A battery is essentially
a can full of chemicals that produce electrons. Chemical reactions
that produce electrons are called electro-chemical reactions.
If you look at any battery, you'll notice that it has
two terminals. One terminal is marked (+), or positive, while the
other is marked (-), or negative. In an AA, C or D cell (normal
flashlight batteries), the ends of the battery are the terminals. In
a large car battery, there are two heavy lead posts that act as the
terminals.
Electrons collect on the negative terminal of the battery. If you
connect a wire between the negative and positive terminals, the
electrons will flow from the negative to the positive terminal as
fast as they can (and wear out the battery very quickly -- this also
tends to be dangerous, especially with large batteries, so it is not
something you want to be doing). Normally, you connect some type of
load to the battery using the wire. The load might be
something like a light bulb, a motor or an
electronic circuit like a radio.
Inside the battery itself, a chemical reaction produces the
electrons. The speed of electron production by this chemical
reaction (the battery's internal resistance) controls how many
electrons can flow between the terminals. Electrons flow from the
battery into wire, and must
travel from the negative to the positive terminal for the chemical
reaction to take place. That is why a battery can sit on a shelf for
a year and still have plenty of power -- unless electrons are
flowing from the negative to the positive terminal, the chemical
reaction does not take place. Once you connect a wire, the reaction
starts.
Battery Chemistry If you want
to learn about the electro-chemical reactions used to create
batteries, it is easy to do experiments at home to try out different
combinations. To do these experiments accurately, you will want to
purchase an inexpensive ($10 to $20) volt-ohm meter down at
the local Radio Shack or hardware store. Make sure that the meter
can read low voltages (in the 1-volt range) and low currents (in the
5- to 10-milliamp range). This way you will be able to see exactly
what your battery is doing.
The first battery was created by Alessandro Volta in 1800. To
create his battery, he made a stack by alternating layers of zinc,
blotting paper soaked in salt water and silver. Like this:
This arrangement was known as a "voltaic pile." The top and
bottom layers of the pile must be different metals, as shown. If you
attach a wire to the top and bottom of the pile, you can measure a
voltage and a current from the pile. The pile can be stacked as high
as you like, and each layer will increase the voltage by a fixed
amount. You can create your own voltaic pile using coins and paper
towel. Mix salt with water (as much salt as the water will hold) and
soak the paper towel in this brine. Then create a pile by
alternating pennies and nickles. See what kind of voltage and
current the pile produces. Try a different number of layers and see
what effect it has on voltage. Then try alternating pennies and
dimes and see what happens. Also try dimes and nickels. Other metals
to try include aluminum foil and steel. Each metallic combination
should produce a slightly different voltage.
Another simple experiment you can try involves a baby food jar
(if you don't have a baby around the house, go buy a few jars of
food at the grocery and empty them out), a dilute acid, wire and
nails. Fill the jar with lemon juice or vinegar (dilute acids) and
place a nail and a piece of copper wire in the jar so that they are
not touching. Try zinc-coated (galvanized) nails and plain iron
nails. Then measure the voltage and current by attaching your volt
meter to the two pieces of metal. Replace the lemon juice with salt
water, and try different coins and metals as well to see the effect
on voltage and current.
In the 1800s, before the invention of the electrical generator
(the generator was not invented and perfected until the 1870s), the
Daniell Cell (which is also known as three other names -- the
"Crowfoot cell" because of the typical shape of the zinc electrode,
the "gravity cell" because gravity keeps the two sulfates separated
and a "wet cell" (as opposed to the modern "dry cell") because it
uses liquids for the electrolytes), was extremely common for
operating telegraphs and doorbells. The Daniell cell is a wet cell
consisting of copper and zinc plates and copper and zinc sulphates.
To make the cell, the copper plate is placed at the bottom of a
glass jar. Copper sulfate solution is poured over the plate to
half-fill the jar. Then a zinc plate is hung in the jar as shown and
a zinc sulfate solution poured very carefully into the jar. Copper
sulfate is denser than zinc sulfate, so the zinc sulfate "floats" on
top of the copper sulfate. Obviously this arrangement does not work
very well in a flashlight, but it works fine for stationary
applications. If you have access to the sulfates, zinc and copper,
you can try making your own Daniell Cell.
Battery Reactions Probably the
simplest battery you can create is called a zinc/carbon battery, and
by understanding the chemical reaction going on inside this battery
you can understand how batteries work in general.
Imagine that you have a jar of sulfuric acid
(H2SO4). Stick a zinc rod in it, and the acid
will immediately start to eat away at the zinc. You will see
hydrogen gas bubbles forming on the zinc, and the rod and acid will
start to heat up. Here's what is happening:
- The acid molecules break up into three ions: two H+
ions and one SO4-- ion.
- The zinc atoms on the surface of the zinc rod lose two
electrons (2e-) to become Zn++ ions.
- The Zn++ ions combine with the
SO4-- ion to create ZnSO4, which
dissolves in the acid.
- The electrons from the zinc atoms combine with the hydrogen
ions in the acid to create H2 molecules (hydrogen gas).
