Saft SBL User manual

Nickel-cadmium block battery
Technical manual

1
Contents
1. Introduction 3
2. Benefits of the block battery 4
2.1 Complete reliability 4
2.2 Long cycle life 4
2.3 Exceptionally long lifetime 4
2.4 Low maintenance 4
2.5 Wide operating temperature range 4
2.6 Fast recharge 4
2.7 Resistance to mechanical abuse 4
2.8 High resistance to electrical abuse 4
2.9 Simple installation 4
2.10 Extended storage 4
2.11 Well-proven pocket plate construction 4
2.12 Environmentally safe 4
2.13 Low life-cycle cost 4
3. Electrochemistry of nickel-cadmium batteries 5
4. Construction features of the block battery 6
4.1 Plate assembly 7
4.2 Separation 8
4.3 Electrolyte 8
4.4 Terminal pillars 8
4.5 Venting system 8
4.6 Cell container 8
5. Battery types and applications 10
5.1 Type L 11
5.2 Type M 11
5.3 Type H 11
5.4 Choice of type 11
6. Operating features 12
6.1 Capacity 12
6.2 Cell voltage 12
6.3 Internal resistance 12
6.4 Effect of temperature on performance 13
6.5 Short-circuit values 14
6.6 Open circuit loss 14
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1. Introduction
The nickel-cadmium battery is the
most reliable battery system available
in the market today. Its unique
features enable it to be used in
applications and environments
untenable for other widely available
battery systems.
It is not surprising, therefore, that the
nickel-cadmium battery has become
an obvious first choice for users
looking for a reliable, long life, low
maintenance, system.
This manual details the design and
operating characteristics of the Saft
Nife pocket plate block battery to
enable a successful battery system to
be achieved. A battery which, while
retaining all the advantages arising
from nearly 100 years of
development of the pocket plate
technology, can be so worry free that
its only major maintenance
requirement is topping-up with water.
For the valve-regulated and
photovoltaic pocket plate ranges,
Ultima and Sunica, specific technical
manuals are available which address
the particular characteristics of these
ranges.
6.7 Cycling 14
6.8 Effect of temperature on lifetime 14
6.9 Water consumption and gas evolution 16
7. Battery sizing principles and sizing method
in stationary standby applications 17
7.1 The voltage window 17
7.2 Discharge profile 17
7.3 Temperature 17
7.4 State of charge or recharge time 17
7.5 Aging 18
7.6 Floating effect 18
8. Battery charging 19
8.1 Charging generalities 19
8.2 Constant voltage charging methods 19
8.3 Charge acceptance 20
8.4 Charge efficiency 22
8.5 Temperature effects 22
8.6 Commissioning charge 22
9. Special operating factors 23
9.1 Electrical abuse 23
9.2 Mechanical abuse 23
10. Installation and storage 24
10.1 Batteries on arrival 24
10.2 Cell oil 24
10.3 Emplacement 25
10.4 Ventilation 25
10.5 Preparation for service 26
11. Maintenance of block batteries in service 27
11.1 Cleanliness/mechanical 27
11.2 Topping up 27
11.3 Capacity check 28
11.4 Changing electrolyte 28
11.5 Recommended maintenance procedure 28
3
2
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3
The nickel-cadmium battery is the
most reliable battery system available
in the market today. Its unique
features enable it to be used in
applications and environments
untenable for other widely available
battery systems.
It is not surprising, therefore, that the
nickel-cadmium battery has become
an obvious first choice for users
looking for a reliable, long life,
low maintenance, system.
This manual details the design and
operating characteristics of the Saft
Nife pocket plate block battery to
enable a successful battery system
to be achieved. A battery which,
while retaining all the advantages
arising from nearly 100 years
of development of the pocket plate
technology, can be so worry free
that its only major maintenance
requirement is topping up with water.
For the valve-regulated and
photovoltaic pocket plate ranges,
Ultima and Sunica, specific technical
manuals are available which address
the particular characteristics of these
ranges.
1. Introduction
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2.1 Complete reliability
The block battery does not suffer from
the sudden death failure associated
with the lead acid battery
(see section 4.1 Plate assembly).
