M
AVTM246004 BITE 2/2P Rev 4 July 2011
64
•
Flooded lead-acid cells having variations (from
baseline) greater than 20%
•
Valve-regulated, lead-acid cell having variations greater
than 50%
•
Nickel-cadmium cells having variations greater than
100%.
A sample curve for a generic valve-regulated, lead-acid cell
is shown in Figure 7-1. Curves may differ for other
manufacturers and battery chemistries, such as nickel-
cadmium.
The initial decrease in
impedance is due to the
completion of the formation
process, i.e., the conversion to
active plate material.
Impedance vs. Expected Battery Life
VRLA Cells
-30
-20
-10
0
10
20
30
40
50
60
0
20
40
60
80
100
120
% of Expected Battery Life
% Impedance Change from Baseline
WEAK
NORMAL
ABNORMAL
Figure 7-1:
Generic
curve of impedance vs. cell life
Temperature Corrections
The internal impedance of a cell is inversely influenced by
temperature. In addition, the degree of influence depends on
battery type and length of exposure to the present ambient
temperature.
Flooded battery types have a significantly more thermal mass
and are slower to react to changing ambient temperatures
than VRLA cells. The actual internal cell temperature can be
measured by inserting a thermometer into the flooded cell via
a fill tube or vent cap to determine the temperature present
during impedance testing. Measure the temperature of VRLA
cells at the negative terminal.
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