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Sunica Ni-Cd batteryTechnical manual

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Contents

1. Introduction 1

2. The photovoltaic application 2

3. Construction features of the Sunica battery 43.1. Plate assembly 43.2. Separation 4

3.3. Electrolyte 43.4. Terminal pillars 53.5. Venting system 53.6. Cell container 5

4. Benefits of the Sunica battery 6

5. Operating features 7

5.1. Capacity 7

5.2. Cell voltage 7

5.3. Internal resistance 75.4. Affect of temperature on performance 8

5.5. Short circuit values 8

5.6. Open circuit loss 9

5.7. Cycling 10

5.8. Effect of temperature on lifetime 11

5.9. Water consumption and gas evolution 12

6. Battery charging 13

6.1. Charging generalities 13

6.1.1. Voltage limit with charge current limited by array 136.1.2. Two step charging 13

6.2. Charge efficiency 14

6.3. Temperature effects 16

6.4. Commissioning requirements 17

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7. Special operating factors 18

7.1. Electrical abuse 18

7.1.1. Ripple effects 18

7.1.2. Over-discharge 18

7.1.3. Overcharge 18

7.2. Mechanical abuse 18

7.2.1. Shock loads 18

7.2.2. Vibration resistance 18

7.2.3. External corrosion 18

8. Battery sizing principles 19

9. Installation and storage 22

9.1. Emplacement 22

9.2. Electrical 23

9.2.1. Batteries supplied charged and filled 23

9.2.2. Batteries supplied discharged and empty 23

10. Maintenance of Sunica batteries in service 24

11. Sunica battery range 25

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1. Introduction

The nickel-cadmium battery is themost reliable battery systemavailable in the market today. Itsunique features enables it to beused in applications andenvironments untenable for other widely available battery systems.

It is not surprising, therefore, that

with the emergence of thephotovoltaic (PV) market and itsrigorous requirements, thenickel-cadmium battery hasbecome an obvious first choicefor users looking for a reliable,low maintenance, system.

This publication details the designand operating characteristics ofthe Saft Sunica battery to enable asuccessful battery system to beachieved. A battery which, whileretaining all the advantages arisingfrom nearly 100 years ofdevelopment of the pocket platetechnology, can be so worry free

that its only maintenancerequirement is topping up withwater.

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2. The photovoltaic application

Telecommunications

emergency telephone postsradio repeater stations

base stations

Rail Transportcrossing guardslighting, signalling

isolated telephone stations

Oil and Gas

cathodic protection for pipelinesemergency lighting on offshore platforms

Utilities

electrification in remote areas

The typical requirements for photovoltaic (PV) applications areruggedness, environmental flexibility, unattended operation, ease ofinstallation, and reliability.

Photovoltaic applications can cover many applications including:

Navigational Aids

offshore

remote lighthousesbeacons

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A photovoltaic system is made upof three distinct parts :

The photovoltaic arraywhich isbuilt to give up to 20 years ofservice life.

Electronic components such asblocking diodes and logic

circuits in power conditionersand as controllers or voltageregulators.

The batterywhich must assure theautonomy required by theinstallation.

Systems are often installed inremote areas, at sites accessibleonly by foot, helicopter or boat,in good weather conditions andwith only limited skilled labouravailable.

Thus, the ideal photovoltaicpower system is a reliableinstallation which requires onlyinfrequent maintenance calls and,clearly, the battery plays acrucial part in this requirement.

Thus, the battery is a critical part ofthe system and, premature failureof the battery results in a totalfailure of the system.

The most importantcharacteristics required in abattery for photovoltaicapplications are:

Ability to withstand cycling,daily and seasonal

Ability to withstand high andlow environmental temperatures

Ability to operate reliably,unattended and with minimalmaintenance

Ruggedness for transportation toremote sites

Easily installed with limited

handling equipment and unskilledlabour

Reliability and availabilityduring the 20 years service life ofthe photovoltaic modules

Resistance to withstand failureof electronic control systems

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3.1.Plate assembly

The nickel-cadmium cell consistsof two groups of plates, onecontaining nickel hydroxide (thepositive plate) and the othercontaining cadmium hydroxide(the negative plate).

The active materials of the

Saft Sunica pocket plate areretained in pockets formed fromnickel plated steel doubleperforated by a patented process.These pockets are mechanicallylinked together, cut to the sizecorresponding to the plate lengthand compressed to the final platedimension. This process leads toa component which is not onlymechanically very strong but alsoretains its active material within a

steel boundary which promotesconductivity and minimiseselectrode swelling.

These plates are then welded to acurrent carrying bus bar assemblywhich further ensures themechanical and electrical stabilityof the product.

