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THE CHARACTERISATION OF NICKEL-CADMIUM BATTERIES FOR
TELECOMMUNICATIONS APPLICATIONS - PART 1
ANTHONY GREEN
SAFT ADVANCED AND INDUSTRIAL BATTERY GROUP
156, AVENUE DE METZ
ROMAINVILLE
93230 FRANCE
ABSTRACT
With the advent of new telecommunication systems there has been a move from large exchanges to
kerbside and remote systems.
This has changed the environment from a temperature controlled, well ventilated location to one
which has large variations in temperature and climatic conditions.
The structure and electrochemistry of the nickel-cadmium battery is such that it is able to withstand
abusive conditions and is generally used where either high reliability and/or the ability to operate
under difficult environmental conditions are required.
This paper briefly describes the design and technology of existing low maintenance nickel-cadmium
products and results are given from recent relevant telecom application testing for high temperature
operation.
This work is being carried as part of a development of new products designed expressly to meet the
requirement of telecom applications.
1. Introduction
developed a new negative plate technology, called
plastic bonded, for sealed cells to give improved
high temperature performance and reliability. This
plate technology was later extended to industrial
products and was first launched for railway
applications in 1987. This was followed by other
ranges including the SPH range designed
specifically for UPS applications and standard
charging systems.
The nickel cadmium battery is probably the most
reliable battery system available in the market
today and, it can be used in applications and
environments unacceptable for other battery
systems.
Thus, in a telecommunications application, which
must be considered to be a critical system, it
would seem that the nickel cadmium battery would
be the obvious choice. However, in a conventional
telecommunications environment the system is
housed in a clean, well ventilated building with a
controlled temperature range. This is not a difficult
situation for the battery and, although they have
not really met their life predictions, the VRLA
(valve regulated lead battery) because of their
« no-maintenance » image, is the preferred
choice.
This sintered positive, plastic bonded negative
technology has advantages over the pocket plate
product in terms of power density and voltage
window, while retaining the inherent robustness of
the nickel cadmium technology.
At the Intelec conference at the Hague in October
1995, the author presented a paper illustrating that
it was possible to achieve the same power density
with these new technology sintered positive,
plastic bonded systems as with the VRLA
products. However, this is not always a
requirement as the dimensions available for the
battery can be larger than the apparent volume
available.
With the advent of cellular telephone systems,
fibre to the curb and other developments, there
has become a requirement for reliable kerb-side
and remote relay systems. These systems are
often housed in units which have limited ventilation
and can have interior temperatures well in excess
of the exterior outside temperature. There is a
need for a battery system which is resistant to
temperature extremes and sudden failure so that
the security of the system can be maintained.
In this paper results are given for testing which
has been carried out on the sintered positive /
plastic bonded negative battery, particularly at
higher temperatures, with a look at other nickel
cadmium technologies.
Saft Nife have, for many years, been
manufacturers of both pocket plate and sintered
plate nickel cadmium products and, in the 1980’s,
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2 Cell Technology
On charge, the reverse reaction takes place until
the cell potential rises to a level where hydrogen is
evolved at the negative plate and oxygen at the
positive plate which results in water loss.
The positive plate used in the cell is of this
sintered type. This is obtained by chemical
impregnation of nickel hydroxide onto a porous
nickel structure, which is obtained by sintering
nickel powder onto a thin, perforated, nickel plated
strip.
There is little change in the electrolyte density
during charge and discharge and this allows large
reserves of electrolyte to be used without
inconvenience to the electrochemistry of the
couple.
The negative electrode is a plastic bonded
cadmium electrode, produced with a continuous
process. This involves blending together the active
material, binder and additives, continuously
spreading this onto a perforated nickel plated steel
substrate, drying and, finally, passing the coated
band through rollers for dimensioning.
Through its electrochemistry, the nickel cadmium
battery has a stable behaviour giving it a long life,
good electrical and mechanical characteristics and
a resistance against abusive conditions.
Positive and negative plates are organised
alternately and separated by a sandwich of
sintered micro-porous polymer and non-woven
felt. This gives a precise spacing between plates
and allows free electrolyte movement within the
stack.
