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A Primer on Storage Cells

Alkaline Cells, Carbon-Zinc cells, Mercury cells, Zinc-Air cells, Silver Oxide cells, Lead Acid cells, Edison cells, Nicad cells, NiMH and Lithium cells are the most commonly used cells today. A concise look at the fundamentals and principles of devices that "confine" energy into small cells.
History

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. As such experiments became fashionable, they 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.

Scientists learn more

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.

Most Common Theory Behind cells

The Storage cell operation is based on "reduction-oxidation (redox)" reactions.

For example, 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 can model Volta's scheme. The light bulb then starts glowing.

Sulfuric acid,(H2SO4) in solution 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 these 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.

Difference Between Cell and Battery

The popular terminology for storage cells is somewhat confusing. They are almost always referred to as "batteries" in common usage, but this is not technically correct.
The storage cell is just that, a "cell", not a "battery". It consists of one cathode and one anode in an electrolyte. A storage cell with specific electrode materials and electrolyte has a certain output voltage, and to get higher voltages with that specific technology, they must be electrically connected together in series as a "battery".

Flashlight cells are just that, cells, but the lead-acid battery used in an automobile consists of several cells packaged and chained together, so it is indeed a battery. A single cell of a lead-acid battery has a voltage of 2 volts, and so a 12-volt lead-acid battery has six cells in series.

Classification of storage cells

Storage cells can also be classified as "wet cells", which have liquid electrolytes; "dry cells", which have electrolytes in the form of a paste; and "solid electrolyte" cells, which as their name indicates use a completely solid electrolyte.

Furthermore there are two classes of storage cells: non-rechargeable or "primary" cells, for example typical cheap throwaway flashlight batteries, and rechargeable or "secondary" cells, for example an automotive lead-acid battery.

There are also standardized form factors for certain classes of storage cells, such as AAA and AA penlight cells; C and D flashlight cells; and the standard nine-volt brick-shaped "transistor radio" battery. Every classification by sizes of storage cells has their output voltages standardized.

Theory behind their output voltages

Many storage cells can maintain their output voltage at a reasonably constant level over a fairly wide range of output currents. In electrical engineering terms, they are said to have a low "internal resistance". Those that have high internal resistance cannot operate at high current loads, since the voltage at their terminals drops below useful levels. Cells with high internal resistance will also burn up a high proportion of their stored electricity with their own resistance at high current loads, draining them prematurely.

This means that another parameter for storage cells is the maximum useful current output. Storage cell makers may also provide curves giving the fall-off in voltage with increasing current drain, from maximum voltage to the "cutoff voltage" specified for the cell. By the way, the low internal resistance, or equivalently current capacity, of big automotive batteries makes them potentially dangerous. While their output voltages are so low that getting a shock off them is not a problem, if the output of a large automotive battery is shorted to the chassis ground the large currents flowing through the short can cause an almost explosive flash and severe burns.

The total energy capacity of a storage cell is measured in the number of hours it can supply a given level of current, or "ampere-hours". This is a straightforward figure of merit for storage cells based on the same technology, since they will all have the same voltage.

However, ampere-hours can be misleading for comparing different storage cell technologies, as the voltages may differ and the power output of a storage cell with a lower voltage is lower for the same level of output current. For this reason, the unit of "watt-hours" is used to compare energy storage capacity between different storage cell technologies.

A related rating is the "specific energy" of a storage cell, which gives the storage capacity of the cell relative to its mass. For example, a storage cell could be said to have a given number of watt-hours per kilogram. A related measure is the "energy density" of the cell, which gives its storage capacity relative to its volume, for example in watt-hours per liter. Specific energy and energy density are used in comparisons between different classes of batteries, particularly for automotive propulsion applications. Electric-powered automobiles have always suffered from the limited energy capacity of electric storage cells compared to gasoline and other chemical fuels, and so obtaining storage cells with greater specific energy has been one of the most important goals of electric-automobile designers.

Parameters to be kept in mind.

Rechargeable storage cells can be run backwards and more or less restored to their original, charged state. The "more or less" is important. The restored state is not a perfect replica of the original state, and so rechargeable storage cells degrade slightly every time until their storage capability fades out. For this reason, rechargeable cells are also described by the number of "charging cycles" they will tolerate. The number of cycles tends to be lower with greater average depth of discharge. Manufacturers may also provide curves showing how the storage cell's capacity slowly falls as the number of cycles increases.

Another parameter specific to rechargeable cells is "efficiency", or the ratio of power available when the cell is fully charged to the power required to recharge it. Other battery parameters include, of course, the physical dimensions and mechanical specifications of the battery; its shelf life; its expected service life, or how long it can be expected to survive in normal operation; and environmental limits on its operation, particularly temperature specs.