2.2 Long cycle life
The block battery has a long cycle
life even when the charge/discharge
cycle involves 100% depth of
discharge (see section 6.7 Cycling).
2.3 Exceptionally long lifetime
A lifetime in excess of twenty years
is achieved by the Saft Nife block
battery in many applications, and at
elevated temperatures it has a lifetime
unthinkable for other widely available
battery technologies (see section 6.8
Effect of temperature on lifetime).
2.4 Low maintenance
With its generous electrolyte reserve,
the block battery reduces the need
for topping up with water, and can be
left in remote sites for long periods
without any maintenance
(see section 6.9 Water consumption
and gas evolution).
2.5 Wide operating
temperature range
The block battery has an electrolyte
which allows it to have a normal
operating temperature of from -20°C
to +50°C, and accept extreme
temperatures, ranging from as low
as -50°C to up to +60°C
(see section 4.3 Electrolyte).
2.6 Fast recharge
The block battery can be recharged
at currents which allow very fast
recharge times to be achieved
(see 8.3 Charge acceptance).
2.7 Resistance to mechanical
abuse
The block battery is designed to have
the mechanical strength required
to withstand all the harsh treatment
associated with transportation over
difficult terrain
(see section 9.2 Mechanical abuse).
2.8 High resistance to electrical
abuse
The block battery will survive abuse
which would destroy a lead acid
battery, for example overcharging,
deep discharging, and high ripple
currents
(see section 9.1 Electrical abuse).
2.9 Simple installation
The block battery can be used with a
wide range of stationary and mobile
applications as it produces no
corrosive vapors, uses corrosion-free
polypropylene containers and has
a simple bolted connector assembly
system
(see section 10 Installation and
storage).
2.10 Extended storage
When stored in the empty and
discharged state under the
recommended conditions, the block
battery can be stored for many years
(see section 10 Installation and
storage).
2.11 Well-proven pocket plate
construction
Saft has nearly 100 years
of manufacturing and application
experience with respect to the nickel-
cadmium pocket plate product, and
this expertise has been built into the
twenty-plus years design life of the
block battery product
(see section 4 Construction features
of the block battery).
2.12 Environmentally safe
More than 99% of the nickel-
cadmium block battery can be
recycled, and Saft operates a
dedicated recycling center to recover
the nickel, cadmium, steel and plastic
used in the battery.
2.13 Low life-cycle cost
When all the factors of lifetime, low
maintenance requirements, simple
installation and storage and
resistance to failure are taken into
account, the Saft Nife block battery
becomes the most cost effective
solution for many professional
applications.
4
2. Benefits of the block battery
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discharge
2 NiOOH + 2H2O + Cd 2 Ni(OH)2+ Cd(OH)2
charge
The nickel-cadmium battery uses
nickel hydroxide as the active
material for the positive plate, and
cadmium hydroxide for the negative
plate.
The electrolyte is an aqueous solution
of potassium hydroxide containing
small quantities of lithium hydroxide
to improve cycle life and high
temperature operation.
The electrolyte is only used for ion
transfer; it is not chemically changed
or degraded during the charge/
discharge cycle. In the case of the
lead acid battery, the positive and
negative active materials chemically
react with the sulphuric acid
electrolyte resulting in an ageing
process.
The support structure of both plates
is steel. This is unaffected by the
electrochemistry, and retains its
characteristics throughout the life of
the cell. In the case of the lead acid
battery, the basic structure of both
plates are lead and lead oxide which
play a part in the electrochemistry
of the process and are naturally
corroded during the life of the battery.
The charge/discharge reaction is as
follows:
During discharge the trivalent nickel
hydroxide is reduced to divalent
nickel hydroxide, and the cadmium
at the negative plate forms cadmium
hydroxide.
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.
Unlike the lead acid battery, there is
little change in the electrolyte density
during charge and discharge. This
allows large reserves of electrolyte
to be used without inconvenience
to the electrochemistry of the couple.
Thus, through its electrochemistry, the
nickel-cadmium battery has a more
stable behavior than the lead acid
battery, giving it a longer life,
superior characteristics and a greater
resistance against abusive conditions.