3 Charge Characteristics
The sintered/PBE nickel cadmium battery can be
charged using either constant current or constant
potential methods although, in most applications,
the latter method is generally used.
The electrolyte is an aqueous solution of
potassium hydroxide (KOH) and lithium hydroxide
(LiOH). During the electrochemical reaction, the
electrolyte is only used for ion transfer and it not
chemically changed or degraded during the
charge/discharge cycle.
The charge/discharge reaction is as follows :
Charging methods can be of two types, single
level charging, where a single voltage is used
which is a compromise between a value high
enough to charge the cell and low enough to give
a low water consumption, or two level charging,
where there is a high level charging voltage to give
a fast charge and a low level maintenance voltage
which gives sufficient current to maintain the
capacity with the minimum water consumption.
discharge
2 NiOOH + 2H2O + Cd
2 Ni(OH)2 + Cd(OH)2
charge
Tests have been carried out for the sintered/PBE
technology to establish the optimum values of
voltages for charging. (Figure 1)
During discharge the trivalent nickel hydroxide is
reduced to divalent nickel hydroxide and the
cadmium at the negative plate forms cadmium
hydroxide.
110
Available capacity (%)
100
90
80
Constant voltage charge
curves at +20°C
current limit 0.2C5A
70
60
50
1.41 volts/cell
1.45 volts/cell
40
30
3
5
7
9
11
13
15
17
19
Time
(hours)
FIGURE 1 - Constant Voltage Charge Curves at °C
2
21
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All this data is based upon a normal temperature
of 20°C. However, temperature will have an effect
since, as the temperature increases then the cells
become more difficult to charge.
The voltage of 1.41 volts per cell is the optimum
value for a single level charge system for this
technology. It can be seen that it is little lower than
the 1.45 volts per cell « high rate » charge voltage
for the first 15 hours, but by 24 hours they are
similar and the battery is over 95% charged. The
tests show that from a fully discharged state the
battery is more that 80% charged within 8 hours.
Figure 2 shows the effect of a higher temperature
on recharge time.
After 5 hours there is only a small percentage
difference, 2 to 3 %, between charging at 20°C
and 40°C. However, as the cell becomes more
fully charged, over 85%, the charging efficiency is
found to be lower at the high temperatures and the
capacity charged is 6 to 7% lower;
.
At 24 hours, therefore, the cells will be 97%
charged if charged at 20°C, but at 40°C, the cells
will only achieve 90% charge.
The 1.45 volts per cell would be used as part of a
two level charge system where fast recharge was
required. Typically the second stage would fall to
1.37 volts per cell for the maintenance level.
This data is based upon a current limit of 0.2C5A,
i.e. for a 100Ah cell (rated at 5 hours to the
IEC623 standard) the current limit would be 20
amperes. If the current limit was reduced to
0.1C5A then the time to recharge would be
correspondingly longer.
Over a period a time the cells at 40°C will become
fully charged but this will take some days.
Capacity charged
100
20°C
95
90
40°C
85
80
75
5
10
20
15
25
30
35
Time (hours)
FIGURE 2 Effect of temperature on recharge at 1.41 volts per cell
example a sealed nickel cadmium consumer
product, the level of recombination normally
approaches 100%. In a nickel cadmium pocket
plate open cell the level of recombination is much
less.
4 Water Consumption
The water consumption of the cell is linked to the
current passing, which converts the water to the
gases hydrogen and oxygen by electrolysis, and
the level of recombination of these gases which
reconverts them back into water.
The current in a battery increases with increasing
voltage and the manner in which this occurs
depends on the technology of battery.
In all electrochemical cells there is a certain level
of recombination. In a fully sealed product, for
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float current (mA)
weight loss (g)
8
100
6.9
80
current
weight
75
6
60
3.6
45
4
40
40
30
20
0
1.1
0.9
pocket
pocket non-iron sinter/plastic
fibre
2
0
FIGURE 3 Effect of technology on water consumption at room temperature
The sintered positive / plastic bonded negative
cells had a current similar to the non-iron pocket
plate but a much reduced water consumption. The
fibre plate had a similar water consumption to the
sintered/plastic bonded cells.