Since the rate of chemical reactions increases at higher temperatures, it is customary to store flashlight cells in a refrigerator to prolong shelf life, though this is becoming less important as improved cell technologies have long shelf lives.

Tuning according to Technology

Companies have now gone beyond simply selling batteries and now sell complete battery-based power modules that can be designed into portable equipment. Such a power module consists of a pack containing cells, power output and recharging control, and control electronics.

The control electronics will include a small cheap digital microcontroller with electrically-programmable ROM to store battery parameters. The module communicates with the rest of the system through a two-wire serial-interface bus called the "SMBus", devised by Intel Corporation. Such schemes are now being standardized by an open specification called the "Smart Battery System (SBS)" that specifies the functionality, interfaces, and software protocols of the battery pack.

Cells, or in the collective series called batteries are here to stay. As technology advances, so does the cell's capacity of storing more and more current increases. Moreover, the current sinking capacity of the devices using cells is decreasing day by day. The day may soon arrive when the cells in our day-to-day devices become so small or thin that they become invisible to the naked eye.

Some Specs of Batteries

CARBON-ZINC (LECLANCHE) CELL:

anode: zinc cup
cathode: manganese dioxide in graphite powder
electrolyte: ammonium chloride & zinc chloride in water
cell voltage: 1.5 volts

Non-rechargeable, poor storage density, but very cheap.

ALKALINE CELL:

anode: nickel-plated steel cup
cathode: manganese dioxide in graphite powder
electrolyte: potassium hydroxide in water
cell voltage: 1.5 volts

(Generally) non-rechargeable, storage density about twice that of the carbon-zinc cell, but several times more expensive.

MERCURY BUTTON CELL:

anode: zinc
cathode: mercuric oxide
electrolyte: potassium hydroxide in paste
cell voltage: 1.35 volts

Non-rechargeable. A variation on this technology used cadmium instead of zinc and provided a cell voltage of 0.91 volts. The first button cell technology, now obsolete due to environmental concerns.

ZINC-AIR BUTTON CELL:

anode: powdered zinc in gel
cathode: carbon disk exposed to air
electrolyte: potassium hydroxide layer
cell voltage: 1.65 volts

Non-rechargeable. Most popular current button cell technology, also some applications in larger cell formats.

SILVER OXIDE BUTTON CELL:

anode: powdered zinc in gel
cathode: silver grid pasted with silver oxide
electrolyte: potassium hydroxide layer
cell voltage: 1.55 volts

Nonrechargeable.

LEAD-ACID CELL:

anode: lead
cathode: lead oxide
electrolyte: sulfuric acid
cell voltage: 2 volts

The standard large capacity battery technology. Can be recharged hundreds of times and very cheap, but bulky and environmentally noxious.

NICKEL-IRON (EDISON) CELL:

anode: iron
cathode: nickel oxide
electrolyte: potassium hydroxide
cell voltage: 1.15 volts

Heavy-duty rechargeable unit, used in some industrial applications.

NICKEL-CADMIUM (NICAD) CELL:

anode: cadmium
cathode: nickel oxide
electrolyte: potassium hydroxide
cell voltage: 1.2 volts

The original rechargeable cell for portable gear, now used mostly in gear that needs high power levels on demand.

NICKEL-METAL HYDRIDE (NIMH) CELL:

anode: metal hydride
cathode: nickel oxide
electrolyte: potassium hydroxide solution in separator sheet
cell voltage: 1.2 volts

Greater capacity than nicads but more expensive.

LITHIUM-MANGANESE DIOXIDE CELL:

anode: lithium foil
cathode: manganese dioxide
electrolyte: separator sheet impregnated with electrolytic salts
cell voltage: 3 volts

The most common non-rechargeable lithium cell.

LITHIUM DISULFIDE CELL:

anode: lithium foil
cathode: iron disulfide with aluminum cathode contact
electrolyte: separator sheet impregnated with electrolytic salts
cell voltage: 1.5 volts

"Voltage compatible" lithium cell as direct replacement for carbon-zinc or alkaline cells.

LITHIUM-ION CELL:

anode: inert carbon sheet
cathode: manganese dioxide
electrolyte: electrolyte separator sheet with lithium ions
cell voltage: 3.6 volts

Rechargeable lithium cell.

References:-

Details were also obtained from the Duracell and Energizer websites, a few chemistry textbooks, and the Microsoft ENCARTA and online BRITTANNICA encyclopedias
By Vishwas Purohit
Published: 7/23/2004
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