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lead-acid battery


A battery is familiar as a device for storing electricity. But, it doesn't hold electrons densely packed. Early experimenters in electricity, such as Benjamin Franklin, had a device that did just that. A Leyden jar is a bottle lined inside and out with metal foil. It holds electricity, but with limitations. A Leyden jar is an early example of what we today call a capacitor.

An actual battery produces electricity by chemical reaction. Alessandro Volta showed this around 1780 with a stack of copper and zinc discs. The discs were separated by cardboard soaked with brine and therefore electrically conductive. A wire connected between the top and bottom of this "voltaic pile" would contain an electric current.

Electricity was a curiosity until the adoption of the telegraph in the nineteenth century. Telegraph offices used a variation of the voltaic pile to generate a current. This was a pot of sulphuric acid containing a zinc rod, all resting in an outer copper pot holding copper sulfate. This came to be called a wet cell, with the copper pot and zinc rod functioning as electrodes.

A wet cell is useful, but it has the same spillage problem as a bucket of water. Efforts to improve upon the wet cell went in two different directions. The lead-acid battery, still familiar for use in cars, was invented by Gaston Planté in 1859. It is still a wet cell and not quite sealed, but it is mostly enclosed and can generate a very high peak current.
The other avenue of research in the late 19th century was the dry cell, that is a battery without liquids. The liquid is replaced by a conductive paste serving as the electrolyte. Various substances were tried. In 1886, German Carl Gassner had success by using plaster of Paris in a zinc cup. The inner metal electrode is manganese dioxide.

Commercial versions of this carbon-zinc battery were produced around 1900 by the National Carbon Company (using coiled cardboard between a zinc casing and carbon electrode). The actual second metal in the device is a manganese dioxide (MnO2) paste around the carbon. Carbon-zinc batteries are still manufactured in great numbers.

Modern battery research looks to create devices that produce more electricity for their weight or volume. Rechargeable batteries are especially desirable for saving on manufacturing and disposal costs. (A lead-acid battery is actually rechargeable.) Specialized cells are designed for a huge number of applications.

What is a Battery

A battery has two electrodes that produce positive and negative electric charges. In between the electrodes is a conductive electrolyte that may be a liquid, a gel, or a solid. The battery works with an internal chemical reaction.

A chemical reaction involves sharing electrons between atoms. Normally, the atoms are close together. Two hydrogen atoms give up an electron each to an oxygen, and the energy release holds the resulting water molecule together.

In a battery, however, the two atoms that would react are separated by a conductive electrolyte. Each can gain or lose electrons, but that causes a voltage to build (and a current to flow if the terminals are connected).

A battery has two terminals or electrodes that are made of different materials. The material that gives up electrons, such as zinc, becomes the negative terminal. The material that gains electrons, such as a manganese-dioxide and carbon paste, becomes the positive terminal.

The voltage produced by a battery is determined by its chemistry. Carbon-zinc batteries produce 1.5 volts, for example. Two batteries in series will produce 3.0 volts, and so on. The actual voltage drops with use as the battery is drained.

Batteries that are not rechargeable eventually react most of the material in one or both electrodes. The battery accumulates chemical products that increase the resistance of the cell, and the voltage drops below a useable value. Most batteries actually undergo some internal reactions just sitting unused, and they are eventually depleted.

Rechargeable batteries use a chemical reaction that can be reversed. The battery is put in a device that applies a large reverse voltage. The actual current is normally kept low, otherwise the battery can heat up and be damaged. Consequently, there is a tradeoff between how long the recharging takes and how safe or complete it is.

Types of Batteries

Alkaline batteries are the familiar batteries that come in sizes such as AA, AAA, and D. These are an improved version of carbon-zinc batteries with a better electrolyte, and they also produce 1.5 volts. Alkaline batteries have been in use for about 60 years and they represent most of the batteries used in consumer products.

Lead-acid batteries are the wet cells used as automobile batteries. They are heavy for their energy output, but they are cheap and can put out the large current that a car starter motor requires. They are also rechargeable.

Nickel-cadmium and nickel-metal hydride batteries are similar rechargeable batteries that are used in consumer devices. Nickel-cadmium (NiCd) batteries use a cadmium electrode and a nickel oxide electrode. Nickel-metal hydride (NiMH) batteries replace toxic cadmium with one of a number of metal alloys. NiMH batteries have a higher capacity. Both types are rechargeable.

Lithium-ion batteries (LIBs) use a porous lithium compound forthe positive terminal and usually graphite for the negative terminal. Lithium ions move within the electrolyte from terminal to terminal during discharge, and this movement is reversed by recharging. LIBs have a very high energy density but can be expensive. They are used in the highest-quality consumer electronics, in electric cars, and in situations where cost is not important (such as military).

Environmental Concerns

Batteries involve both good and bad environmental concerns. On the good side, efficient batteries allow electricity to be generated in clean, sustainable ways when and where it is available. The power can then be applied when and where it is needed. Solar power is the prime example of this, available in the day and needed at night. Batteries also make possible electric cars.

At least some batteries present a disposal hazard. Lead and cadmium in particular should be turned in for recycling. Disposable batteries are simply waste, unless they contain metals such as copper that are valuable to recycle.
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