Nickel-cadmium batteries have anexceptionally good cycle life

because their plates are notgradually weakened by repeatedcycling as the structuralcomponent of the plate is steel.The active material of the plate isnot structural, only electrical. Thealkaline electrolyte does not reactwith steel, which means that thesupporting structure of the Sunicabattery stays intact andunchanged for the life of thebattery. There is no corrosion and

no risk of sudden death.

In contrast, the lead plate of alead acid battery is both thestructure and the active materialand this leads to shedding of thepositive plate material andeventual structural collapse.

3.2.Separation

Separation between plates isprovided by injection mouldedplastic separator grids, integratingboth plate edge insulation andplate separation.

By providing a large spacingbetween the positive and negativeplates and a generous quantity ofelectrolyte between plates, goodelectrolyte circulation and gasdissipation are provided andthere is no risk of the stratificationof electrolyte found with lead acidbatteries.

3.3.Electrolyte

The electrolyte used in Sunica,which is a solution of potassiumhydroxide and lithium hydroxide,is optimised to give the bestcombination of performance, life,energy efficiency and a widetemperature range.

The concentration is such as toallow the cell to be operateddown to temperature extremes aslow as 50C and as high as+ 60C. This allows the very hightemperature fluctuations found incertain remote regions to beaccommodated.

It is an important consideration ofSunica, and indeed all nickel-

cadmium batteries, that the

The construction of the SaftSunica cell is basedupon the Saft pocketplate technology but withspecial features to enhanceits use in the specialisedphotovoltaic application.

3. Construction features ofthe Sunica battery

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electrolyte does not changeduring charge and discharge. Itretains its ability to transfer ionsbetween the cell platesirrespective of the charge level.

In most applications theelectrolyte will retain itseffectiveness for the life of the

battery and will never needreplacing. However, undercertain conditions, such asextended use in high temperaturesituations, the electrolyte canbecome carbonated. If this occursthe battery performance can berestored by replacing theelectrolyte.

3.5. Venting system

Sunica is fitted with a special flamearresting vent to give aneffective and safe venting system.

3.6.Cell container

Sunica is built up using the well

proven block battery construction.The tough polypropylenecontainers are welded together byheat sealing and the assembly ofthe blocks are completedby a clip-on cover enclosing thetop of the Sunica block, giving anon conducting , easy to clean,top surface.

3.4. Terminal pillars

Short terminal pillars are weldedto the plate bus bars using thewell proven block batteryconstruction. These posts aremanufactured from steel bar,internally threaded for bolting onconnectors and are nickel plated.

The terminal pillar to cover seal isprovided by a compressed visco-elastic sealing surface held inplace by compression lockwashers. This assembly isdesigned to provide satisfactorysealing throughout the life ofthe product.

Flame arresting vent plug

Protective cover

to prevent short circuits

Plate group busto ensure reliable connections between projection-weldedplate tabs and terminal posts

Plate groups

Plate tab spot welded for greater strength to platesideframes and upper edge of pocket plate

Separating gridsto allow free flow of electrolyte between plates

Plate frame

robust seal for plate pockets, serves as current collector

Plate

with horizontal pockets of double-perforatednickel-plated steel strips

Active materials

Nickel hydroxide and cadmium oxide,balanced to match charge-dischargecharacteristics of photovoltaic applications

Electrolyte

Potassium hydroxide with additives forbetter charging efficiency

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4. Benefits of the Sunica battery

The benefits of the Saft Sunicaphotovoltaic battery are:

Complete reliability

Does not suffer from the suddendeath failure associated with otherbattery technologies.

Long cycle life

Sunica has a long cycle life evenwhen the charge/discharge cycleinvolves full discharges.

Exceptional long life

Sunica incorporates all the designfeatures associated with theconventional Saft twenty yearlife products.

Low maintenanceWith its increased electrolytereserve, Sunica reduces the needfor topping up with water and canbe left in remote sites for longperiods without any maintenance.

Low installation costs

Sunica can be used with a widerange of photovoltaic systems as itproduces no corrosive vapours,uses corrosion free polypropylenecontainers and has a simplebolted assembly system.

Well proven pocket plateconstruction

Saft has nearly 100 yearsof manufacturing and applicationexperience with respect to thenickel-cadmium pocket plateproduct and this expertise hasbeen built into the twenty plusyears design life of the Sunicaproduct.

Wide operating temperaturerange

Sunica has a special optimisedelectrolyte which allows it to havea normal operating temperature offrom 30C to + 50C, andaccept extreme temperatures,ranging from as low as 50C toup to + 60C.

Resistance to mechanical abuse

Sunica is designed to have themechanical strength required towithstand all the harsh treatmentassociated with transportationover difficult terrain.

