Fig. 121 is a view of the special AMBU Voltmeter, which is designed to be used specially in making Cadmium tests. Fig. 122 shows the Cadmium leads. The four red lines marked "Neg. Charged," "Neg. Discharged," "Pos. Charged," and "Pos. Discharged," indicate the readings that should be obtained. Thus, in testing the positives of a battery on charge, the pointer will move to the line which is marked "Pos. Charged," if the positive plates are fully charged. In testing the negatives, the pointer will move to the line marked "Neg. Charged," which is to the left of the "0" line, if the negatives are fully charged, and so on. Figs. 123, 124, 125 and 126 show the pointer in the four positions on the scale which it takes when testing fully charged or discharged plates. In each figure the pointer is over one of the red lines on the scale. These figures also show the readings, in volts, obtained in making the cadmium tests on fully charged or completely discharged plates.
If Pointer Is Not Over the "0" Line: It sometimes happens, in shipping the instrument, and also in the use of it, that the pointer does not stand over the "0" line, but is a short distance away. Should you find this to be the case, take a small screwdriver and turn the screw which projects through the case, and which is marked "Correct Zero," so as to bring the pointer exactly over the "0" line on the scale while the meter has no wires connected to its binding posts.
Connections of Cadmium Leads: In making Cadmium Tests, connect the prod which has the cadmium fastened to it to the negative voltmeter binding post. Connect the plain brass prod to the positive voltmeter binding post. The connections to the AMBU Cadmium Voltmeter are shown in Fig. 127.
Testing a Battery on Discharge
The battery should be discharging continuously, at a constant, fixed rate, see page 265.
Generally, on a starting ability test (see page 267), the positive Cadmium readings will start at about 2.05 volts for a hard or very new set of positives, and at 2.12 volts or even higher for a set of soft or somewhat developed positives, and will drop during the test, ending at 1.95 volts or less. The negative Cadmium readings will start at 0.23 volt or higher, up to 0.30, and will rise gradually, more suddenly toward the end if the plates are old, ending anywhere above 0.35 and up to 0.6 to 0.7 for poor negatives.
Short Circuited Cells: In cases of short circuited cells, the voltage of the cell will be almost down to zero. The Cadmium readings would therefore be nearly zero also for both positives and negatives. Such a battery should be opened for inspection and repairs.