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Powered by batteries

Batteries are all over the place - in our Walkmans, computers, laptops, portable MP3 players and cell phones. A battery is essentially a can full of chemicals that produce electrons.

Today we will discuss batteries - from the basic concept at work, to the actual chemistry going on inside a battery.

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 should 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 a 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.

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. This arrangement was known as a voltaic pile. The top and bottom layers of the pile must be different metals. 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.

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 by three other names - the "Crowfoot cell" because of the typical shape of the zinc electrode, the "gravity cell" because gravity keeps the two sulphates 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 Daniell cell, the copper plate is placed at the bottom of a glass jar. Copper sulphate solution is poured over the plate to half-fill the jar. Then a zinc plate is hung in the jar and a zinc sulphate solution poured very carefully into the jar.

Copper sulphate is denser than zinc sulphate, so the zinc sulphate "floats" on top of the copper sulphate. Obviously, this arrangement does not work very well in a flashlight, but it works fine for stationary applications. If you have access to zinc sulphate and copper sulphate, you can try making your own Daniell cell.

Probably the simplest battery you can create is called a zinc/carbon battery. 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 sulphuric 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."

Battery chemistries

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 are used in cameras because of their 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 - The electrodes are nickel-hydroxide and cadmium, with potassium-hydroxide as the electrolyte (rechargeable).

* Nickel-metal hydride battery - This battery is rapidly replacing nickel-cadmium because it does not suffer from the memory effect that nickel-cadmiums do (rechargeable).

* Lithium-ion battery - With a very good power-to-weight ratio, this is often found in high-end laptop computers and cell phones (rechargeable).

* Zinc-air battery - This battery is lightweight and rechargeable.

* Zinc-mercury oxide battery - This is often used in hearing-aids.

* Silver-zinc battery - This is used in aeronautical applications because the power-to-weight ratio is good.

* Metal-chloride battery - This is used in electric vehicles.

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