High resistance to electricalabuse

While the use of a voltageregulator is recommended toobtain maximum overall efficiencyof the system, the failure of thiscomponent will not damage thebattery. It will simply cause anovercharging of the battery andso use extra water. The Sunicabattery is resistant to overchargeand over-discharge conditions.

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5.1.Capacity

The Sunica battery capacity israted in ampere hours (Ah) and isthe quantity of electricity which itcan supply for a 100 hourdischarge to 1.2 volts after beingfully charged. This figure waschosen as being the most useful forsizing photovoltaic applications.

5.2.Cell voltage

The cell voltage of nickel-cadmiumcells results from theelectrochemical potentials ofthe nickel and the cadmiumactive materials in the presenceof the potassium hydroxideelectrolyte. The nominal voltage forthis electrochemical couple

is 1.2 volts.

and when 90% discharged it isabout 80% higher. Theinternal resistance of a fullydischarged cell has very littlemeaning. Reducing thetemperature also increases theinternal resistance and, at 0C,the internal resistance isabout 40% higher.

5.3. Internal resistance

The internal resistance of a cellvaries with the type of service andthe state of charge and is,therefore, difficult to define andmeasure accurately.

The most practical value for

normal applications is thedischarge voltage response to achange in discharge current. Theinternal resistance of a Sunica cellwhen measured at normaltemperature is approximately140 milliohm divided by the capacity (Ah).

The above figures are for fullycharged cells. For lower states ofcharge the values increase. Forcells 50% discharged the internal

resistance is about 20% higher

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5.4.Affect of temperature onperformance

Variations in ambient temperatureaffects the performance of Sunicaand this must be allowed for in thebattery engineering.

Low temperature operation has theeffect of reducing the performance

but the higher temperaturecharacteristics are similar to thoseof normal temperatures. The effectof temperature is more marked athigher rates of discharge.

The factors which are required insizing a battery to compensate fortemperature variations are given ina graphical form in Figure 1for operating temperatures from 40C to + 60C.

5.5.Short circuit values

The typical short circuit value inamperes for a Sunica cell isapproximately 15 times theampere-hour capacity.

40C 20C 0C + 40C+ 20C + 60C

40%

60%

80%

100%

120%Available capacity

Temperature

Typical temperature derating factors for available capacity

for fully charged cells

The Sunica battery withconventional bolted assemblyconnections will withstand a shortcircuit current of this magnitude formany minutes without damage.

Figure 1: Typical capacity derating factors versus temperature

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5.6.Open circuit loss

The state of charge of the Sunicacell on open circuit stand slowlydecreases with time due to selfdischarge. In practice thisdecrease is relatively rapidduring the first two weeks butthen stabilises to about 2%

per month at + 20C.

The self discharge characteristicsof a nickel-cadmium cell are

affected by the temperature.At low temperatures the chargeretention is better than at normaltemperature and so the opencircuit loss is reduced. However,the self discharge is significantlyincreased at higher temperatures.

The open circuit loss for Sunica

for a range of temperatures whichmay be experienced in aphotovoltaic application is shownin Figure 2.

Figure 2: Typical open circuit loss variation with time

Open circuit period (days)

Percentage of initial capacity

0 50 100 150 200 250 300 350 400

50%

60%

70%

80%

100%

90%

+ 40C

+ 25C

0C

Open circuit capacity loss as a function oftime for different temperatures

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5.7.Cycling

Sunica is designed to withstand thewide range of cycling behaviourencountered in photovoltaicapplications. This can vary fromlow depth of discharges todischarges of up to 100% and thenumber of cycles that the product

will be able to provide will dependon the depth of discharge required.

The less deeply a battery is cycledthen the greater the number ofcycles it is capable of performingbefore it is unable to achieve theminimum design limit. A shallowcycle (say 20%) will give morethan 8000 operations, whereas adeep cycle (say 80%) will giveabout 1000 operations.

Figure 3 gives the effect of depthof discharge on the available cyclelife and, it is clear, that whensizing the battery for anapplication, the number and depthof cycles have an importantconsequence on the predicted lifeof the system.

Figure 3: Typical cycle life versus depth of discharge at + 20C

Cycle depth

20%10% 30% 40% 50% 60% 70% 80% 90% 100% 110%

Number of cycles

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Cycle life as a function of cycle depth of dischargeexpressed as a percentage of the rated capacity

Temperature +20C

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5.8. Effect of temperature onlifetime

Sunica is designed as a twentyyear life product but, as with everybattery system, increasingtemperature reduces the expectedlife. However, the reduction inlifetime with increasing

temperature is very much lower forthe nickel-cadmium battery thanthe lead acid battery.