Nickel-cadmium cells have a nominal
voltage of 1.2 volts.
5
3. Electrochemistry of nickel-cadmium
batteries
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6
4. Construction features of the block
battery
Connector covers
Material: hard PVC
plastic.
Flame arresting
vents
Material: polypropylene.
Cell container
Material: translucent
polypropylene.
Splash guard
Prevents electrolyte
splash, and possible
short-circuiting by objects
entering the cell.
Plate tab
Spot-welded both to the
plate side-frames and to
the upper edge of the
pocket plate.
The cells are welded
together to form rugged
blocks of 1-10 cells
depending on the cell size.
Plate group bus
Connects the plate tabs
with the terminal post.
Plate tabs and terminal
post are projection
welded to the plate group bus.
Separating grids
Separate the plates
and insulate the plate frames
from each other.
The grids allow free
circulation of electrolyte
between the plates.
Plate frame
Seals the place pockets
and serves as a current collector.
Plate
Horizontal pockets
of double-perforated
steel strips.
Saft Nife battery cells
fulfill all requirements
specified by IEC,
publication 623.
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Nickel-cadmium batteries have an
exceptionally good lifetime and cycle
life because their plates are not
gradually weakened by corrosion, as
the structural component of the plate
is steel. The active material of the
plate is not structural, only electrical.
The alkaline electrolyte does not react
with steel, which means that the
supporting structure of the block
battery stays intact and unchanged
for the life of the battery. There is no
corrosion and no risk of “sudden
death”.
In contrast, the lead plate of a lead
acid battery is both the structure and
the active material and this leads to
shedding of the positive plate material
and eventual structural collapse.
7
4.1 Plate assembly
The nickel-cadmium cell consists
of two groups of plates, the positive
containing nickel hydroxide and
the negative containing cadmium
hydroxide.
The active materials of the Saft
Nife pocket plate block battery are
retained in pockets formed from steel
strips double perforated by a
patented process. These pockets are mechanically
linked together, cut to the size
corresponding to the plate width and
compressed to the final plate
dimension. This process leads to a
component which is not only
mechanically very strong but also
retains its active material within
a steel containment which promotes
conductivity and minimizes electrode
swelling.
These plates are then welded to a
current carrying bus bar assembly
which further ensures the mechanical
and electrical stability of the product.
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4.2Separation
Separation between plates is
provided by injection molded plastic
separator grids, integrating both plate
edge insulation and plate separation.
By providing a large spacing
between the positive and negative
plates and a generous quantity
of electrolyte between plates, good
electrolyte circulation and gas
dissipation are provided, and there
is no stratification of the electrolyte
as found with lead acid batteries.
4.3Electrolyte
The electrolyte used in the block
battery, which is a solution of
potassium hydroxide and lithium
hydroxide, is optimized to give the
best combination of performance, life,
energy efficiency and a wide
temperature range.
The concentration of the standard
electrolyte is such as to allow the cell
to be operated down to temperature
extremes as low as -20°C and as
high as +60°C. This allows the very
high temperature fluctuation found in
certain regions to be accommodated.
For very low temperatures a special
high density electrolyte can be used.
It is an important consideration of the
block battery, and indeed all nickel-
cadmium batteries, that the electrolyte
does not change during charge and
discharge. It retains its ability to
transfer ions between the cell plates,
irrespective of the charge level.
In most applications the electrolyte
will retain its effectiveness for the life
of the battery and will never need
replacing. However, under certain
conditions, such as extended use in
high temperature situations, the
electrolyte can become carbonated.
If this occurs the battery performance
can be improved by replacing the
electrolyte (see section 11.4).
The standard electrolyte used for
the first fill in cells (see 10.5
Discharged and empty cells)
is E22 and for replacement in
service is E13.
4.4Terminal pillars
Short terminal pillars are welded
to the plate bus bars using a well
established and proven method.
These posts are manufactured from
steel bar, internally threaded for
bolting on connectors and nickel
plated.
The sealing between the cover
and the terminal is provided by
a compressed visco-elastic sealing
surface held in place by compression
lock washers. This assembly
is designed to provide satisfactory
sealing throughout the life of
the product.
