6.1
Capacity
The block battery capacity is rated in
ampere hours (Ah) and is the quantity
of electricity which it can supply for
a 5 hour discharge to 1.0 volts after
being fully charged for 7.5 hours at
0.2C
5
A. This figure conforms to
the IEC 623 standard.
6.2
Cell voltage
The cell voltage of nickel-cadmium
cells results from the electrochemical
potentials of the nickel and the
cadmium active materials in the
presence of the potassium hydroxide
electrolyte. The nominal voltage
for this electrochemical couple is
1.2 volts.
6.3
Internal resistance
The internal resistance of a cell varies
with the temperature and the state
of charge and is, therefore, difficult
to define and measure accurately.
The most practical value for normal
applications is the discharge voltage
response to a change in discharge
current.
The internal resistance of a block
battery cell depends on the
performance type and at normal
temperature has the values given
in Table 3 in milliohms per Ah
of capacity.
To obtain the internal resistance of a
cell it is necessary to divide the value
from the table by the rated capacity.
For example, the internal resistance of
a SBH 118 (module type 3) is given
by:
39 = 0.33 m
Ω
118
The figures of Table 3 are for fully
charged cells. For lower states
of charge the values increase.
For cells 50 % discharged the internal
resistance is about 20 % higher, and
when 90 % discharged, it is about
80 % higher. The internal resistance
of a fully discharged cell has very
little meaning.
Reducing the temperature also
increases the internal resistance, and
at 0°C, the internal resistance is
about 40 % higher.
12
6.
Operating features
Table 3 - Internal resistance in relation to rated capacity
Cell type
Module plate size (see table 1)
1
2
3
4
SBL
84
105
123
142
SBM
55
62
78
86
SBH
N/A
30
39
43
BlockBat 3/11/98 10:12 Page 15
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