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Storage Cell Fundamentals

In 1786, the Italian physiologist Luigi Galvani noticed by chance that when he stuck a copper hook into the spinal cord of a frog, which was in turn hanging from an iron hook, the frog's legs twitched. Galvani performed experiments that showed other pairs of dissimilar metals caused similar effects. He felt that he was seeing the discharge of some sort of "animal electricity" from the frog's muscles. Such experiments became fashionable, and led to a popular belief that electricity was an elemental "life force". This belief was illustrated by Mary Shelley's Gothic horror novel FRANKENSTEIN, with the monster brought to life by electricity, and by a range of electrical quack medical equipment that remained popular into the 20th century.

More practically, an Italian physicist named Count Alessandro Volta conducted further experiments with electrical currents produced by dissimilar metals. Volta concluded that the frog's muscle could be replaced by a salt solution or an acid solution, and the two dissimilar metals would still be able to produce an electric current. Volta made a stack of zinc and silver disks, with a zinc-silver pair separated by wet cloth containing a salt or weak acid solution, and was able to generate steady, fairly strong direct currents (DC) with this "Voltaic pile". This was an important advance in electrical research, since up to that time the only way to produce electricity was through building up static charges, say by rubbing fur with a rubber rod. This could produce substantial voltages but not sustained currents.

Volta's work was put onto a solid scientific basis in the 1830s, when the brilliant English scientist Michael Faraday established the fundamental principles of electrochemistry, which underlie the operation of storage cells as well as other electrochemical processes such as electroplating. In 1836, the English chemist John Daniell developed the first modern storage cell using Faraday's principles.

* Storage cell operation is based on "reduction-oxidation (redox)" reactions. For example, Volta's scheme can be modeled by placing a bar of zinc at one side of a beaker containing a solution of weak sulfuric acid; placing a bar of silver at the other side of the beaker; and then wiring the two "electrodes" through a light bulb outside the beaker. The light bulb then starts glowing.

Sulfuric acid has the formula H2SO4, and in solution it breaks down into two H+ ions and a single SO4-- ion. These ions allow electric current to flow through the solution, and so an ionic acid (or basic or salt) solution used to support an electrochemical reaction is known as an "electrolyte".

The SO4-- ion easily reacts with, or "oxidizes", zinc to form zinc sulfate (ZnSO4), which is released into the solution, eating away the zinc electrode. As each zinc sulfate molecule leaves the electrode, it leaves behind two electrons that flow through the external wire as a current to the silver electrode.

At the silver electrode, the electrons combine with or "reduce" the hydrogen ions in the solution to form diatomic hydrogen gas. The silver is inert and not consumed in the reaction. The negative zinc electrode is called the "anode", while the positive silver electrode is called the "cathode".

All modern storage cells use similar oxidation-reduction schemes, though the specific implementations vary widely. Some classes of storage cells can be "recharged" by running an electric current through them backwards, which reverses the chemical reactions and more or less restores things to their original condition.

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