4.5Venting system
The block battery is fitted with a
special flame arresting flip top vent
to give an effective and safe venting
system.
4.6Cell container
The battery is built up using
well-proven block battery construction.
The tough polypropylene containers
are welded together by heat sealing.
8
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9
The block battery uses 4 plate sizes
or plate modules. These are
designated module type 1, 2, 3
and 4. They can be recognized from
the block dimensions as follows:
Block width (mm) Block height (mm) Plate module
123 194 1
123 264 2
195 349 3
195 405 4
Table 1 - Correlation between block dimensions and plate module number
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5.1 Type L
The SBL is designed for applications
where the battery is required to
provide a reliable source of energy
over relatively long discharge
periods. Normally, the current is
relatively low in comparison with the
total stored energy, and the
discharges are generally infrequent.
Typical uses are power backup and
bulk energy storage.
5.2 Type M
The SBM is designed for applications
where the batteries are usually
required to sustain electrical loads for
between 30 minutes to 3 hours or for
“mixed”loads which involve a
mixture of high and low discharge
rates. The applications can have
frequent or infrequent discharges.
The range is typically used in power
backup applications.
5.3 Type H
The SBH is designed for applications
where there is a demand for a
relatively high current over short
periods, usually less than 30 minutes
in duration. The applications can
have frequent or infrequent
discharges. The range is typically
used in starting and power backup
applications.
5.4 Choice of type
In performance terms the ranges
cover the full time spectrum from
rapid high current discharges of a
second to very long low current
discharges of many hours. Table 2
shows in general terms the split
between the ranges for the different
discharge types. The choice is related
to the discharge time and the end of
discharge voltage. There are, of
course, many applications where
there are multiple discharges, and so
the optimum range type should be
calculated. This is explained in the
chapter “Battery Sizing”.
10 11
5. Battery types and applications
In order to provide an optimum solution for the wide range of battery applications
which exist, the block battery is constructed in three performance ranges.
Saft Battery
types SBL SBM SBH
Autonomy mini 1 h 15 min 1 s
maxi 100 h 2 h 30 min
Capacity mini 7.5 11 8.3
range maxi 1540 1390 920
Power Power Starting,
backup backup Power
Use of battery Bulk energy backup
storage
Applications Engine starting - Switchgear - UPS - Process control -
Data and information systems - Emergency lighting -
Security and fire alarm systems -
Switching and transmission systems - Signalling
Railways
intercity and ▼ ▼ ▼
urban transport
Stationary
Utilities
electricity, gas,
water production ▼ ▼ ▼
and distribution
Oil and gas
offshore & onshore,
petrochemical ▼ ▼ ▼
refineries
Industry
chemical, mining, ▼ ▼ ▼
steel metal works
Buildings
public, private ▼ ▼ ▼
Medical
hospitals, ▼ ▼ ▼
X-ray equipment
Telecom
radio,
satellite, cable, ▼ ▼
repeater stations,
cellular base stations
Railroad
substations ▼ ▼ ▼
& signalling
Airports ▼ ▼ ▼
Military
all applications ▼ ▼ ▼
Table 2 - General selection of cell range
10 min 15 min 30 min 60 min 2 h 3 h 5 h 8 h
1.14 V
1.10 V
1.05 V
1.00 V
H
M
L
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1.14V
1.10V
1.05V
1.00V
10 min 15 min 30 min 60 min 2 h 3 h 5 h 8 h
11
5.1Type L
The SBL is designed for applications
where the battery is required to
provide a reliable source of energy
over relatively long discharge
periods. Normally, the current is
relatively low in comparison with
the total stored energy, and the
discharges are generally infrequent.
Typical uses are power backup and
bulk energy storage.
5.2Type M
The SBM is designed for applications
where the batteries are usually
required to sustain electrical loads
for between 30 minutes to 3 hours
or for “mixed” loads which involve a
mixture of high and low discharge
rates. The applications can have
frequent or infrequent discharges.
The range is typically used in power
backup applications.
5.3Type H
The SBH is designed for applications
where there is a demand for a
relatively high current over short
periods, usually less than 30 minutes
in duration. The applications can have
frequent or infrequent discharges.
