Fig. 265.—The positive plate of a storage cell.
Fig. 266.—The negative plate of a storage cell.
Fig. 267.—A complete storage cell.
Fig. 268.
About 75 per cent. of the energy put into the storage cell in charging can be obtained upon discharging. Therefore the efficiency of a good storage cell is about 75 per cent. Fig. 268 represents a storage battery connected to charging and discharging circuits. The lower is the charging circuit. It contains a dynamo and a resistance (neither of which are shown in the figure) to control the current sent into the cell. The charging current enters the positive pole and leaves by the negative pole. The current produced by the cell, however, flows in the opposite direction through it, that is, out from the positive and in at the negative pole. This current may be controlled by a suitable resistance and measured by an ammeter. Storage cells have several advantages: (a) They can be charged and discharged a great many times before the material placed in the perforations in the plates falls out. (b) The electrical energy used in charging the plates costs less than the plates and electrolyte of voltaic cells. (c) Charging storage cells takes much less labor than replacing the electrolyte and plates of voltaic cells. (d) Storage cells produce larger currents than voltaic cells. The two principal disadvantages of storage cells are that (a) they are very heavy, and (b) their initial cost is considerable.
Fig. 269.—The Edison storage cell.
Fig. 270.—The plates of the Edison storage cell.
289. The Edison storage cell (Figs. 269 and 270) has plates of iron and nickel oxide. The electrolyte is a strong solution of potassium hydroxide. These cells are lighter than lead cells of the same capacity and they are claimed to have a longer life.
290. Energy and Power of a Storage Cell.—In a storage cell, the electrical energy of the charging current is transformed into chemical energy by the action of electrolysis. It is this chemical energy that is transformed into the energy of the electric current when the cell is discharged. The capacity of storage cells is rated in "ampere hours," a 40 ampere hour cell being capable of producing a current of 1 ampere for 40 hours, or 5 amperes for 8 hours, etc. The production and extensive use of electric currents have made necessary accurate methods for measuring the energy and power of these currents. To illustrate how this is accomplished, let us imagine an electric circuit as represented in Fig. 268. Here four storage cells in series have an E.M.F. of 8 volts and in accordance with Ohm's law produce a current of 2 amperes through a resistance in the circuit of 4 ohms. Now the work done or energy expended by the current in passing through the resistance between the points M and N depends upon three factors (1) the E.M.F. or potential difference; (2) the current intensity and (3) the time. The energy is measured by their product. That is, electrical energy = potential difference × current intensity × time. This represents the electrical energy in joules, or
Joules = volts × amperes × seconds, or
j = E × I × t.