Ieee 485 2020

IEEE Recommended Practice
STANDARDS
for Sizing Lead-Acid Batteries
for Stationary Applications
IEEE Power and Energy Society
Developed by the
Energy Storage and Stationary Battery Committee
IEEE Std 485™-2020

(Revision of IEEE Std 485-2010)
Authorized licensed use limited to: PUC MG. Downloaded on July 07,2020 at 16:55:19 UTC from IEEE Xplore. Restrictions apply.
IEEE Std 485™-2020
(Revision of IEEE Std 485-2010)

Sponsor
Energy Storage and Stationary Battery Committee

of the
IEEE Power and Energy Society
Approved 6 May 2020
IEEE SA Standards Board
Abstract: Methods for defining the dc load and for sizing a lead-acid battery to supply that load
for stationary battery applications in float service are described in this recommended practice.
Some factors relating to cell selection are provided for consideration. Installation, maintenance,
qualification, testing procedures, and consideration of battery types other than lead-acid are
beyond the scope of this recommended practice. Design of the dc system and sizing of the battery
charger(s) are also beyond the scope of this recommended practice.
Keywords: battery duty cycle, cell selection, dc load, full-float operation, IEEE 485™, lead-acid
batteries, rated capacity, sizing, stationary applications, valve-regulated lead-acid (VRLA) cell,
vented battery, vented lead-acid (VLA)
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5
Participants
At the time this IEEE recommended practice was completed, the Vented Lead Acid Sizing Working Group had
the following membership:
James Midolo, Chair

Sepehr Mogharei, Vice Chair
Amber Aboulfaida Rufus Lawhorn Kenneth Sabo

Robert Beavers Daniel Martin Surendra Salgia
Steven Belisle Tania Martinez Navedo Joseph Stevens
Thomas Carpenter Thomas Mulcahy Richard Tressler
Ali Heidary Volney Naranjo Lesley Varga
Ken Hill Jason Wallis
The following members of the individual balloting committee voted on this recommended practice. Balloters
may have voted for approval, disapproval, or abstention.
Amber Aboulfaida James Graham Arthur Neubauer

William Ackerman Randall Groves Michael O’Brien
Satish Aggarwal Hamidreza Heidarisafa Bansi Patel
Samuel Aguirre James Houston Christopher Petrola
Steven Alexanderson Alan Jensen Anthony Picagli
Edward Amato Wayne Johnson John Polenz
Curtis Ashton Jim Kulchisky Jan Reber
Gary Balash Mikhail Lagoda Charles Rogers
Thomas Barnes Chung-Yiu Lam Art Salander
Robert Beavers Jeffrey LaMarca Bartien Sayogo
Christopher Belcher Daniel Lambert Robert Schuerger
Thomas Blair Thomas La Rose Nikunj Shah
William Bloethe Jon Loeliger David Smith
Mark Bowman Debra Longtin Joseph Stevens
Derek Brown Jose Marrero Thomas Stomberski
William Bush Daniel Martin Richard Tressler
William Byrd Michael May Lesley Varga
William Cantor William McBride John Vergis
Thomas Carpenter Stephen Mccluer Donald Wengerter
Randy Clelland James Mcdowall Kenneth White
Peter Demar Larry Meisner Hughes Wike
Robert Fletcher John Merando Jian Yu
John Gagge Jr Thomas Mulcahy Luis Zambrano
Haissam Nasrat
6
When the IEEE SA Standards Board approved this recommended practice on 6 May 2020, it had the following
membership:
Gary Hoffman, Chair

Jon Walter Rosdahl, Vice Chair
Jean-Philippe Faure, Past Chair
Konstantinos Karachalios, Secretary
Ted Burse David J. Law Mehmet Ulema

