Batteries

In science and technology, a battery is a device that stores chemical energy and makes it available in an electrical form. Batteries consist of electrochemical devices such as one or more galvanic cells, fuel cells or flow cells. more...

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The earliest known artifacts that may have been batteries are the Baghdad Batteries, from some time between 250 BC and 640 AD. The modern development of batteries started with the Voltaic pile, announced by the Italian physicist Alessandro Volta in 1800. The worldwide battery industry generates US$48 billion in sales annually (2005 estimate).

Battery concepts

A battery is a device in which chemical energy is directly converted to electrical energy. It consists of one or more voltaic cells, each of which is composed of two half cells connected in series by the conductive electrolyte. In the figure to the right, the battery consists of one or more voltaic cells in series. (The conventional symbol does not necessarily represent the true number of voltaic cells.) Each cell has a positive terminal, shown by a long horizontal line, and a negative terminal, shown by the shorter horizontal line. These do not touch each other but are immersed in a solid or liquid electrolyte.

The electrolyte is a conductor which connects the half-cells together. It also contains ions which can react with chemicals of the electrodes. Chemical energy is converted into electrical energy by chemical reactions that transfer charge between the electrode and the electrolyte at their interface. Such reactions are called faradaic, and are responsible for current flow through the cell. Ordinary, non-charge-transferring (non-faradaic) reactions also occur at the electrode-electrolyte interfaces. Non-faradaic reactions are one reason that voltaic cells (particularly the lead-acid cell of ordinary car batteries) "run down" when sitting unused.

Around 1800, Alessandro Volta studied the effect of different electrodes on the net electromotive force (emf) of many different types of voltaic cells. (Emf is equivalent to what was called the internal voltage source in the previous section.) He showed that the net emf (E) is the difference of the emfs Ε1 and Ε2 associated with the electrolyte-electrode interfaces within the two half-cells. Hence identical electrodes yield Ε=0 (zero emf). Volta did not appreciate that the emf was due to chemical reactions. He thought that his cells were an inexhaustible source of energy, and that the associated chemical effects (e.g., corrosion) were a mere nuisance -- rather than, as Michael Faraday showed around 1830, an unavoidable by-product of their operation.

Voltaic cells, and batteries of voltaic cells, are rated in volts, the SI unit of electromotive force. The voltage across the terminals of a battery is known as its terminal voltage. The terminal voltage of a battery that is neither charging nor discharging (the open-circuit voltage) equals its emf. The terminal voltage of a battery that is discharging is less than the emf, and that of a battery that is charging is greater than the emf.

Read more at Wikipedia.org