These batteries have already been mentioned as secondary batteries. They are sometimes called electric accumulators. The electricity is stored or accumulated, not by reason of the destruction of an electrode as in the primary cell but by the chemical change that takes place in the plates as the charging current is sent through the cell. When the battery is discharged, the current from the dynamo is sent through the battery circuit in the reverse direction to that of the discharge and the plates are restored to their original condition. The action that takes place in charging and discharging is due to chemical changes that take p}ace in the plates and also in the solution or electrolyte in which the plates are immersed.

There are two types of storage batteries, those made of lead plates immersed in an acid electrolyte and the Edison battery which is composed of iron-nickel cells immersed in a caustic potash electrolyte. The former type is most commonly used and is the one to be described.

The lead-plate cell illustrated in Fig. 262 shows all of the parts of a working element. The plates are made in the form of lead grids which when filled to suit the requirements of their action, form the positive and negative electrodes. The negative plates are filled with finely divided metallic lead which when charged are slate gray in color. The positive plates are filled with lead oxide. When charged they are chocolate brown in color. In the figure there are three positive and four negative plates which together form the element, then with their separators are placed in a solution of sulphuric acid electrolyte. The separators are thin pieces of wood and perforated rubber plates that keep the positive and negative plates from touching each other and keep in place the disintegration produced by the electro-chemical action of the cell.

The unit of electric capacity in batteries is the ampere-hour. The cell illustrated will accumulate 80 ampere-hours of energy. It will discharge an ampere of current for 80 hours. If desired it may be discharged at the rate of two amperes for 40 hours, or four amperes for 20 hours, or at any other rate of amperes and hours, the product of which is 80. The number of ampere-hours a cell will accumulate will depend on the area of the positive and negative plates; large cells will store a greater number of ampere-hours than those of small size.

The cells, no matter what size, give an average electric pressure of 2 volts.

The plates are joined by heavy plate-straps connecting all of the positives on one end and all of the negative kind on the opposite end. To insure rigidity the two sets are secured to the rubber cover by locknuts. In this cell the plates are suspended from the cover. The plate terminals are made of heavy lead connectors that when formed into a battery are joined together with lead bolts and nuts.

The electrolyte is a solution of pure sulphuric acid in distilled water and on its purity depends, in a great measure, its action and length of life. The electrolyte is made of a definite density which is expressed as its specific gravity. When fully charged the electrolyte will test 1220 by the hydrometer. That is, it will be 1.220 heavier than water. When discharged it will test by the hydrometer 1185. This means that in discharging the density has been reduced to 1.185 that of water. The chemical change in the electrolyte is, therefore, an important part of the charge and discharge of the cell. The density of an electrolyte may be determined by a hydrometer such as Fig. 261. This is an ordinary glass hydrometer such as is used for determining the density of fluids, enclosed in a glass tube, to which is attached a rubber bulb. The point of the tube is inserted into the opening at the top of the cell and the electrolyte drawn into the tube by the reopening of the collapsed bulb. The density is then read from the stem of the hydrometer.

Fig. 261.   Hydrometer for testing storage battery electrolyte.

Fig. 261. - Hydrometer for testing storage battery electrolyte.