The reduction in lifetime for boththe nickel-cadmium battery and,for comparison, a high quality"10 year life" lead acid batteryis shown graphically in Figure 4.

In general terms for every 9Cincrease in temperature over thenormal operating temperature of

+ 25C, the reduction in service

life for a nickel-cadmium batterywill be 20% and for a lead acid

Temperature

+ 25C + 30C + 40C + 45C + 50C + 55C + 60C+ 20C + 35C

Figure 4: Typical battery life expectancy at high temperatures

0

2

4

6

8

10

12

14

16

18

20

battery will be 50%. In hightemperature situations, therefore,special consideration must be givento dimensioning the nickel-cadmiumbattery. Under the sameconditions, the lead acid battery isnot a practical proposition due toits very short lifetime.

Rated life in years

Sunica nickel-cadmium

High quality 10 year life lead-acid

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5.9. Water consumption andgas evolution

During charging, more ampere-hours is supplied to the batterythan the capacity available fordischarge. These additionalampere-hours must be provided toreturn the battery to the fully

charged state and, since they arenot all retained by the cell and donot all contribute directly to thechemical changes to the activematerials in the plates, they mustbe dissipated in some way.This surplus charge, or overcharge,breaks down the water content ofthe electrolyte into oxygen andhydrogen, and pure distilled waterhas to be added to replace this loss.

Water loss is associated with thecurrent used for overcharging.

A battery which is constantlycycled, i.e. is charged anddischarged on a regular basis,will consume more water than abattery on standby operation.

In theory, the quantity of waterused can be found by the Faradicequation that each ampere hour of

overcharge breaks down 0.366 ccof water. However, in practice, thewater usage will be less than thisas the overcharge current is alsoneeded to support self dischargeof the electrodes.

The overcharge current is afunction of both voltage andtemperature and so both havean influence on the consummationof water.

Figure 5 gives typical waterconsumption valuesover a range of voltagesand temperature.

The gas evolution is a functionof the amount of water electrolysedinto hydrogen and oxygen and arepredominately given off at the end

of the charging period. The batterygives off no gas during discharge.

The electrolysis of 1 cc of waterproduces about 2000 cc of gasmixture and this gas mixture is inthe proportion of 2/3 hydrogenand 1/3 oxygen. Thus theelectrolysis of 1 cc of waterproduces about 1300 cc ofhydrogen.

Temperature+ 10C + 20C + 30C + 40C

0.1

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

Water consumption (g/month, Ah)

Figure 5: Typical water consumption values

Typical water consumption values in gramsper month per Ah of capacity as a function oftemperature and voltage e. g. a battery of120 Ah floating at 1.50 volts at +20C

would use over a period of 6 months:6 (months) x 120 (Ah) x 0.42 (factor for1.50 volts at +20C) = 6 x 120 x 0.42 =302 g of water per cell

1.55 volts/cell

1.45 volts/cell

1.40 volts/cell

1.50 volts/cell

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6. Battery charging

6.1.Charging generalities

The photovoltaic array convertssolar irradiance into dc electricalpower at a predetermined rangeof voltages whenever sufficientsolar radiation is available.Unlike a mains connected systemthe output from a photovoltaic

array is variable and, to obtainthe best efficiency from thesystem, it is quite normal to havesome form of charge control.

Two techniques for chargingnickel-cadmium batteries usedin photovoltaic systems are asfollows:

6.1.1. Voltage limit with chargecurrent limited by array

In this system, the battery ischarged at a current which isdetermined by the arraycapability. When the batteryhas reached a predeterminedvoltage limit, the charge current

tapers off. Ideally, to obtain thebest efficiency from the system,the voltage limit should betemperature compensated.The battery will continue tocharge until the photovoltaicpower available is less than thatrequired to charge it.

The upper voltage limit for thenickel-cadmium battery is usuallychosen in the range of 1.55 to

1.65 volts/cell depending on thesystem requirements.

6.1.2. Two step charging

This differs from the above methodby allowing the batteryto charge up to a high presetvoltage and then dropping to alower maintenance level voltageto reduce the water consumptionof the battery. This second low

level "floating" voltage should bein the range 1.42 to 1.45 voltsto ensure that the battery will stillcontinue to accept a small level ofcharge. Again, as the previousmethod, it is recommended thattemperature compensation shouldbe used to reduce waterconsumption of the battery.

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6.2. Charge efficiency

The charge efficiency of Sunicais dependent on the state ofcharge of the battery and thetemperature.For much of its charge profile it isrecharged at a high level ofefficiency.