The range is typically used in starting
and power backup applications.
5.4Choice of type
In performance terms the ranges
cover the full time spectrum from
rapid high current discharges of
a second to very long low current
discharges of many hours. Table 2
shows in general terms the split
between the ranges for the different
discharge types. The choice is related
to the discharge time and the end of
discharge voltage. There are, of
course, many applications where
there are multiple discharges, and
so the optimum range type should be
calculated. This is explained in the
chapter “Battery Sizing”.
Table 2 - General selection of cell range
H
M
L
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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
0.2C5A. 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 mΩ
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)
1234
SBL 84 105 123 142
SBM 55 62 78 86
SBH N/A 30 39 43
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13
6.4 Effect of temperature on
performance
Variations in ambient temperature
affect the performance of the cell,
and this must be allowed for in
battery engineering.
Low temperature operation has the
effect of reducing the performance,
but the higher temperature
characteristics are similar to those
at normal temperatures. The effect
of low temperature is more marked
at higher rates of discharge.
The factors which are required in
sizing a battery to compensate for
temperature variations are given in a
graphical form in Figure 1(a), H type,
Figure 1(b), M type and Figure 1(c) L
type for operating temperatures from
-30°C to +50°C.
Figure 1(a) - Temperature derating factors for H type plate
1.2
1
0.8
0.6
0.4
0.2-40 -20 0 20 40
Derating factor
special electrolyte required for
continuous use below -20°C
5 hour rate
1 minute rate
30 minute rate
Temperature (°C)
Figure 1(b) - Temperature derating factors for M type plate
1.2
1
0.8
0.6
0.4
0.2-40 -20 0 20 40
Derating factor
special electrolyte required for
continuous use below -20°C
5 hour rate
15 minute rate
1 hour rate
Temperature (°C)
Figure 1(c) - Temperature derating factors for L type plate
1.2
1
0.8
0.6
0.4
0.2-40 -20 0 20 40
Derating factor
special electrolyte required for
continuous use below -20°C
5 hour rate
1 hour rate
Temperature (°C)
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6.5 Short-circuit values
The typical short-circuit value in
amperes for a block battery cell is
approximately 9 times the ampere-
hour capacity for an L type block,
16 times the ampere-hour capacity
for an M type block and 28 times
the ampere-hour capacity for an
H type block.
The block battery with conventional
bolted assembly connections will
withstand a short circuit current of this
magnitude for many minutes without
damage.
6.6 Open circuit loss
The state of charge of the block cell
on open circuit slowly decreases
with time due to self-discharge. In
practice this decrease is relatively
rapid during the first two weeks, but
then stabilizes to about 2% per month
at 20°C.
The self-discharge characteristics of
a nickel-cadmium cell are affected by
the temperature. At low temperatures,
the charge retention is better than at
normal temperature, and so the open
circuit loss is reduced. However, the
self-discharge is significantly
increased at higher temperatures.
The typical open circuit loss for
the block battery for a range
of temperatures which may be
experienced in a stationary
application is shown in Figure 2.
6.7 Cycling
The block battery is designed to
withstand the wide range of cycling
behavior encountered in stationary
applications. This can vary from low
depth of discharges to discharges of
up to 100% and the number of cycles
that the product will be able to
provide will depend on the depth
of discharge required.
The less deeply a battery is cycled,
the greater the number of cycles it is
capable of performing before it is
unable to achieve the minimum
design limit. A shallow cycle will give
many thousands of operations,
whereas a deep cycle will give only
hundreds of operations.
Figure 3 gives typical values for the
effect of depth of discharge on the
available cycle life, and it is clear
that when sizing the battery for a
cycling application, the number and
depth of cycles have an important
consequence on the predicted life
of the system.
6.8 Effect of temperature
on lifetime
The block battery is designed as
a twenty year life product, but as
with every battery system, increasing
temperature reduces the expected life.
However, the reduction in lifetime
with increasing temperature is very
much lower for the nickel-cadmium
battery than the lead-acid battery.
