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Batteries consist of one or more electric cells which produce electric currents directly from chemical reactions. The first battery was the Voltaic Pile, named after Alessandro Volta (1745 - 1827). Volta discovered that, by using dissimilar metals immersed in water with a little acid added, an electric current was produced. Batteries have come a long way since then and are now used in everything from a watch to powerful fuel cells.
The simplest form of electrical battery comprises a vessel containing an electrolyte such as a solution of salt in water and two dissimilar metals immersed in the electrolyte, but not touching. In this situation a voltage difference can be measured between the two plates which depends on the positions of the metals in the electrochemical services. (This is a list of metals in a specific order from most electropositive to most electronegative. The further apart the metals are in the series the greater the voltage developed). If a copper plate and a zinc plate are connected by a conductor current will flow.
The so-called dry battery replaces the liquid solution with a conducting jelly but the principle of its function is the same.
If high voltages are required a number, N, of such cells can be connected in series copper to zinc at each junction and the voltage between the ends will then be N times the voltage across one cell. The disadvantage of these batteries is that they cannot provide large currents due to the internal resistance of each cell and the energy stored is small. The cost per kilowatt hour of electricity from a battery may be several hundred times that of electrical mains energy.
Where electrical energy has to be available from a portable source the lead accumulator may be used. This is formed by two lead plates with a series of rectangular recesses on their surface. One of these plates has the recesses filled with lead oxide and the vessel is filled with sulphuric acid. If the cell has a constant voltage applied across the plates current flows and energy is stored. When the cell is fully charged it will retain this for long periods and deliver the energy when required. This is the basis of most batteries in motor cars. The charge in the battery developed by the generator when the engine is running is available to restart the engine when required even after periods of weeks or months. Several cells are joined together to form an accumulator to provide whatever voltage is required. The disadvantages of the lead accumulator restrict the areas in which it can be used. These are its weight, the presence of the corrosive acid which is dangerous if spilled, the fact that when charging hydrogen gas is produced which can be explosive in the presence of a flame, the fact that the lead plates will be irreparably damaged if too large a current is drawn from the battery and therefore that the battery can only be recharged at a slow rate. This slow rate of charge is also necessary because under rapid charging gas bubbles collect on the plates and prevent the acid from having free contact. AA
Further reading P. Dunsheath, A History of Electrical Engineering. |
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