J. Travis Griffith Howard Li Lei Wang
Grace Gu Dong Liu Sha Wei
Guido R. Hiertz Kevin Lu Philip B. Winston
Joseph L. Koepfinger* Paul Nikolich Daidi Zhong
John D. Kulick Damir Novosel Jingyi Zhou
Dorothy Stanley
*Member Emeritus
7
Introduction
This introduction is not part of IEEE Std 485-2020, IEEE Recommended Practice for Sizing Lead-Acid Batteries for
Stationary Applications.
The storage battery is of primary importance for the satisfactory operation of stationary applications including
but not limited to generating stations, substations, telecommunications, and other stationary applications.
This recommended practice is based on commonly accepted methods used to define the load and determine
adequate battery capacity. The method described is applicable to all installations and battery sizes.
The installations considered herein are designed for operation with a battery charger serving to maintain the
battery in a charged condition as well as to supply the normal dc load. This recommended practice does not
apply to “cycling” applications. (See IEEE Std 1660™ [B7].1)
This recommended practice was prepared by the Vented Lead Acid Sizing Working Group of the Energy Storage
and Stationary Battery Committee. It may be used separately, but when combined with IEEE Std 450™ 2 and
IEEE Std 484™ (for vented lead acid batteries) or IEEE Std 1187™ and IEEE Std 1188™ (for valve-regulated
lead-acid [VRLA] batteries), it provides the user with a general guide to designing, placing in service, and
maintaining the applicable lead-acid battery installation.
1
The numbers in brackets correspond to those of the bibliography in Annex H.
2
Information on references can be found in Clause 2.
8
Contents
1. Scope�� 12
2. Normative references�� 12
3. Definitions�� 13
4. Defining loads�� 14

4.1 General considerations�� 14
4.2 Load classification�� 14
5. Cell selection�� 16
6. Determining battery size�� 17

6.1 General�� 17
6.2 Number of cells�� 17
6.3 Additional considerations�� 18
6.4 Cell size�� 20
6.5 Cell sizing worksheet�� 23
7. Cell voltage/time profile calculation�� 25
Annex A (informative) Battery and cell sizing examples�� 26
Annex B (informative) Calculating cell voltage during discharge�� 32
Annex C (informative) Consideration of cell types�� 41
Annex D (informative) Constant power and constant resistance sizing�� 42
Annex E (informative) Development and use of battery discharge curves�� 50
Annex F (informative) Random loads�� 59
Annex G (informative) Full-size worksheet�� 65
Annex H (informative) Bibliography�� 67
9
List of Figures
Figure 1—Diagram of a duty cycle�� 16
Figure 2—Generalized duty cycle�� 21
Figure 3—Cell sizing worksheet�� 24
Figure A.1—Sample worksheet using Kt�� 27
Figure A.2—Duty cycle diagram�� 29
Figure A.3—Hypothetical composite rating curve for XYZ cell manufactured by ABC Company�� 30
Figure A.4—Sample worksheet using Rt capacity factor�� 31
Figure B.1—Discharge characteristics of ABC-type cell (Fan Curve)�� 33
Figure B.2—Discharge characteristics of DEF-type cell (S curve)�� 34
Figure B.3—Calculated voltage/time profile from “fan” curves�� 38
Figure B.4—Calculated voltage/time profile from “S” curves�� 40
Figure D.1—Voltage versus time constant power load�� 42
Figure D.2—Constant power discharge characteristic curve cell type: ABC�� 43
Figure D.3—Typical voltage versus time curve with calculated average volts cell type: ABC�� 46
Figure D.4—Cell type: ABC-33 average volts to final volts—curve fit�� 47
Figure D.5—Cell type: ABC-33 average volts to final volts—final�� 48
Figure E.1—Typical discharge characteristic curve for AB battery�� 51
Figure E.2—Test data curve�� 51
Figure E.3—Typical discharge characteristics to 1.75 V�� 53
Figure E.4—Coup de fouet at various discharge rates for cell type ABC-33�� 54
Figure E.6—Time lines�� 55
Figure E.7—Completed discharge characteristic curve�� 56
Figure E.8—One hour sizing calculation�� 57
Figure E.9—100 A/positive plate load calculation�� 57
Figure E.10—Sizing calculation 3�� 58
Figure F.1—Random load in last hour�� 59
Figure F.2—Battery sizing for random load in last hour�� 60
Figure F.3—Random load in first minute�� 61
Figure F.4—Random load at end of first hour�� 62
Figure F.5—Battery sizing for random load in first minute�� 63
Figure F.6—Battery sizing for random load at end of first hour�� 64