The reduction in lifetime for the nickel-
cadmium battery, and for comparison,
a high quality lead acid battery is
shown graphically in Figure 4. The
values for the lead acid battery are
as supplied by the industry and found
in Eurobat and IEEE documentation.
14
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In general terms, for every 9°C
increase in temperature over the
normal operating temperature of
25°C, the reduction in service life
for a nickel-cadmium battery will
be 20%, and for a lead acid battery
will be 50%. In high temperature
situations, therefore, special
consideration must be given to
dimensioning the nickel-cadmium
battery. Under the same conditions,
the lead-acid battery is not a practical
proposition, due to its very short
lifetime. The VRLA battery, for
example, which has a lifetime of
about 7 years under good conditions,
has this reduced to less than 1 year,
if used at 50°C.
15
Figure 2 - Capacity loss on open circuit stand
100
90
80
70
60
50 050 100 150 200 250 300 350 400
Percentage of initial capacity (%)
+ 40°C
Open circuit period (days)
0°C
+20°C
Figure 3 - Typical cycle life versus depth of discharge
10 000
9 000
8 000
7 000
6 000
5 000
4 000
3 000
2 000
1 000
0
10 % 20 % 30 % 40 % 50 % 60 % 70 % 80 % 90 % 100 %
Cycles
Depth of discharge
Cycle life versus depth of discharge expressed
as a percentage of the rated capacity
Temperature +20°C
Figure 4 - Effect of temperature on lifetime
100
90
80
70
60
50
40
30
20
10
025 30 35 40 45 50 55
Percentage of 25°C lifetime
Temperature °C
Nickel-cadmium
Lead acid
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6.9Water consumption
and gas evolution
During charging, more ampere-hours
are supplied to the battery than the
capacity available for discharge.
These additional ampere-hours must
be provided to return the battery to
the fully charged state and, since they
are not all retained by the cell and
do not all contribute directly to the
chemical changes to the active
materials in the plates, they must be
dissipated in some way. This surplus
charge, or over-charge, breaks down
the water content of the electrolyte
into oxygen and hydrogen; and pure
distilled water has to be added to
replace this loss.
Water loss is associated with the
current used for overcharging. A
battery which is constantly cycled,
i.e. is charged and discharged on
a regular basis, will consume more
water than a battery on standby
operation.
In theory, the quantity of water used
can be found by the faradic equation
that each ampere hour of overcharge
breaks down 0.366 cm3of water.
However, in practice, the water usage
will be less than this, as the
overcharge current is also needed
to support self-discharge of the
electrodes.
The overcharge current is a function
of both voltage and temperature,
so both have an influence on the
consumption of water. Figure 5 gives
typical water consumption values over
a range of voltages for different plate
types.
16
Figure 5 - Water consumption values for different voltages and plate types
1
0.11.4 1.42 1.44 1.46 1.48 1.5 1.52 1.54
cm /month, Ah Temperature +25°C
L type M type H type
3
Example: An SBM 161 is floating at
1.43 volts per cell. The electrolyte
reserve for this cell is 500 cm3.
From Figure 5, an M type cell at
1.43 volts per cell will use
0.27 cm3/month for one Ah of
capacity. Thus an SBM 161 will use
0.27 x 161 = 43.5 cm3per month
and the electrolyte reserve will be
used in 500 = 11.5 months.
43.5
The gas evolution is a function of the
amount of water electrolyzed into
hydrogen and oxygen and are
predominantly given off at the end
of the charging period. The battery
gives off no gas during a normal
discharge.
The electrolysis of 1 cm3of water
produces 1865 cm3of gas mixture
and this gas mixture is in the
proportion of 2/3 hydrogen and
1/3 oxygen. Thus the electrolysis
of 1 cm3of water produces about
1240 cm3of hydrogen.
BlockBat 3/11/98 10:12 Page 19
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7.Battery sizing principles in stationary
standby applications
There are a number of methods
which are used to size nickel-
cadmium batteries for standby
floating applications. These include
the “Hoxie”sizing method, the IEEE
1115 .
All these methods must take into
account multiple discharges,
temperature de-rating, performance
after floating and the voltage window
available for the battery. All methods
have to use certain methods of
approximation and each does this
more or less successfully.
