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1. 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 given
size. If you want to use a given set of curves for batteries of different sizes, you
can indicate this in the Battery Method Case Editor.
26.4.3 Battery Sizing Sizing Study
The battery sizing calculation includes determining the number of cells
to meet the system voltage requirement and determining the battery size
and number of strings to meet the load duty cycle requirement.
Number of Cells
The number of cells should be determined to satisfy system
minimum and maximum voltage requirements:
1. When charging the battery, the voltage to be applied to the battery should
not
be greater than the maximum system voltage.

2. Let N be the number of cells. The voltage requirements can be given in
the 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 deviation
Vsys,max
is 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 deviation
Vcell,disch is the battery discharge voltage in V/Cell entered in the
Battery 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 requirement
entered in the Battery Sizing Study Case Editor. It can happen that for
some incompatible values, we cannot determine a value for N to satisfy
the above equation. When this situation occurs, ETAP will display a
message indicating that it cannot determine the number of cells.
In practical cases, there is often a range of values that N can take to
satisfy the above equation. In this
case, ETAP will select the value for N that results in the battery rated
voltage being closest to its terminal bus nominal voltage.

3. Cell Size
In determining the battery size, ETAP will find the smallest size that
can provide sufficient power for the
specified duty cycle. The capacity of a battery can be increased
either by using a larger size or by adding more strings. Since ETAP
allows you to enter different characteristic curves for different sizes
of batteries, in the battery sizing calculation, the program starts with
one string and the smallest size available for the calculation. If it fails
to meet the load requirement, the program first increases the size and
performs calculations with the characteristic curves for the new size.
When no available sizes can meet the load requirement for the given
number of strings, it then increases the string number and performs
the calculation with the smallest size again. This process continues
until a battery size and a string number are found to meet the load
requirement.
Load Sections in Battery Duty Cycle
A battery duty cycle generally can be represented as a square
waveform. It consists of a number of time periods, with a constant
current value during a period. The figure below shows a sample
duty
i
S i a= ∑ It P j
cycle for battery.1 consists of six periods, designated as P1, P2,
j=
… P6. A load section Si is a combination of a number of load
periods, defined as:

4. In the sample duty cycle there are six load sections.
Load Sections for A Sample Battery Duty Cycle
Determination of Cell Size Based on Battery Characteristic Curves
Based 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 size
calculated for the ith load section. The calculation of Fi depends on the
where Ap is the load current value in period P. RT is the value obtained from the
type of characteristic curve, which is the number of amperes that each positive
battery battery library curves.

5. Random Load and Non-Random Load
In general, the duty cycle for a battery consists of random loads and
non-random loads. The program determines the cells for random and
non-random loads separately in the same way as described in the
previous section. The sum of the two cell size values is the uncorrected
cell 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, aging
Calculation Cycle
It is clear from the equations for determining cell size that the cell size is
calculated based on a given set of battery characteristic curves, which is for a
given cell size. If the calculated cell size is different from the one
corresponding to the characteristic curves used. We have to do the calculation

6. 26.4.4 Battery Discharging Calculation Method
The purpose of battery discharge calculation is to determine battery
performance for a specified duty cycle. One of the key parameters for battery
performance is the battery terminal voltage. When the battery is supplying
the load as the sole source, it should be able to maintain voltage level for the
whole period of the specified duty cycle.
Battery Characteristic Curves for Voltage Interpolation
The terminal voltage of a battery is dependent on the current drawing from the
battery and the ampere-hour capacity contained in the battery. This
relationship is described by the battery characteristic curve and is very
nonlinear. In ETAP, the battery characteristics are described in the battery
library as discrete points. Because no closed form equation is available to
describe the battery characteristics, numerical interpolation methods have to
Battery Combined Duty Cycle
be used to find the points missing in the curves. Apparently, the more curves
When the load powered by the battery includes random load, the random load
are entered in the battery library, the more accurate the calculated results will
should be added to the non-
random load at the worst point, which is the time the battery has the lowest
be.
voltage value when only the non-random load is considered. To identify this
time moment, the program first performs battery discharge calculation
excluding the random load. It then determine the worst point, add the random
load to the non-random load and perform discharge calculation from the time
when the random load takes effect all the way to the end of battery duty cycle.

7. Battery Voltage Calculation
An iterative process is conducted to calculated battery discharge
voltage values. A battery voltage value
is reported at each time step specified in the battery sizing study case
and at each moment when there is a change in the load duty cycle. By
changing the step size from the battery sizing study case, the user can
adjust the level of detail information on discharge calculation to be
reported.
If the battery duty cycle is calculated by the load current summation
method, the battery current will
change only when there is a change in any load duty cycle. When the
load flow method is selected in the study case, even if there is no
change in the load duty cycle, the battery current will change due to
decrease in the battery voltage. In this case the battery current is
calculated by a full load flow calculation, considering different types
of loads and system losses. In this load flow calculation, the battery is
modeled as a constant voltage source with the voltage calculated in the
previous step. The calculated battery current will be used in the
current step for battery voltage calculation.
Along with battery voltage and current, the battery discharge program
also calculates battery discharge
capacity. When there is change in the load current, two values of
voltage
When the battery is calculated using load flow method, the battery
and current are calculated, at t –
discharge calculation also provides a
and t+, one for before the load change and one for after the load change.