10
List of Tables
Table 1—Cell size correction factors for temperature for vented and VRLA cells�� 19
Table A.1—Sample cell sizing data�� 28
Table B.1—Cell voltage over time using “fan” curve�� 37
Table B.2—Cell voltage over time using “S” curve�� 39
Table C.1—Representative battery types�� 41
Table D.1—Sample sizing chart—values shown in kW per cell�� 44
Table D.2—7 h discharge data�� 45
Table E.1—Preliminary test data�� 52
Table E.2—Initial voltage points�� 53
11
1. Scope
Methods are described for defining the dc load and for sizing a lead-acid battery to supply that load for
stationary battery applications in float service. Some factors relating to cell selection are provided for
consideration. Installation, maintenance, qualification, testing procedures, and consideration of battery types
other than lead acid are beyond the scope of this recommended practice. The design of the dc system and
sizing of the battery charger(s) are also beyond the scope of this recommended practice.
2. Normative references

The following referenced documents are indispensable for the application of this document (i.e., they shall
be understood and used, so each referenced document is cited in text and its relationship to this document is
explained). For dated references, only the edition cited applies. For undated references, the latest edition of the
referenced document (including any amendments or corrigenda) applies.
IEEE Std 450TM, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-
Acid Batteries for Stationary Applications.3,4
IEEE Std 484TM, IEEE Recommended Practice for Installation Design and Installation of Vented Lead-Acid
Batteries for Stationary Applications.
IEEE Std 1184TM-2006, IEEE Guide for Batteries for Uninterruptible Power Supply Systems.
IEEE Std 1187TM, IEEE Recommended Practice for Installation Design and Installation of Valve-Regulated
Lead-Acid Storage Batteries for Stationary Applications.
IEEE Std 1188TM, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Valve-
Regulated Lead-Acid (VRLA) Batteries for Stationary Applications.
IEEE Std 1881TM, IEEE Standard Glossary of Stationary Battery Terminology.
3
This publication is available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854, USA
(http://standards.ieee.org/)
4
The IEEE standards or products referred to in this clause are trademarks owned by the Institute of Electrical and Electronics Engineers,
Incorporated.
12
IEEE Std 485-2020
IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications
3. Definitions
For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary
Online5, or IEEE Std 1881™, IEEE Standard Glossary of Stationary Battery Terminology, should be consulted
for terms not defined in this clause.
cell size: The rated capacity of a cell or the number of positive plates in a cell.
continuous load: Loads that are energized throughout the duty cycle.
coup de fouet: Initial voltage drop and recovery experienced when discharging a lead-acid battery.
duty cycle: The sequence of loads a battery is expected to supply for specified time periods.
equalizing charge: A charge, at a level higher than the normal float voltage, applied for a limited period of
time, to correct inequalities of voltage, specific gravity, or state of charge that may have developed between the
cells during service.
float service: Operation of a dc system in which the battery spends the majority of the time on float charge with
infrequent discharge. Syn: standby service.
NOTE—The primary source of power is normally the battery charger or rectifier.6
momentary load: loads that can occur one or more times during the duty cycle, but are of short duration, not
to exceed one minute during any occurrence.
non-continuous load: loads energized only during a portion of the duty cycle.
period: An interval of time in the battery duty cycle during which the current (or power) is assumed to be
constant for purposes of cell sizing calculations.
rated capacity: The capacity assigned to a cell by its manufacturer for a given discharge rate, at a specified
electrolyte temperature, to a given end-of-discharge voltage.
valve-regulated lead-acid (VRLA) cell: A lead-acid cell that is sealed with the exception of a valve that
opens to the atmosphere when the internal pressure in the cell exceeds atmospheric pressure by a preselected
amount. VRLA cells provide a means for recombination of internally generated oxygen and the suppression of
hydrogen gas evolution to limit water consumption.
vented cell: A cell in which the products of electrolysis and evaporation are allowed to escape to the atmosphere
as they are generated. Syn: flooded cell.
NOTE—vented cell is the preferred term that should be used in place of wet cell or flooded cell.
5
IEEE Standards Dictionary Online subscription is available at: http://ieeexplore.ieee.org/xpls/dictionary.jsp. An IEEE Account is
required for access to the dictionary, and one can be created at no charge on the dictionary sign-in page.
6
Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement this
standard.
13
IEEE Std 485-2020
4. Defining loads

