact reactions depend on the materials from which the electrodes and electrolyte are made, and we won't go into them here. (If you want to know what they are, Google the type of the battery you're interested in followed by the words "anode cathode reactions". For example, Google "zinc carbon battery anode cathode reactions".) Whatever chemical reactions take place, the general principle of electrons going around the outer circuit and ions flowing in the opposite direction through the electrolyte happens in all batteries. As the battery generates power, the chemicals inside it are gradually converted into different chemicals. Their ability to generate power dwindles, the battery's voltage slowly falls, and the battery eventually runs flat. In other words, if the battery cannot produce positive ions because the chemicals inside it have become depleted, it can't produce electrons for the outer circuit either.

Now you may be thinking: "Hang on, this doesn't make any sense! Why don't the electrons just take a short cut and hop straight from the negative electrode through the electrolyte to the positive electrode? It turns out that, because of the chemistry of the electrolyte, electrons can't flow through it in this simple way. In fact, so far as the electrons are concerned, the electrolyte is pretty much an insulator: a barrier they cannot cross. Their easiest path to the positive electrode is actually by flowing through the outer circuit.

Types of batteries

Although there are lots of different kinds of batteries, there are really only two types: disposable and rechargeable. They contain two different kinds of cells. Primary cells make the power in ordinary, disposable batteries. They produce electricity by slowly using up the chemicals from which the electrodes and electrolyte are made. Secondary cells power rechargeable batteries. You can find them in the big lead-acid batteries that start cars and the nickel-cadmium (NiCd or "nicad") batteries that power cellular phones. Unlike primary cells, secondary cells can be recharged simply by passing a current through them in the reverse direction to normal. When you charge your cellphone, you are really just running the battery (and the chemical reactions inside it) in reverse.

Examples of disposable batteries (primary cells)

Zinc-chloride batteries

In a zinc-chloride long-life battery, the positive electrode is made from a carbon rod surrounded by a mixture of powdered carbon and manganese dioxide, the negative electrode is made from an alloy of zinc and the electrolyte between them is a jelly or paste of ammonium chloride. The whole battery may be sealed inside a metal or plastic case and, because there is no liquid that can be spilled, it is often referred to as a dry cell. The cheapest, ordinary, everyday batteries you get for things like flashlights are zinc carbon ones.

Alkaline batteries

Inside an alkaline battery, manganese dioxide molecules are converted into manganese oxide and hydroxyl ions. The hydroxyl ions then react with zinc to form zinc oxide and water, releasing electrons. The electrons move toward the carbon rod and flow out around the circuit, producing an electric current. The battery stops producing electricity when all the manganese dioxide is used up. Alkaline batteries look much the same as zinc carbon ones but last longer and cost more.

Button cells

Button cells are used inside calculators and watches (and you find really tiny ones in hearing aids). The top of the cell is the negative electrode, made from powdered zinc trapped between two metal layers. The bottom of the cell and the case make up the positive electrode, made from mercury oxide and graphite. In between the electrodes is an alkaline electrolyte of potassium hydroxide. During operation, the zinc loses electrons to become zinc oxide and the mercury oxide changes to mercury metal.

A photo of several of the more popular types of batteries

Examples of rechargeable batteries (secondary cells)

This a quick overview of rechargeables. You can read a more detailed account in our main article on how battery chargers work.

Nickel cadmium (NiCd) and nickel metal hydride (NiMH) batteries

Until recently, virtually all rechargeable batteries were nickel-cadmium (NiCd, usually pronounced "nicad"). Although very dependable, it's often said that they need to be discharged fully before you charge them up or the amount of charge they will store (and their effective lifespan) can be greatly reduced. Nickel metal hydride work in a similar way, but suffer less from this so-called "memory effect." Another problem with NiCd batteries is the toxic cadmium metal they contain. If they are buried in a landfill, instead of properly recycled the cadmium can escape into the soil and could potentially pollute watercourses nearby.

Lithium-ion batteries

Lithium is a lightweight metal that easily forms ions, so it is excellent for making batteries. The latest lithium-ion batteries can store about twice as much energy as traditional NiCd rechargeables, work at higher voltages, and are more environmentally friendly, but do not last as long. There are probably lithium-ion batteries in your cellphone, MP3 player, and laptop computer.

How do they work? When you plug a cellphone or laptop into the power supply, the lithium-ion battery inside starts buzzing with chemical activity. The battery's job is to store as much electricity as possible, as fast as possible. It does this through a chemical reaction that shunts lithium ions (lithium atoms that have lost an electron to become positively charged) from one part of the battery to another. When you unplug the power and use your laptop or phone, the battery switches into reverse: the ions move the opposite way and the battery gradually loses its charge. Lithium-ion batteries also have special electronic circuits that can interrupt charging and discharging. These switch off the power to prevent overcharging and overheating and to prevent too much discharging, which makes the battery unstable and harder to charge up again. Read more in our main article on how lithum-ion batteries work.

A typical car battery

Accumulators

Accumulators are most familiar to us as large, powerful car batteries. A lead-acid accumulator contains three or six separate cells inside a tough plastic casing. Each cell contains lead electrodes and an electrolyte of sulfuric acid and water. During operation, the sulfuric acid is gradually turned into water, the lead electrodes are converted into lead sulfate, and the battery becomes unable to supply more charge. But unlike a dry cell, it can be recharged simply by passing a current through it in the opposite direction.

Photo: A typical lead-acid car battery (accumulator).

How do fuel cells differ from batteries?

NASA photo of a bus powered by fuel cells

Photo of a bus powered by fuel cells. The fuel cells are just inside the open door on the right. Picture courtesy of NASA Dryden Flight Research Center.

Unlike a battery, which gradually loses its ability to make electricity from the chemicals inside it, a fuel cell converts chemicals into electricity from a continuous supply of fuel outside it. Like a battery, a fuel cell has positive and negative electrodes and an electrolyte in between.

Because fuel cells are at least twice as efficient as internal combustion (gasoline and diesel) engines and produce nothing more polluting than water, they are expected to be used inside environmentally friendly electric cars in the future. Fuel cells are already used to generate power inside unmanned space probes and the Space Shuttle.

Inside a fuel cell

A fuel cell has a fuel electrode and an oxygen electrode. As it passes over the negative fuel electrode, hydrogen turns into hydrogen ions and electrons. The electrons move through the circuit to the positive oxygen electrode, while the ions move through the electrolyte. At the oxygen electrode, electrons combine with hydrogen ions and oxygen gas to make water.

Read more in our main article on fuel cells.

Websites

Energizer Learning Center: How batteries work: Includes an animated guide.

Books

Battery Science: Make Widgets that Work and Gadgets that Go by Doug Stillinger.
Electric Mischief: Battery-Powered Gadgets Kids Can Build by Alan Bartholomew.
Eyewitness: Electricity by Steve Parker.

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