Figure 6 shows the chargingefficiency for batteries 50%,70% and 90% charged for awide range of temperatures and,it should be noted that, althoughthe charging efficiency is at itlowest at +40C it still remains atabove 75% for a battery already70% charged.

In general, at states of charge lessthan 80% the charge efficiencyremains high but asthe battery approaches a fullycharged condition, its thecharging efficiency falls off.This is illustrated graphicallyin Figure 7.

In practice, a photovoltaicsystems battery normally has astate of charge between 20% and80% and so the chargingefficiency of Sunica remains high.

The charging efficiency of Sunicais not reduced with time and sothis does not have to be takeninto account in battery sizing.

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Figure 7: Typical ampere-hour efficiency at + 25

C

Charge at constant current (current0.2 C

5A)

10C + 20C + 40C+ 30C+ 10C0C30%

40%

50%

60%

70%

80%

90%

100%

110%Charging efficiency

Temperature

Figure 6: Charging efficiency as a function of temperature

Charging efficiency of the Sunica battery as afunction of temperature and the state of chargeof the battery

Battery 50% charged

Battery 90% charged

Battery 70% charged

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

110%

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140Charge input (% rated capacity)

20C

Percentage

Charge efficiency (%)

Available capacity (% rated)

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6.3. Temperature effects

As the temperature increases,then the electrochemicalbehaviour becomes more activeand so, for the same floatingvoltage, the current increases.As the temperature is reduced,then the reverse occurs. Increasing

the current, increases the waterloss and reducing the currentcreates the risk that the cell willnot be sufficiently charged.

Thus, as it is clearly advantageousto maintain the same currentthrough the cell, it is necessaryto modify the floating voltage asthe temperature changes.The change in voltage required,or "temperature compensation",is given in Figure 8.

40C 20C + 20C + 60C+ 40C0C

8%

4%

0%

4%

8%

12%

Voltage adjustment

Figure 8: Charging voltage adjustment for sustained temperatures

Temperature compensation recommendedfor sustained temperatures above +30Cand below +10C

If these values cannot be exactlymet with a particular systemthen temperature compensationvalues of between 2 mV/C and3.5 mV/C are acceptable.

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6.4.Commissioningrequirements

It is recommended that a goodfirst charge should be given tothe battery.

This is particularly true fordischarged and empty cells which

have been filled as they will be ina totally discharged state.

A constant current first charge ispreferable and this should be suchas to supply 300% of the ratedcapacity of the cell.Thus, a 250 Ah cell will require750 ampere hours inpute.g. 50 amperes for 15 hours.

In cases where it is not possible toprovide constant currentcharging, it is possible to achievethis with a constant voltage byusing a high voltage level. e.g.1.65 voltage limit may be usedfor 20 to 30 hours if the currentlimit is approximately equivalentto the 5 hour charge current (cell

rated capacity 5).If the current rating is lower thenthe charge time should beincreased accordingly.

When the charger maximumvoltage setting is too low tosupply constant current charging,divide the battery into two partsto be charged individually at ahigh voltage.

In the case of remote areas, wherethe only charger available is thephotovoltaic array, charging maybe possible.The battery should be connectedto the system with no connectedload and no voltage limit.The battery should then becharged, in good sunshine

conditions, for approximately threetimes the calculated full chargecapability of the solar array.

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7. Special operating factors

7.1. Electrical abuse

7.1.1. Ripple effects

The nickel-cadmium battery istolerant to high ripple and the onlyeffect is that of increased waterusage. In general, anycommercially available chargeror generator can be used forcommissioning or maintenancecharging of Sunica.

7.1.2. Over-discharge

If more than the designed capacityis taken out of a battery then itbecomes over-discharged.This is considered to be an abusesituation for a battery and shouldbe avoided.

In the case of lead acid batteriesthis will lead to failure of thebattery and is unacceptable.

The Sunica battery is designed tomake recovery from this situationpossible.

7.1.3. Overcharge

In the case of Sunica, with itsgenerous electrolyte reserve, asmall degree of overcharge willnot significantly alter themaintenance period. In the case ofexcessive overcharge, waterreplenishment is required but therewill be no significant effect on thelife of the battery.

7.2. Mechanical abuse

7.2.1. Shock loads

The Sunica block batteryconcept has been tested to bothIEC 68-2-29 (bump tests at 5 g,10 g and 25 g) and IEC 77(shock test 3 g).

7.2.2. Vibration resistance

The Sunica block batteryconcept has been tested toIEC 77 for 2 hours at 1 g.

7.2.3. External corrosion

Sunica nickel-cadmium cellsare manufactured in durablepolypropylene, all external metalcomponents are nickel plated

and these components areprotected by a rigid plastic cover.

