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Batteries<^< Batteries, Fuses, Lamps and Switches | Course Index | Examples: Batteries >^>  Batteries are energy storage devices that convert electrical energy to chemical energy when charging, and chemical energy to electrical energy when discharging. Batteries are usually made up from a number of cells. Cells may be connected in series to provide larger voltages (e.g. a nominal 12V or 24V d.c. supply) in which case the terminal voltage is the sum of the individual cell voltages. Parallel connection of individual cells is not usually recommended. The cells that make up a battery fall into one or two principal categories, primary, which end their useful life when their chemical constituents are are exhausted, and secondary, in which the chemical reaction may be reversed, thus permitting recharging. Leclanche (zinc carbon) cells are perhaps the most common example of the former type whilst nickel cadmium cells are an example of the latter type. The capacity of a battery is the amount of electrical energy which it can deliver and is simply the product of its rated discharge current (A) and discharge time (hours). Capacity is thus normally quoted in ampere-hours (Ah). Unfortunately, the capacity of a battery is somewhat dependent upon the rate of discharge and thus, to be strictly meaningful, the ampere-hour rate should be specified for a given discharge rate. Depending upon the application envisaged, a secondary cell is normally rated for discharge periods of between four and sixteen hours. The energy content of a battery is specified in watt-hours (Wh) and is the product of the cell's capacity (Ah) and the average voltage of the cell (V) on discharge. The voltages produced by some common types of cell are shown in the table:
<^< Batteries, Fuses, Lamps and Switches | Course index | Examples: Batteries >^> |