Figure 3 shows the results from a recent test
comparing pocket plate, sintered positive, plastic
bonded negative, and fibre plate technologies
which have floated for 12 weeks at constant
voltage. The pocket plate and fibre technologies
were floated at 1.43 volts per cell, which is the
recommended value, and the sintered positive,
plastic bonded negatives were floated at 1.41 volts
per cell, which is the recommended single level
value. All cells tested were of the high
performance type.
However, the water consumption is not the only
criteria, and the effect on performance is given in
section 5.
The sintered/plastic bonded product has the ability
to operate at these low voltages and so have only
low water usage while, at the same time,
maintaining the advantages of a flooded cell
concept.
Two pocket plate technologies were evaluated,
one with conventional negative active material and
the other with a newly developed iron-free
negative active material. It should be noted that
both the plastic bonded and the fibre negatives
have iron-free active materials.
Figure 4 shows the water usage over time based
on recent tests for the two principle floating
voltages, 1.41 volts per cell for single level
charging and 1.37 volts per cell for the
maintenance voltage level of a two level charging
system.
The effect of introducing an iron-free negative to
the pocket plate is to effectively half the current
flowing and so reduce the water consumption
correspondingly.
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Water consumption (cm3/Ah of capacity)
0.6
Results from tests at room temperature
0.5
1.41 volts/cell
0.4
0.3
0.2
0.1
1.37 volts/cell
0
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36
Months
FIGURE 4 - Water consumption from tests results at +20/25°C for standard float voltages
has the opposite effect. Thus, as it is the current
which is an important factor with regard to charge
and water usage, it is desirable to use
temperature compensation in the charge system
which decreases the voltage as the temperature
increases and increases the voltage as the
temperature reduces.
It can be seen that after an initial rise in water
consumption, the water consumption steadies to a
linear relationship, with the 1.41 volts per cell
regime consuming about twice that of the 1.37
volts per cell regime. Over the 3 year period
floating at 1.41 volts per cell consumes about 0.55
3
cm per Ah of capacity and at 1.37 volts per cell
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about 0.28 cm per Ah of capacity
From data of current flowing at different
temperatures, actual data at room temperatures
and allowing for a security margin compared to
actual results, it is possible to construct a water
consumption table for these two different voltages
at different temperatures (Figure 5)
As with the charging, the temperature has an
effect on the water consumption and reducing the
temperature has the effect of reducing the current
for a set voltage and increasing the temperature
Temperature °C
0°C
10°C
1.41 V/cell
1.37 V/cell
20°C
30°C
40°C
50°C
0
1
2
3
4
5
6
Topping-up interval in years per cm3 of reserve per Ah
5
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FIGURE 5 - Typical topping-up intervals at floating voltages of 1.37 and 1.41 volts per cell
reduce the performance to correspond to its
normal usage after floating. This de-rating factor
can vary from 1 (i.e. no de-rating) for long
discharges to low voltages to up to 0.65 for very
short discharges to high end of discharge
voltages. It is a requirement of the IEEE1155
sizing standard that the data should be supplied
with its performance after floating.
This demonstrates that at normal temperature,
20°C, a voltage of 1.37 volts per cell will give over
3 years without water replenishment for each
cubic centimetre per Ah of water reserve. Thus a
100 Ah cell with 600 cc of water reserve will give
20 years without requiring water replenishment
when operated at 1.37 volts per cell maintenance
voltage. If it is operated with a single level 1.41 volt
per cell system, the same cell will go 10 years
without requiring water replenishment.
It can be seen that initially, before floating, when
discharged at twice the C5 capacity, (i.e. 160
amperes to 1.0 volts per cell for these 80Ah cells)
the sintered/plastic bonded cells and the fibre cells
have about the same performance. The pocket
plate cells have approximately 15% lower
performance, which is normal for the technology.
However, when discharged after floating, it is
found that the pocket plate have improved a little
in performance, the sintered / plastic bonded cells
have remained exactly the same, but the fibre has
fallen to a lower level than the pocket plate.