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The type of use of the PV systemand the required reliability is ofparamount importance in sizingthe system.Professional applications(emergency systems, sea-lights,radio beacons etc.) have to beoversized according to theirimportance and it is necessary to

take into account the workingconditions of the system.

It is not the purpose of this manualto give sizing methods forcomplete photovoltaic systems.However, this is an applicationwith specific performancerequirements and it is useful todiscuss the different factors whichcan effect the design of the systemand the battery sizing.

The array and battery size arerelated since the photovoltaicsystem must have array andbattery sizes which are sufficientfor the load to operate at all therequired times throughout the year.The system could have a smallarray and a large battery or viceversa. However, there are limits tothese sizes as, while the minimumarray size is that which can deliver

the annual dailyload in the average dailyinsolation, the minimum batterysize is that which can supplythe overnight load.

The final choice of a particularcombination of array size andbattery size is related to therelative costs of each, theirrespective maintenance costsand their respective replacement

frequencies.

The following factors have amajor influence when sizing aphotovoltaic system:

Insolation

Photovoltaic array tilt angle

Load size and profile

Ambient temperature

Maximum permissible depthof discharge of the battery

System availability (autonomy)

Numerous other factors have alesser but still significant effect:

Battery charging efficiency

Battery charging current

Ageing of array

Safety margin Rate of self discharge of

batteries

Water consumption

The importance of each factoris discussed below and it isimportant that a batterymanufacturer should supply suchinformation where required.

The insolation is very importantand the annual variation inaverage daily insolation isrequired. This is often availableas average monthly insolationvalues and should be preferableaverages over several years.For some types of sizing programsthe number of consecutive days oflow insolation is required.

The size and angle of tilt of the

photovoltaic array affects the

quantity of insolation which isconverted into usable electricity.This is also influenced by theambient temperature.

The load characteristics have amajor effect on battery size. If theload demand is mostly during themiddle of the day then the battery

size will be smaller than if theload is during the night.

If it is possible to discharge thebattery fully, then the battery sizerequired will be smaller than if thebatteries may only be dischargedto a predetermined value. Indeciding the maximum dischargedepth, consideration must begiven to its effect on battery life(section 5.7) e.g. if the battery

size is halved by doubling thedepth of discharge but the life ofthe battery is reduced four-foldthen perhaps it may be better torestrict the depth of discharge anduse a larger battery. Again, theambient temperature affects thetotal available battery capacity(section 5.4).

The charging efficiency is the ratioof the energy supplied to

the energy stored in the battery.This is dependent both on theambient temperature and the stateof charge of the battery.It is important to allow for differentcharging efficiencies at extremesof ambient temperature and depthof discharge (section 6.2).

8. Battery sizing principles

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The charging current has aninfluence on charging efficiencyand, as in photovoltaicapplications the charging currentis very variable, values usedshould be typical for low chargingcurrents and it should take intoaccount the ambient temperature(section 6.3).

The water consumption, has anindirect effect on the sizing of thebattery as, if reduced maintenancevisits to the site are required, itmay be necessary to oversize thebattery.Correct choice of battery type andcharging voltages can reduceconsiderably the waterconsumption (section 5.9).

Over the life of the photovoltaicsystem, due to ageing effects, thearray output will decease slightlyand the fully charged capacity ofthe battery will also decrease.In addition, high ambienttemperatures will have an affecton the lifetime of the battery.Thus an over-sizing of the batteryis required to allow for this(section 5.8).

In some applications where theremay be periods of several weeksor more when the system is neithercharged or discharged, the selfdischarge, or open circuit loss,of the batteries should also beconsidered (section 5.6).

Thus the battery information

required for sizing a photovoltaicapplication is generally moreextensive than that required forconventional systems.

Sizing methods

With regard to photovoltaic systemsizing, there are fundamentally twodifferent methods used. Onemethod calculates the array size fora stated system availability and arange of battery sizes, the otheruses the number of days ofautonomy required for the system(based upon the maximumexpected number of low insolationdays) which determines the batterysize and then calculates the arraysize. The former method is oftenknown as availability sizing, thelatter as autonomy sizing.

Autonomy sizing

Autonomy sizing is generallyapplicable where the insolation isfairly constant during the year,typically in the tropics, but it canbe extended to any situationwhere a non-sunny period can beexpected. Thus, it relies on astatistical analysis of consecutivenon-sunny days and depends onan optimisation between cost andsystem availability.

For the battery sizing, it is fairlysimple to carry out.

