Page 122 - Battery Reference Book

4/6 Nickel batteries

is inhibited and therefore, under normal circumstances, the internal resistance of a sealed nickel-cadmium bat-
charging at low temperatures is not recommended. In tery is dependent mainly on plate surface area, distance
addition, at sub-zero temperatures hydrogen evolution between the plates, separator resistance, amount and
may be promoted (cathode overcharge), causing vent- density of electrolyte, and temperature. The internal
ing of gas and consequent electrolyte loss. The cells resistance will also change with the state of charge.
have a negative temperature coefficient of approxi- As an example, the d.c. internal resistance of typically
mately 4mVkell per 0°C. When selecting cells for fully charged 1 Ah sealed cells is as follows:
low-temperature operation, both their reduced capacity
and changed cell voltage should be taken into consid- Standard button cell 110 mQ/cell
50 mWcell
eration. Heavy-duty button cell
Rolled sintered cell 19 mWcell
Eficiency In some applications it is an advantage to know the a.c.
internal resistance. As a comparison, the a.c. resistance
Sealed nickel-cadmium cells with mass plate elec- at 1OOOHz is:
trodes have an ampere hour efficiency of approxi-
mately 72%, while those with sintered electrodes have Standard button cell 42 mWcel1
approximately 84% efficiency at about 20°C. (The Heavy-duty button cell 14 mQ/cell
effect of temperature will be discussed later.) For this Rolled sintered cell 17 mWcell
reason the charging factor for mass plate cells is 1.4
and for sintered plates 1.2; therefore to fully charge Cyclic life
a fully discharged cell, either 40% (1.4) or 20% (1.2)
more capacity than has been withdrawn must be put The life of a sealed battery depends on the operat-
back into the cell. ing conditions. It is principally affected by the depth
The watt hour efficiency is the quotient of the of discharge (capacity dischargedrated capacity) in a
discharged energy and the necessary energy to fully charge/discharge application, or in ampere hours over-
charge a cell. Comparing the mean discharge voltage charged in a permanent charge application. Life is also
with the mean charge voltage confirms a figure of affected by temperature, though less so at high temper-
approximately 61% for a cell with mass electrodes atures, and by end-point requirements regarding rate
and 73% for a cell with sintered electrodes when and capacity (increased cycle life will ordinarily be
discharged at the Zlo rate (both at approximately 20°C). the result of a shallow discharge regimen).
Any treatment that causes a cell to vent itself is
harmful. Frequent or extended venting of even prop-
Storage
erly valved cells eventually destroys them. In rating
Sealed nickel-cadmium batteries can be stored cycle life, the end of life for a sealed nickel-cadmium
indefinitely, ideally in a clean, dry atmosphere, cell is considered to be when it no longer provides
in a discharged state, unlike lead-acid batteries 80% of its rated capacity. The discharge currents used
which will sulphate if stored in a discharged state. in determining the cycle lives listed below are the 10 h
Nickel-cadmium batteries can be stored in any state rate for button cells and the 1 h rate for cylindrical
of charge, without a significant loss of life. types. The charge current is terminated after return
Maintenance charging is not required. However, of approximately 140% of the capacity previously
after prolonged storage, up to three cycles of charge removed. If a cell can be considered to be satisfac-
and discharge may be required to achieve the battery’s tory while delivering less than the arbitrary 80% end-
rated capacity. It is recommended that the first charge point figure, the reliability characteristic of the sealed
following storage should be for 24 h at the Zl0 rate. nickel-cadmium battery corresponds to the ‘bathtub’
During operation and storage, it is possible that curve of electronics components. In cycling applica-
crystals may form in the area of the sealing between tions, as in the supply to a portable transceiver, the
the positive and the negative poles. This is due to service life of a battery is most conveniently expressed
minute pores in the sealing ring allowing electrolyte as the number of cycles of charge and discharge, and
to combine with carbon dioxide in the air to form the failure rate in failures per cycle.
potassium carbonate crystals. This crystallization has In applications with permanent maintenance charge,
no detrimental effect on the electrical properties or the life of a battery is expressed as the multiple of
life expectancy of the battery but, if it is thought to Cs Ah of overcharge or as hours of operation, and
be aesthetically unacceptable, removal of the crystals the failure rate as failures per operating hour. As an
with a dry cloth and then a smear of silicone grease example, Figure 4.5 shows the estimated cycling life
will inhibit further growth. at 20°C as a function of discharge for SAFT sealed
nickel-cadmium batteries.
Internal resistance The following results have been obtained at 20°C:
bum-in period lasts a few cycles or a few tens of
Unlike open-cell batteries, where the internal resist- C5 Ah of overcharge; failure rate of the order of
ance decreases linearly with an increase of capacity, per cycle with rapid charge at 2C5A and discharge