A significant advantage of the nickel-
cadmium battery compared to a lead
acid battery, is that it can be fully
discharged without any inconvenience
in terms of life or recharge. Thus, to
obtain the smallest and least costly
battery, it is an advantage to
discharge the battery to the lowest
practical value in order to obtain the
maximum energy from the battery.
The principle sizing parameters which
are of interest are:
7.1The voltage window
This is the maximum voltage and the
minimum voltage at the battery
terminals acceptable for the system.
In battery terms, the maximum
voltage gives the voltage which is
available to charge the battery, and
the minimum voltage gives the lowest
voltage acceptable to the system to
which the battery can be discharged.
In discharging the nickel-cadmium
battery, the cell voltage should be
taken as low as possible in order to
find the most economic and efficient
battery.
7.2Discharge profile
This is the electrical performance
required from the battery for the
application. It may be expressed in
terms of amperes for a certain
duration, or it may be expressed in
terms of power, in watts or kW, for a
certain duration. The requirement
may be simply one discharge or many
discharges of a complex nature.
7.3Temperature
The maximum and minimum
temperatures and the normal ambient
temperature will have an influence on
the sizing of the battery. The
performance of a battery decreases
with decreasing temperature and
sizing at a low temperature increases
the battery size. Temperature de-
rating curves are produced for all cell
types to allow the performance to be
re-calculated.
7.4State of charge
or recharge time
Some applications may require that
the battery shall give a full-duty cycle
after a certain time after the previous
discharge. The factors used for this
will depend on the depth of
discharge, the rate of discharge, and
the charge voltage and current. A
requirement for a high state of charge
does not justify a high charge voltage
if the result is a high end of discharge
voltage.
16 17
6.9Water consumption
and gas evolution
During charging, more ampere-hours
are supplied to the battery than the
capacity available for discharge.
These additional ampere-hours must
be provided to return the battery to
the fully charged state and, since they
are not all retained by the cell and
do not all contribute directly to the
chemical changes to the active
materials in the plates, they must be
dissipated in some way. This surplus
charge, or over-charge, breaks down
the water content of the electrolyte
into oxygen and hydrogen; and pure
distilled water has to be added to
replace this loss.
Water loss is associated with the
current used for overcharging. A
battery which is constantly cycled, i.e.
is charged and discharged on a
regular basis, will consume more
water than a battery on standby
operation.
In theory, the quantity of water used
can be found by the faradic equation
that each ampere hour of overcharge
breaks down 0.366 cm 3 of water.
However, in practice, the water usage
will be less than this, as the
overcharge current is also needed to
support self-discharge of the
electrodes.
The overcharge current is a function
of both voltage and temperature, so
both have an influence on the
consumption of water. Figure 5 gives
typical water consumption values
over a range of voltages for different
plate types.
Example: An SBM 161 is floating at
1.43 volts per cell. The electrolyte
reserve for this cell is 500 cm3. From
Figure 5, an M type cell at 1.43 volts
per cell will use 0.27 cm3/month for
one Ah of capacity. Thus an SBM
161 will use 0.27 x 161 = 43.5 cm3
per month and the electrolyte reserve
will be used in 500 = 11.5 months.
43.5
The gas evolution is a function of the
amount of water electrolyzed into
hydrogen and oxygen and are
predominantly given off at the end of
the charging period. The battery
gives off no gas during a normal
discharge.
The electrolysis of 1 cm3of water
produces 1865 cm3of gas mixture
and this gas mixture is in the
proportion of 2/3 hydrogen and 1/3
oxygen. Thus the electrolysis of 1 cm3
of water produces about 1240 cm3
of hydrogen.
Figure 5 - Water consumption values for different voltages and plate types
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8.1 Charging generalities
The block battery can be charged by
all normal methods. Generally,
batteries in parallel operation with
charger and load are charged with
constant voltage. In operations where
the battery is charged separately from
the load, charging with constant
current or declining current is
possible. High-rate charging or
overcharging will not damage the
battery, but excessive charging will
increase water consumption to some
degree.