If the battery is assumed to be fullycharged every day the followingformula can be used:

Capacity = A x L x kt x kd x kawhere:A = required autonomy (hours)L = daily loadkt = temperature compensationfactor (see Figure 1)kd = compensation factor formaximum allowable depth ofdischarge (see Figure 3)ka = compensation factor forageing (see Figure 4)

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found from the number and depthof discharge and the optimumsystem produced.

Clearly, this iterative methodrequires many differentparameters and is timeconsuming. Computer programswhich carry out such an analysis

are used by all major systemsmanufacturers and are beyond thescope of this manual. It isrecommend that system sizingshould be referred to suchsuppliers but if you do havedifficulty to obtaining theinformation you need or findinga suitable system supplierdo nothesitate to contact your Saftrepresentative.

Availability sizing

Where the insolation data is notconstant enough throughout theyear to use autonomy sizing, thenavailability sizing must be used.This method of sizing requiresmeteorological data (e.g. TypicalReference Year for Europe or

Typical Meteorological Year forUS to give insolation and daytimetemperatures) to be available and,for a particular tilt angle,the load and battery type areentered. The minimum array sizeis calculated from the arrayparameters such that it will supplyjust enough electrical energy inthe year to meet the annual load.Using this minimum array size, the

array size to meet the worst monthfor different battery sizes can thenbe calculated.Once the array/battery size isfound the battery life may be

Thus, as an example, we willtake an application which requiresan autonomy of 3 days (72 hours),but is usually only dischargedto its design limit once every2 weeks, has a normal ambienttemperature of + 30C and hasa normal average daily loadof 100 watts/48 volts.

(see example below).

Autonomy sizing can be of twodistinct types. The inter-seasonalstorage type, where the arraysize (small) is chosen to produceenough energy to match theyearly load and the battery (large)is required to store sufficientenergy produced in summer tosupply the energy deficit in winter,or, the short term storage type,where the array size is chosen tocover the load demand in the worstsunlight period, and the batterysize is chosen to supply energyover a prescribed number ofdays autonomy.

Autonomy = 3 x 24 = ............................................................. 72 hoursDaily load = 100/48 = ...............................................................2.1 ATemperature compensation factor (Figure 1) = 1/1.04 = ................. 0.96Maximum allowable depth of discharge is calculated from liferequirement (20 years) and number of cycles (26 per year) =........... 520so discharge depth allowable (Figure 3) = ......................................90%and so factor = 1/0.9 = ............................................................... 1.11Compensation factor for aging at + 30C (figure 4) = 20/18 = ....... 1.11So battery capacity = 72 x 2.1 x 0.96 x 1.11 x 1.11 = ...........178.8 AhNumber of cells required = 48/1.2 = ............................................... 40

and so the battery will be 8 blocks of Sunica type SUN 17-5.

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9. Installation and storage

9.1.Emplacement

The battery should be installed ina dry and clean location awayfrom direct sunlight, strongdaylight and heat.

Sunica batteries can be fitted ontostands, can be floor mounted or

can be fitted into cabinets.Local Standard or Codes normallydefine the mounting arrangementsof batteries, and these must befollowed if applicable.However, if this is not the case,the following comments shouldbe used as a guide.

When the battery is housed ina cubicle or enclosedcompartment it is necessary toprovide adequate ventilation.

A typical figure for roomventilation is about 2.5 airchanges per hour and undersuch conditions it is satisfactoryto install 700 watt hours of batterycapacity per cubic meterif the final charge current is at0.1 C5 amperes.

Note that special codes forventilation may be applicablein your area and/or batteryapplication. In case of doubt,please contact Saft for advice.

When mounting Sunica it isdesirable to maintain an easyaccess to all blocks and they

should be situated in a readilyavailable position.Distances between stands, andbetween stands and walls, shouldbe sufficient to give good accessto the battery.

The overall weight of the batterymust be considered and the loadbearing on the floor taken intoaccount in the selection of thebattery accommodation. In caseof doubt, please contact Saftfor advice.

When mounting the battery ensurethat the cells are correctlyinterconnected with theappropriate polarity. The batteryconnection to load should be withnickel plated cable lugs.

The connectors and terminalscrews should be corrosion-protected by coating with a thinlayer of neutral vaseline or anti-corrosion oil.

Recommended torque forconnecting screws are:

M6 = 11 1.1

N.m M 8 = 20 2 N.m M10 = 30 3 N.m

To avoid accelerated ageing ofthe plastic due to UV-light,batteries with plastic cellcontainers should not be exposedto direct sunlight or strongdaylight for a prolonged period.

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9.2.Electrical

Batteries may be suppliedcharged and filled or dischargedand empty.

9.2.1. Batteries supplied chargedand filled

When the battery is receivedremove immediately the plastictransport seals. The battery isready for installation.

