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# Chapter 26 battery sizing and discharge analysis norestriction

## by Mostafa Khaled, Working at ROV Egypt Competition on Jun 30, 2012

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## Chapter 26 battery sizing and discharge analysis norestrictionDocument Transcript

• In the above sample curves, the set of curves may apply to batteries of different sizes or to only one size.In ETAP PowerStation, you specify a set of characteristic curves for a givensize. If you want to use a given set of curves for batteries of different sizes, youcan indicate this in the Battery Method Case Editor. 26.4.3 Battery Sizing Sizing StudyThe battery sizing calculation includes determining the number of cellsto meet the system voltage requirement and determining the battery sizeand number of strings to meet the load duty cycle requirement.Number of CellsThe number of cells should be determined to satisfy systemminimum and maximum voltage requirements:1. When charging the battery, the voltage to be applied to the battery should notbe greater than the maximum system voltage.
• Let N be the number of cells. The voltage requirements can be given inthe following equation V sys, ≤N≤ V sys, V mincell, V cell, max disch ch Where Vsys,min is the minimum system voltage that is equal to the nominal voltage of the battery terminal bus multiplied by the minimum system voltage deviationVsys,maxis the maximum system voltage that is equal to the nominal voltage of the entered in the Battery Sizing Study Case Editor.battery terminal bus multiplied by the maximum system voltage deviationVcell,disch is the battery discharge voltage in V/Cell entered in theBattery Sizing Study Case Editor.entered in the Battery Sizing Study Case Editor.It is clear that the number of cells of the battery is dependent on the four values for voltage requiremententered in the Battery Sizing Study Case Editor. It can happen that forsome incompatible values, we cannot determine a value for N to satisfythe above equation. When this situation occurs, ETAP will display amessage indicating that it cannot determine the number of cells. In practical cases, there is often a range of values that N can take tosatisfy the above equation. In thiscase, ETAP will select the value for N that results in the battery ratedvoltage being closest to its terminal bus nominal voltage.
• Cell SizeIn determining the battery size, ETAP will find the smallest size that can provide sufficient power for thespecified duty cycle. The capacity of a battery can be increasedeither by using a larger size or by adding more strings. Since ETAPallows you to enter different characteristic curves for different sizesof batteries, in the battery sizing calculation, the program starts withone string and the smallest size available for the calculation. If it failsto meet the load requirement, the program first increases the size andperforms calculations with the characteristic curves for the new size.When no available sizes can meet the load requirement for the givennumber of strings, it then increases the string number and performsthe calculation with the smallest size again. This process continuesuntil a battery size and a string number are found to meet the loadrequirement.Load Sections in Battery Duty CycleA battery duty cycle generally can be represented as a squarewaveform. It consists of a number of time periods, with a constant current value during a period. The figure below shows a sampleduty i S i a= ∑ It P jcycle for battery.1 consists of six periods, designated as P1, P2, j=… P6. A load section Si is a combination of a number of loadperiods, defined as:
• In the sample duty cycle there are six load sections. Load Sections for A Sample Battery Duty CycleDetermination of Cell Size Based on Battery Characteristic CurvesBased on a given set of battery characteristic curves, we can determine the required battery size for a specified duty cycle. Let F represent cell size. It is F= Max Fi i=1,..Sm equal to:where Sm is the total number of load sections and Fi is the sizecalculated for the ith load section. The calculation of Fi depends on thewhere Ap is the load current value in period P. RT is the value obtained from thetype of characteristic curve, which is the number of amperes that each positivebattery battery library curves.
• Random Load and Non-Random LoadIn general, the duty cycle for a battery consists of random loads andnon-random loads. The program determines the cells for random andnon-random loads separately in the same way as described in theprevious section. The sum of the two cell size values is the uncorrectedcell size for the given duty cycle. Adjusting Factors In the Battery Sizing Study Case Editor, you can select several adjusting factors to be considered in calculating battery size. These factors include temperature factor, design margin factor, agingCalculation CycleIt is clear from the equations for determining cell size that the cell size iscalculated based on a given set of battery characteristic curves, which is for agiven cell size. If the calculated cell size is different from the onecorresponding to the characteristic curves used. We have to do the calculation