(6) Check the speed of the high or low speed driving head using either a
hand tachometer or strobotac of known accuracy. The tachometer circuit selector
remains in the DIRECT DRIVE position during this check. The tachometer must agree
with the hand tachometer or strobotac.
(7) Set the drive speed control clockwise (reducing varidrive speed).
(8) Depress the STOP button.
(9) Place the circuit breaker switch to OFF.
f. The preceding checks must be made each time the test stand is operated.
Once these checks are completed the operator can begin the testing procedure
required by a specific item of equipment. The mounting of units under test and the
specific tests required will be discussed in later lessons.
11. LEAD-ACID BATTERIES (GENERAL CONSTRUCTION).
a. Storage batteries of the lead-acid type are called wet-cell batteries.
The chemical action between two unlike conductors and an electrolyte produces
electrical energy. Stiff lead alloy is used to construct the grid network of both
positive and negative plates. This grid network gives the plates strength. The
active material is applied to the grids in paste form and allowed to dry. Brown
lead peroxide is applied to the positive plates and grey spongy lead to the
negative plates. Plates are combined into positive and negative groups by welding
lugs on the top corner of each plate to a lead plate strap. The plate strap on
each group has a terminal post which serves as an outside electrical connection.
b. An element is formed by putting positive and negative plate groups
together. Then separators of wood, rubber, or glass are inserted between positive
and negative plates to prevent them from touching. The separators have many small
holes so that the electrolyte can pass through them. When an element is immersed
in electrolyte, it becomes a cell. The voltage of a fully charged cell of this
type is about 2.1 volts.
c. The electrolyte is a mixture of sulfuric acid and pure water. The
electrolyte of a fully charged cell contains about 38 percent sulfuric acid by
weight (27 percent by volume). The condition of a cell can be checked by measuring
the specific gravity of the electrolyte. Fully charged cells have a specific
gravity of 1.280 at 80 F. This figure states that the electrolyte is 1.280 times
heavier than pure water.
d. Several cells are connected in series to form batteries larger than 2
volts. Three cells are required for 6-volt batteries and six cells for 12-volt
batteries. The cell terminals are connected by welding them to connector straps.
Cells are placed in a one-piece container that is divided into compartments. Each
compartment is the container for one cell. Figure 16 shows a cutaway view of a
single cell. The element rests on raised ribs. The area between the ribs serves
as sediment space. During use, the active material on the plates gradually sheds
and falls into the sediment space. The container is constructed of insulating
material which is resistant to acid corrosion and mechanical shock.
OS 010, 3-P21
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