8.2 Constant voltage charging
methods
Batteries in stationary applications are
normally charged by a constant
voltage float system and this can be
of two types: the two-rate type, where
there is an initial constant voltage
charge followed by a lower voltage
floating voltage; or a single rate
floating voltage.
The single voltage charger is
necessarily a compromise between a
voltage high enough to give an
acceptable charge time and low
enough to give a low water usage.
However it does give a simpler
charging system and accepts a
smaller voltage window than the two-
rate charger.
The two-rate charger has an initial
high voltage stage to charge the
battery followed by a lower voltage
maintenance charge. This allows the
battery to be charged quickly, and
yet, have a low water consumption
due to the low voltage maintenance
level.
The values used for the block battery
ranges for single and two-rate charge
systems are as shown in Table 5
below.
To minimize the water usage, it is
important to use a low charge
voltage, and so the minimum voltage
for the single level and the two level
charge voltage is the normally
recommended value. This also helps
within a voltage window to obtain the
lowest, and most effective, end of
discharge voltage (see Battery sizing
chapter 7).
The values given as maximum are
those which are acceptable to the
battery, but would not normally be
used in practice, particularly for the
single level, because of high water
usage.
18 19
7.5 Aging
Some customers require a value to be
added to allow for the aging of the
battery over its lifetime. This may be a
value required by the customer, for
example 10 %, or it may be a
requirement from the customer that a
value is used which will ensure the
service of the battery during its
lifetime. The value to be used will
depend on the discharge rate of the
battery and the conditions under
which the discharge is carried out.
7.6 Floating effect
When a nickel-cadmium cell is
maintained at a fixed floating
voltageover a period of time, there is
a decrease in the voltage level of the
discharge curve. This effect begins
after one week and reaches its
maximum in about 3 months. It can
only be eliminated by a full
discharge/charge cycle, and it cannot
be eliminated by a boost charge. It is
therefore necessary to take this into
account in any calculations
concerning batteries in float
applications. This is used in the sizing
program, the IEEE sizing method and
the published data.
8. Battery charging
single level (V) two level (V)
min max min max floating
SBH 1.43 1.50 1.45 1.70 1.40
SBM 1.43 1.50 1.45 1.70 1.40
SBL 1.43 1.50 1.47 1.70 1.42
Table 5 - Charge and float voltages for the block battery ranges
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8.1Charging generalities
The block battery can be charged
by all normal methods. Generally,
batteries in parallel operation with
charger and load are charged
with constant voltage. In operations
where the battery is charged
separately from the load, charging
with constant current or declining
current is possible. High-rate charging
or overcharging will not damage the
battery, but excessive charging will
increase water consumption to some
degree.
8.2Constant voltage charging
methods
Batteries in stationary applications
are normally charged by a constant
voltage float system and this can be
of two types: the two-rate type, where
there is an initial constant voltage
charge followed by a lower voltage
floating voltage; or a single rate
floating voltage.
The single voltage charger is
necessarily a compromise between
a voltage high enough to give an
acceptable charge time and low
enough to give a low water usage.
However it does give a simpler
charging system and accepts a
smaller voltage window than the
two-rate charger.
The two-rate charger has an initial
high voltage stage to charge the
battery followed by a lower voltage
maintenance charge. This allows the
battery to be charged quickly, and
yet, have a low water consumption
due to the low voltage maintenance
level.
The values used for the block battery
ranges for single and two-rate charge
systems are as shown in Table 5
below.
To minimize the water usage, it is
important to use a low charge voltage,
and so the minimum voltage for the
single level and the two level charge
voltage is the normally recommended
value. This also helps within a voltage
window to obtain the lowest, and most
effective, end of discharge voltage (see
Battery sizing chapter 7).
The values given as maximum are
those which are acceptable to the
battery, but would not normally be
used in practice, particularly for
the single level, because of high
water usage.
19
8.Battery charging
single level (V) two level (V)
min max min max floating
SBH 1.43 1.50 1.45 1.70 1.40
SBM 1.43 1.50 1.45 1.70 1.40
SBL 1.43 1.50 1.47 1.70 1.42
Table 5 - Charge and float voltages for the block battery ranges
BlockBat 3/11/98 10:12 Page 24
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