Battery

Between 1790 and 1800, Luigi Galvani, lecturer in anatomy at the University of Bologna, and Alessandro Volta, professor of physics at Pavia University, began the science of electrochemistry. Galvani observed the effect of a copper probe on the muscles of a frog hung from an iron hook (the muscles twitched), and Volta interpreted this phenomena as the result of two metals being near each other, separated by an electrolyte (the blood of the frog).

Volta later built a stack of alternating zinc and silver disks separated by layers of paper or cloth soaked in a solution of sodium hydroxide or brine. He thus created a stable source of electrical current.

In 1834 Michael Faraday, inspired by Volta's results, derived the quantitative laws of electrochemistry. These established the fundamental relationships between chemical energy and electrical energy. Following Faraday's work, the following cells were developed:

• Copper and zinc in sulfuric acid (1836)
• Platinum cathode immersed in dilute nitric acid with a zinc anode in another compartment containing sulfuric acid
• Carbon cathode immersed in dilute nitric acid with a zinc anode in another compartment containing sulfuric acid
• Lead/acid battery (1859)
• LeClanche wet cell with a zinc anode and a cathode of naturally occurring manganese dioxide (1866)
• The first dry cell, consisting of a moistened cathode and a swollen starch or plaster of paris separator (1888) battery, dry cell nickel/cadmium and nickel/iron cells developed (1895-1905)
• Silver oxide/zinc cell (1930s and 1940s)

If an incandescent lamp is connected to the two poles of a battery, an electric current flows through the lamp, illuminating it. As current flows through the electrolyte from the positive electrode to the negative one, gas bubbles are deposited on the electrodes, and an internal resistance to current flow builds up. To prevent this depolarization, the buildup of hydrogen gas at the positive electrode (anode) must be prevented to keep the cell functioning.

In the LeClanche cell, depolarization is prevented by enclosing the carbon anode with a mixture of manganese dioxide and graphite. The cell uses a zinc negative pole (cathode) and an ammonium chloride electrolyte. The potential difference between the poles is 1.3 volts. The potential difference does not depend on the size of the cell. (However, the size of the cell does affect the current intensity A collection of batteries seen from above, with the top of the center battery cut away to show the interior. This type of battery, a dry version of the LeClanche cell, is called a carbon-zinc primary cell. A carbon rod sits upright in the center and acts as a cathode (negative electrode). The outer vessel is made of zinc, which allows it to serve as both a container and as an anode (positive electrode). The electrodes are in contact with a moist electrolytic paste (the grey material that fills the battery) consisting of ammonium and zinc chlorides, manganese dioxide and carbon particles. © Dr. Jeremy Burgess/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission.

or amperage that can be delivered.) Chemical energy, which is converted into electrical energy, results as the zinc electrode dissolves and is consumed. Thus the zinc must be renewed from time to time. Cells in which the electrodes are consumed are called primary cells.

A secondary cell can be restored to its original state by charging it, i.e., passing an electric current through it so that the electrodes are regenerated. These cells are also called storage cells or accumulators. They are usually used as groups of two or more cells. A commonly used storage cell consists of lead plates with a dilute sulfuric acid electrolyte. A layer of lead sulphate forms on the plates. When the storage cell is charged, the layer on the anode plate changes to lead dioxide, and the cathode is reduced to lead. Thus one electrode consists of lead and the other of lead dioxide. The electrodes and electrolyte together function as a galvanic cell. The stored chemical energy is converted back to electrical energy on discharging. The nickel-iron storage cell, another secondary cell, uses a potassium hydroxide electrolyte. The lead storage cell produces a potential difference of about 2 volts; the nickel-iron cell a difference of 1.36 volts.

In a dry cell, the electrolyte is in the form of a paste instead of a liquid. Higher voltages are produced by connecting the cells in series. Higher current intensities are produced by connecting cells in parallel. All cells produce direct current, i.e., electric current that flows in one direction.