Direct Current Power Systems 167
charge and the operating temperature of the battery. The normal operat-
ing temperature is considered to be 800 Fahrenheit.Nickel-iron Cells
The nickel-iron (or “Edison”) cell, shown in Figure 7-7 is a secondary
cell constructed with an unbreakable case of welded nickel-plated sheet
steel. The positive plate is made of nickel tubes about V. inch in diam-
eter and 4-1/4 inches long. Nickel oxide (NiO 2 ) is contained in the tubes,
which are mounted into steel grids and forced into position under high
pressure. The positive plates are assembled into groups. The negative plates
of the Edison cell are made of flat nickel-plated steel pockets, which are
composed of iron oxide. They are built into steel grids in groups, in the
same way as the positive plates. The electrolyte is a solution of potassium
hydroxide (KOH) with lithium hydroxide added.
Chemical action of the nickel-oxide cell is very complex as compared
to that of the lead-acid cell; however, they are similar in many respects.
The potassium hydroxide electrolyte (KOH) breaks up into negative and
positive ions. The negative ions move toward the iron plate, oxidize the iron,
and give up excess electrons to the plate. This plate becomes negatively
charged. The positive ions move to the nickel-oxide plate and take elec-
trons from the plate. The deficiency of electrons causes the nickel-oxide
plate to become positively charged. The voltage produced by the chemical
action of a nickel-iron cell is about 1.4 volts.
The internal resistance of the nickel-iron (Edison) cell is higher than
that of the lead-acid cell. This resistance increases quickly as the cell dis-
charges. Unlike the lead-acid cell, the nickel-iron cell cannot be tested us-
ing a hydrometer to show its discharge level. A voltmeter is used to indi-Figure 7-7. Cutaway drawing of a nickel-iron (Edison) secondary cell