However, these are calculated values based on
water consumption at 20°C and the current flowing
at different temperatures. Practical tests are now
in progress to verify this data over floating periods
of several months.
5 Performance after floating
Shown in the previous section in figure 3 was the
consumption of water on floating over a period of
12 weeks. However, after floating in this way the
cell will not necessarily give the same
performance as it would after being fully charged
at constant current to the international standard
IEC623. Generally a de-rating factor is used to
Thus, the effect of floating a cell over a period of
time, rather than constant current charging, can
have a significant effect on the performance
available.
% C5 capacity available discharged at 160 amperes to 1.0 vpc
120
100
80
before floating
80 Ah
cells
after floating
76.9
66.2
71
76.9
76.3
69.8
63.9
61.5
60
40
20
0
pocket
pocket non-iron sinter/plastic
fibre
FIGURE 6 - Effect of floating on performance for different plate technologies
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an ion carrier and does not chemically take part in
the charge discharge reaction. In addition, the
structural material of the nickel cadmium battery is
steel and not the active material. Thus the ageing
of the active material does not compromise the
structure of the electrode assembly. As a result,
the ageing process of the nickel cadmium battery
is controlled and is not a corrosion phenomena.
Due to this the nickel cadmium battery has a long
lifetime and, in a standby application, 20 years is
the normal minimum.
6 Lifetime and Temperature
The lifetime of a battery can be expressed simply
in terms of its duration in time or, alternatively, by
the number of discharge cycles it is able to
perform.
The nickel cadmium battery and, in particular, the
sintered positive / plastic bonded negative battery
is able to perform many deep discharge cycles. In
the case of the sintered positive / plastic bonded
negative battery it is possible to perform in excess
of 3,500 cycles to 80% depth of discharge, and,
this has led to its use by the major electric vehicle
manufacturers.
However, as the temperature increases, the
electrochemical activity increases and so the
speed of the natural ageing of the active material
also increases. However, this does not effect the
steel structural components of the electrode
assembly which maintain their long life
characteristics. The rate of ageing is about 20%
reduction in life for 10°C increase in temperature.
This compares to lead acid batteries, where the
rate of ageing is about 50% for each 10°C rise in
temperature.
Although this is not a normal criteria for
telecommunication applications in developed
countries, it can be a factor in countries where the
mains electricity supply is suspect or, even, only
available for a few hours per day.
However, what is generally important is the
lifetime duration of the product and its variation
with temperature.
This is shown graphically in Figure 7, where the
life relative to the life at 25°C for nickel cadmium
and lead acid cells are given.
A characteristic of the nickel cadmium couple is
that the electrolyte, potassium hydroxide, is only
Percentage (% ) of 25°C lifetime
100
90
Lifetime at elevated temperatures
relative to lifetime at 25°C
80
70
60
50
N ickel cadmium battery
40
30
20
L e a d a c i d b a ttery
10
0
25
30
35
40
T e m p e r a ture °C
45
50
FIGURE 7 - Effect of continuous operation temperature on lifetime
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Thus a 20 year life at normal temperature nickel
cadmium cell when run at an average ambient
temperature of 45°C would give 15 years and a 10
year life lead acid product under the same
conditions would be reduced to a little over 2 years
life.
In practice, with the valve regulated lead acid
battery, the end of life at high temperatures is
often due to drying out which can occur before its
normal failure mechanisms occur. Thus, in this
situation, the time to failure can be shorter than
Figure 7 would normally predict.
7 Conclusions
In this paper the most recent testing of nickel
cadmium cells under float conditions has been
presented. In particular, testing has been carried
out at higher temperatures since that is the
environment now being encountered in outdoor
telecommunication applications.
The approach which has been taken to preserve
the robustness of the product is to retain the
flooded technology but to optimise the charge
conditions and reserve of electrolyte to give many
years without maintenance, even under difficult
temperature conditions.
The testing has shown that the nickel cadmium
battery is compatible with difficult environments
and, if correctly engineered for the application, is a
feasible solution.
Further testing is in progress to demonstrate
lifetime and water consumption under difficult
conditions and these will be presented at a later
time.
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