Secondary Battery

A voltaic battery whose positive and negative electrodes are formed or deposited by a current from a separate source of electricity by electrolysis. On disconnection the battery is ready to yield a current, in the reverse direction of that of the charging current. The usual type has lead plates on one of which lead binoxide and on the other of which spongy lead is formed. The lead binoxide seems to be the negative element, and it also acts as the depolarizer. The spongy lead is the positive electrode. The solution is dilute sulphuric acid of specific gravity 1.17. The action consists first in the oxidation of the spongy lead. The hydrogen set free by the reaction, and which by electrolytic transfer goes to the other plate, reduces the lead binoxide to protoxide. The sulphuric acid then attacks the oxides and converts the oxides into sulphates.

The charging process consists in sending a current in the reverse direction through the battery. If there are several cells they are arranged in series, so that each one receives the same intensity of current. An electrolytic decomposition takes place, the lead sulphateon one plate is reduced to metallic lead, and that on the other plate is oxidized to lead binoxide. It is then ready for use.

The plates in a lead plate battery are of very large area per cell, and are placed close together. Sometimes, as in Planté's battery,large flat plates are laid together with a separating insulator between them, and are then rolled into a spiral. Sometimes, the most usual arrangement, the plates are in sets, the positive and negative ones alternating, and each cell containing a number of plates.

To secure a good quantity of active material, the plates are sometimes perforated, and the perforations are filled with oxide of lead. This gives a good depth of material for the charging current to act on, and avoids the necessity for a tedious "forming," q. v.

The electro-motive force of such a battery per cell is 2 volts. Its resistance may only be one or two-hundredths of an ohm. An intense current of many amperes can be supplied by it, but to avoid injuring the cell a current far less than the maximum is taken from it.

To charge it, a slightly greater electro-motive force, the excess being termed spurious voltage, is required.

Fig. 48. SIEMENS' AND HALSKE'S PAPER PULP BATTERY.