Secondary Batteries

Secondary Batteries, ACCUMULATORS, or STORAGE CELLS. When an electric current is passed from one metallic plate to another through an electrolyte, the electrodes are polarised [POLARISATION], and, by suitably arranging matters, a considerable amount of energy may be stored up in this polarisation and subsequently be available as an electric current. In Plante's original secondary cell two large sheets of lead were immersed in dilute sulphuric acid; on connecting these to a source of electric current, the anode becomes covered with a film of lead peroxide, while hydrogen reduces any oxide which may be present on the cathode. If now the plates are disconnected from the charging dynamo or battery, they will be found to differ in potential by rather more than 2 volts, and, if connected, will give a current which is in the reverse direction to the charging current. In discharging, the peroxide plate is reduced and the lead plate is oxidised. Such a cell has a very small capacity, which may be largely increased by repeated charging and discharging, the polarity of the plates being reversed between each operation. This process of forming reduces the surface of the plates to a spongy condition, whereby their active area is much increased. In order to effect the same result more easily, Faure coated the plates with red-lead, held in place by felt, which was packed between them, and which was converted by the forming process into lead peroxide on one plate and spongy lead on the other. An improvement on this plan was to cast the plates in the form of grids with many small holes, and to fill these with a paste of lead oxide and sulphuric acid. Many methods have also been devisen for making spongy lead plates, the result to be aimed at in either case being a plate exposing a very large area of active material to the electrolyte. The chemical reactions which occur in secondary cells are of an exceedingly complicated cbaracter, and have been much discussed, but the subject is too extensive to be entered upon here. Commercial accumulators usually consist of a number of plates alternatively positive and negative placed in glass boxes, suitable arrangements being made for connecting similar and insulating dissimilar plates. Such cells have important applications in electric lighting; in private installations the machinery may be run, say, one day in seven, as much energy being then stored as is needed during the week. In supplying electricity from central stations, the demand is practically confined to a few hours in the twenty-four, so that much less plant is needed if the machinery is run continuously, and the energy stored in accumnlators for use when required; but, owing to the initial cost and maintenance of, and losses in, secondary batteries, it is as yet doubtful if any real economy is attained by this means. When in good order, the current efficiency of storage cells (that is, the ratio of ampere hours of discharge to ampere hours of charge) is about 0.85; but, as the charging pressure must always be in excess of the discharging pressure, the watt efficiency is only about 0.75. Accumulators are also used for the propulsion of electric launches, and have been often tried for street railways, but with donbtful success in the latter case. Some other forms of secondary cells, which are practically reversed primary cells - such as Daniells (q.v.) have been proposed, but are of small practical importance.