n of the capacity of a
battery, and the repairman must be careful in drawing conclusions from
Cadmium tests.

In general it is not always safe to depend upon Cadmium tests on a
battery which has not been opened, unless the battery is almost new.
Plates having very little active material, due to shedding, or due to
the active material being loosened from the grid, will often give good
Cadmium readings, and yet a battery with such plates will have very
little capacity. Such a condition would be disclosed by an actual
examination of the plates, or by a capacity discharge test.

How Cadmium Tests Are Made

To measure the voltages of the positives and negatives separately,
Cadmium is used. The Cadmium is dipped in the electrolyte, and a
voltage reading is taken between the Cadmium and the plates which are
to be tested. Thus, if we wish to test the negatives, we take a
voltage reading between the Cadmium and the negatives, as shown in
Fig. 119. Similarly, if we wish to test the positives, we take a
voltage reading between the Cadmium and the positives, as shown in
Fig 120.

[Fig. 119 Making cadmium test on negative plates]

[Fig. 120 Making cadmium test on positive plates]

In dipping the Cadmium into the electrolyte, we make two cells out of
the battery cell. One of these consists of the Cadmium and the
positives, while the other consists of the Cadmium and the negatives.
If the battery is charged, the Cadmium forms the negative element in
the Cadmium-Positives cell, and is the positive element in the
Cadmium-Negatives cell. The voltage of the Cadmium does not change,
and variations in the voltage readings obtained in making Cadmium
tests are due to changes in the state of charge of the negative and
positive plates which are being tested.

What Cadmium Is: Cadmium is a metal, just like iron, copper, or lead.
It is one of the chemical elements; that is, it is a separate and
distinct substance. It is not made by mixing two or more substances,
as for instance, solder is made by mixing tin and lead, but is
obtained by separating the cadmium from the compounds in which it is
found in nature, just as iron is obtained by treatment of iron ore in
the steel mill.

When Cadmium Readings Should Be Made

1. When the battery voltage drops to 1.7 per cell on discharge before
the battery has delivered its rated ampere-hour capacity, at the
5-hour rate when a discharge test is made.

2. When a battery on charge wil