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Batteries
by Chris Woodford. Last updated: June 12, 2013.
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?
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.
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, enter the type of the battery you're interested in followed by the words "anode cathode reactions" in your favorite search engine.) 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"), nickel metal hydride (NiMH)
and lithium-ion 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.
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.
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.
How do fuel cells differ from batteries?
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.
Find out more
Related articles
You might enjoy some of these other articles on Explain that Stuff covering similar ground:
Books for older readers
Books for younger readers
These are all aimed at younger readers (9–12 or so):
History of batteries
- Voltaic pile: The earliest battery, developed in 1800 by Italian physicist Alessandro Volta.
- Volta's Life and Works: This great website by the University of Pavia has lots of good information about Alessandro Volta's pioneering experiments with electricity and batteries.
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