A filled battery can be stored fora maximum of 1 year and shouldbe given a commissioning chargeas described in section 6.4 beforeputting into service.

Cells delivered filled have already

the cell oil in place.

9.2.2. Batteries supplieddischarged and empty

When the battery is received donot remove the plastic transportseals until ready to fill the batterywith electrolyte.

A fully discharged, empty batterywith its transport seal can bestored for many years.

When filling the cells refer tothe "Electrolyte Instructions"data sheet supplied with theelectrolyte.

The electrolyte type to be usedfor the first filling is: E30.

The cell oil must be added toreduce water loss due toevaporation and self discharge.Add the oil with a syringe,according to the quantity indicatedin the Installation and operatinginstruction sheet.

After filling with electrolytethe battery should be given afull charge discharge cycle (300%charge, discharge to1.0 volt/cell) before passingto the normal commissioningcharge described in section 6.4.

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10.Maintenance of Sunica batteries

in service

In a correctly designed standbyapplication Sunica requires theminimum of attention.

However, it is good practice withany system to carry out aninspection of the system once peryear or at the recommendedtopping up interval period to

ensure that the charger, thebattery and the ancillaryelectronics are all functioningcorrectly.

When this system service iscarried out it is recommended thatthe Sunica cell electrolyte levelsshould be checked visually toensure that the level is above theminimum and if necessary thecells should be topped up.

The batteries should also bechecked for external cleanliness,and if necessary cleaned with adamp cloth and the cover shouldbe removed to check that theconnections are tight, that theprotective grease on the terminalsremains intact and that the ventsare clean.

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W L

H

11.Sunica battery range

Type

Voltage Capacity (Ah) Dimensions (mm) Weight Electrolyte (l)

(V) 100 h to 1.2 V L W H (kg) Total Reserve

SUN 3-2 2.4 35 63 195 349 5 0.74 0.22SUN 3-3 3.6 35 88 195 349 8 0.74 0.22SUN 3-5 6.0 35 137 195 349 13 0.74 0.22SUN 7-2 2.4 70 85 195 349 8 1.08 0.35SUN 7-3 3.6 70 121 195 349 12 1.08 0.35SUN 7-5 6.0 70 192 195 349 21 1.08 0.35

SUN 10-2 2.4 105 109 195 349 11 1.31 0.48SUN 10-3 3.6 105 157 195 349 17 1.31 0.48

SUN 10-56.0 105 252 195 349 28 1.31 0.48

SUN 14-2 2.4 143 133 195 349 14 1.84 0.61SUN 14-3 3.6 143 193 195 349 21 1.84 0.61SUN 14-5 6.0 143 312 195 349 37.5 1.84 0.61SUN 17-2 2.4 178 165 195 349 17 2.17 0.75SUN 17-3 3.6 178 238 195 349 25.5 2.17 0.75SUN 17-5 6.0 178 377 195 349 42.5 2.17 0.75

SUN 21-2 2.4 214 189 195 349 19 2.60 0.88SUN 21-3 3.6 214 274 195 349 29 2.60 0.88SUN 21-5 6.0 214 437 195 349 48 2.60 0.88SUN 24-2 2.4 251 228 195 349 24 3.25 1.10SUN 24-3 3.6 251 336 195 349 36 3.25 1.10SUN 28-2 2.4 286 252 195 349 27 3.78 1.20SUN 28-3 3.6 286 372 195 349 40.5 3.78 1.20

SUN 31-2 2.4 322 278 195 349 31 4.00 1.35SUN 31-3 3.6 322 411 195 349 46.5 4.00 1.35

SUN 35-2 2.4 358 304 195 349 32 4.24 1.50SUN 35-3 3.6 358 450 195 349 48 4.24 1.50SUN 38-1 1.2 393 171 195 349 18 4.77 1.65SUN 42-1 1.2 429 183 195 349 19 5.20 1.75SUN 52-1 1.2 537 232 195 349 25.5 6.40 2.25SUN 63-1 1.2 645 268 195 349 29 7.80 2.65SUN 70-1 1.2 717 304 195 349 34 8.60 3.00SUN 84-1 1.2 860 352 195 349 38.5 10.40 3.50SUN 87-1 1.2 896 377 195 349 42.5 10.60 3.75

SUN 104-1 1.2 1070 437 195 349 48 13.00 4.40

+

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Industrial Battery Group

156, avenue de Metz - 93230 Romainville - FranceTelephone : +33 (0)1 49 15 36 00 Fax : +33 (0)1 49 15 34 00 Web: www.saft.alcatel.com

Doc NRM 12.99.21028.2.Data in this document is subject to change without notice and becomes contractual only after written confirmation. So

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