We see the hydrogen gas as bubbles forming on the zinc rod.
If you now stick a carbon rod in the acid, the acid does
nothing to it. But if you connect a wire between the zinc rod and
the carbon rod, two things change:
- The electrons flow through the wire and combine with hydrogen
on the carbon rod, so hydrogen gas begins bubbling off the carbon
rod.
- There is less heat. You can power a light bulb or similar load
using the electrons flowing through the wire, and you can measure
a voltage and current in the wire. Some of the heat energy is
turned into electron motion.
The electrons go to the
trouble to move to the carbon rod because they find it easier to
combine with hydrogen there. There is a characteristic voltage in
the cell of 0.76 volts. Eventually the zinc rod dissolves completely
or the hydrogen ions in the acid get used up and the battery "dies."
In any battery, the same sort of electrochemical reaction occurs
so that electrons move from one pole to the other. The actual metals
and electrolytes used control the voltage of the battery -- each
different reaction has a characteristic voltage. For example, here's
what happens in one cell of a car's lead-acid battery:
- The cell has one plate made of lead and another plate made of
lead dioxide, with a strong sulfuric acid electrolyte that the
plates are immersed in.
- Lead combines with SO4 to create PbSO4
plus one electron.
- Lead dioxide, hydrogen ions and SO4 ions, plus
electrons from the lead plate create PbSO4 and water on
the lead dioxide plate.
- As the battery discharges, both plates build up
PbSO4 (lead sulfate), and water builds up in the acid.
The characteristic voltage is about 2 volts per cell, so by
combining six cells you get a 12-volt battery.
A lead-acid
battery has a nice feature -- the reaction is completely
reversible. If you apply current to the battery at the right
voltage, lead and lead dioxide reform on the plates so you can reuse
the battery over and over. In a zinc-carbon battery, there is no
easy way to reverse the reaction because there is no easy way to get
hydrogen gas back into the electrolyte.
Modern batteries use a variety of chemicals to power their
reactions. Typical battery chemistries include:
- Zinc-carbon battery - Also known as a standard
carbon battery. Zinc-carbon chemistry is used in all
inexpensive AA, C and D dry-cell batteries. The electrodes are
zinc and carbon, with an acidic paste between them that serves as
the electrolyte.
- Alkaline battery - Used in common Duracell and
Energizer batteries. The electrodes are zinc and manganese-oxide,
with an alkaline electrolyte.
- Lithium photo battery - Lithium, lithium-iodide and
lead-iodide, used in cameras because of its ability to supply
power surges.
- Lead-acid battery - Used in automobiles. The electrodes
are made of lead and lead-oxide with a strong acidic electrolyte.
Rechargeable.
- Nickel-cadmium battery - Uses nickel-hydroxide and
cadmium electrodes, with potassium-hydroxide as the electrolyte.
Rechargeable.
- Nickel-metal hydride battery - Rapidly replacing
nickel-cadmium because it does not suffer from the memory effect
that nickel-cadmiums do. Rechargeable.
- Lithium-ion battery - Very good power-to-weight ratio,
often found in high-end laptop computers and cell phones.
Rechargeable.
- Zinc-air battery - Lightweight, rechargeable.
- Zinc-mercury oxide battery - Often used in hearing-aid
batteries.
- Silver-zinc battery - Used in aeronautical applications
because the power-to-weight ratio is good.
- Metal Chloride battery - Used in electric vehicles
Battery Arrangements In almost
any device that uses batteries, you do not use just one cell at a
time. You normally group them together serially to form higher
voltages, or in parallel to form higher currents. In a serial
arrangement, the voltages add up. In a parallel arrangement, the
currents add up. The following diagram shows these two arrangements:
The upper arrangement is called a parallel
arrangement. If you assume each cell produces 1.5 volts, then
four batteries in parallel will also produce 1.5 volts, but the
current supplied will be four times that of a single cell. The
lower arrangement is called a serial arrangement. The four
voltages add together to produce 6 volts.
Normally, when you buy a pack of batteries, the package will tell
you the voltage and current rating for the battery. For example, my
digital camera uses four nickel-cadmium batteries that are rated at
1.25 volts and 650 milliamp-hours for each cell. The milliamp-hour
rating means that the cell can produce 650 milliamps for one hour.
In general, you can scale milliamp-hours linearly -- this battery
could produce 325 milliamps for two hours or 1,300 milliamps for
half an hour. It is not completely linear -- all batteries have a
maximum current they can produce, and many battery chemistries have
longer or shorter than the expected life at very low currents -- but
it is generally linear over a normal range. Using the amp-hour
rating, you can estimate how long the battery will last under a
given load.
If you arrange four of these batteries in a serial arrangement,
you get 5 volts (1.25 x 4) at 650 milliamp-hours. If you arrange
them in parallel, you get 1.25 volts at 2,600 (650 x 4)
milliamp-hours.
Have you ever looked inside a normal 9-volt battery? It contains
six, very small batteries producing 1.5 volts each in a serial
arrangement.
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