8. 26.5Required Data
26.5.1 Source
In battery sizing calculation, the only source is the battery to be
sized. Batteries may only besized/discharged one at a time as
specified in the study case. A UPS may be considered as a load to the
Battery
system when its input bus is not connected to an energized bus.
•ID
•Bus connection data
•Battery library type data. This information is used to retrieve library
data for calculations.
If only the battery discharge calculation is conducted, the
following additional information is also
•Battery number of plates and Capacity.
required:
•Number of cells
•Number of Strings
•SC page battery external resistance.

9. 26.5.2 Load
UPS
When a UPS is not connected to an energized input AC bus, it is
considered a load in battery sizing calculations.
•ID
•Bus connection data
•DC rated voltage kW and kVA. Duty Cycle Page
•If no duty cycle data is entered, this load will be assumed to be zero.
DC Motor
•ID
•Bus connection data
•Quantity
•Rated voltage
•kW or HP and Efficiency. Duty Cycle Page

10. Lumped Load
•ID
•Bus connection data
•Rated voltage kW Rating
•Duty Cycle Page
•If no duty cycle data is entered, this load will be assumed to be zero.
Static Load
•ID
•Bus connection data kW Rating.
•Rated voltage
•Duty Cycle Page
•If no duty cycle data is entered, this load will be assumed to
be zero.
Elementary Diagram (ED) Load
•ID
•Bus connection data
•Rated voltage kW Rating.
•Duty Cycle Page
•If no duty cycle data is entered, this load will be assumed to
be zero.

11. Inverter
•ID
•Bus connection data
•DC rated voltage
•kVA, PF, DC kW rating
•Duty Cycle Page
•If no duty cycle data is entered, this load will be assumed to be
zero.
26.5.3 Branch
DC Cable
•ID
•Bus connection data
•Cable length
•Resistance and Inductance and cable length
units
DC Impedance
•ID
•Bus connection data
•Resistance and inductance impedance
information.

12. Tie PD (CB, Fuse, & Single-Throw & Double-ThrowSwitches)
•ID
•Bus connection data
DC Converter
•ID
•Bus connection data
•kW Rating and Rated kV Input and output.
Library
•Library type data
•Battery characteristic curve data
Study Case
When you initiate a battery sizing calculation, PowerStation uses the
study case currently selected from
the Study Case Toolbar. Every field in the Study Case Editor is set to
its default value. However, it is important to set the values in the study
case correctly to meet your calculation requirements.

13. The battery sizing calculation results are reported graphically on the one-line diagram, in plots and in the
Crystal Reports format. The graphical one-line display shows the number of cells, number of strings, cell
size, etc. You can use the Display Options Editor to specify the content to be displayed.
26.6 Output Reports
The Crystal Reports format provides you with detailed information for a
battery sizing study. You can
utilize the Battery Sizing Report Manager to help you view the output
26.6.1 Battery Sizing Report Manager
report.
To open the Battery Sizing Report Manager, simply click on the View
Output File button on the Battery
Sizing Study Toolbar.
The editor includes four pages (Complete, Input, Result, and
Summary)
representing different sections of the output report. The Report
Manager
allows you to select formats
available for different portions of the report and view it via Crystal
Reports. There are several fields and buttons common to every page, as
described below.

14. Output Report Name
This field displaysName to the output report you want to view.
Project File the name
This field displays the name of the project file based on which report was
Help
generated, along with the directory where the project file is located.
Click on this button to access Help.
OK / Cancel
Click on the OK button to dismiss the editor and bring up the Crystal
Reports view to show the selected portion of the output report. If no
selection is made, it will simply dismiss the editor. Click on the Cancel
Complete Report Page
button to dismiss the editor without viewing the report.
In this page there is only one format available, Complete, which brings
up the complete report for the
battery sizing study. The complete report includes input data, results,
and summary reports.

15. Input Page
This page allows you to select formats to view different input data,
grouped according to type. They
include the following available formats:
•Battery Characteristics
•Branch Connection
•Bus and Connected Load
•Cable
•Cover
•DC Converter Impedance Inverter
•Load Duty Cycle
•UPS

16. Result Page
This page allows you to select formats to view the result portion
of the output report, including Calculation Results, Battery Load
Profile, and Battery Characteristics. The Calculation Results portion
prints the uncorrected cell size for each load section in non-random
load and random load. The Battery Load Profile is the battery duty
cycle generated based on load duty cycles. The Battery Characteristic
s are mostly data entered by the user