168 Electrical Power Systems Technology
cate the state of charge of the nickel-iron cell. This cell is also charged by
applying a DC voltage to the positive and negative terminals and causing
current within the cells to flow in reverse. The chemicals are thus reacti-
vated so that the cells may be used again.
Other Types of Secondary Cells
Another type of secondary cell that has gained recent popularity is the
nickel-cadmium cell, sometimes called a “Ni-cad” battery. These cells are avail-
able in large or small sizes with capacities up to 2000 ampere-hours. Figure
7-8 shows the small rechargeable type of nickel-cadmium cell, which is used
extensively in portable equipment. The positive plate of this cell is nickel hy-
droxide, the negative plate is cadmium hydroxide, and the electrolyte is po-
tassium hydroxide. These cells are extremely reliable, operate over a wide
range of temperatures, and have a long life expectancy. A fully charged
nickel-cadmium cell has a voltage of approximately 1.35 volts per cell.
There are a few other types of secondary cells in use today. Most of
these types are for specialized applications. Other cells include the silver-
oxide-zinc cell and the silver-cadmium cell. These cells have the highest out-
put per physical size and the longest life, but they are more expensive than
other cells.
Applications of Secondary Cells
Secondary cells, in the form of storage batteries, are used in industry
and commercial buildings to provide emergency power in the event of a
power failure. Such standby systems are necessary to sustain lighting and
some critical operations when power is not available. Industrial trucks
and loaders use storage batteries for their everyday operation. Many
types of instruments and portable equipment rely upon batteries for pow-
er. Several of these instruments get their power from rechargeable second-
ary cells rather than primary cells. Railway cars use batteries for lighting
when they are not in motion. Of course, automotive systems of all kinds
use secondary batteries to supply DC power for starting and lighting.
DC GENERATING SYSTEMS
Mechanical generators are used in many situations to produce direct
current. These generators convert mechanical energy into DC electrical
energy.