4.1 General considerations
The duty cycle imposed on the battery by any of the conditions described herein depends on the dc system
design and the requirements of the installation. The battery supplies the dc power requirements when one or
more of the following conditions occur:
a) Load on the dc system exceeds the maximum output of the battery charger
b) Output of the battery charger is interrupted
c) AC power to the battery charger is lost [may result in a greater dc power demand than item b)]
The most severe of these conditions, in terms of battery load and duration, should be used to determine the
battery size for the installation.
4.2 Load classification

The individual dc loads supplied by the battery during the duty cycle are classified as continuous or
noncontinuous. Noncontinuous loads lasting 1 min or less are designated “momentary loads” and should be
given special consideration (see 4.2.3).
4.2.1 Continuous loads
Continuous loads are energized throughout the duty cycle. These loads are those normally carried by the
battery charger and those initiated at the inception of the duty cycle. Typical continuous loads are as follows:
a) Lighting
b) Continuously operating motors
c) Converters (e.g., inverters)
d) Indicating lights
e) Continuously energized coils
f) Annunciator loads
g) Communication systems
h) Power Supplies (e.g., Relay protection, security systems, battery monitors)
4.2.2 Noncontinuous loads
Noncontinuous loads are energized only during a portion of the duty cycle. These loads come on at any time
within the duty cycle and remain on for a set length of time, or be removed automatically or by operator action,
or continue to the end of the duty cycle. Typical noncontinuous loads may include:
a) Emergency pump motors

b) Critical ventilation system motors
c) Fire protection systems actuations
d) Motor-driven valve operations (stroke time > 1 min)
e) Other ac loads on the output of inverters
14
IEEE Std 485-2020
4.2.3 Momentary loads
Momentary loads can occur one or more times during the duty cycle but are of short duration, not exceeding
1 min at any occurrence. Although momentary loads may exist for only a fraction of a second, it is common
practice to treat each load as if it lasts for a full minute because the battery voltage drop after several seconds
often determines the battery’s 1-min rating. When several momentary loads occur within the same 1-min
period and a discrete sequence cannot be established, the load for the 1-min period should be assumed to be
the sum of all momentary loads occurring within that minute. If a discrete sequence can be established, the
load for the period is the maximum load at any instant. Sizing for a load lasting only a fraction of a second,
based on the battery’s 1-min performance rating, results in a conservatively sized battery. Consult the battery
manufacturer for ratings of discharge durations less than 1 min. Typical momentary loads may include:
a) Switchgear operations
b) Motor-driven valve operations (stroke time < 1 min)
c) Motorized switch operations
d) Field flashing of generators
e) Motor starting currents
f) Inrush currents
4.2.4 Other considerations
The loads applied to the battery are normally categorized as constant power, constant resistance, or constant
current. However, for sizing purposes, the loads are treated as constant power or constant current. The designer
should review each system to be sure all possible loads and their variations are included. If the loads are solely
constant power loads, sizing as described in Annex D is appropriate and simplifies the sizing process.
4.2.5 Duty cycle diagram
A duty cycle diagram showing the total load at any time during the cycle is an aid in the analysis of the duty
cycle. To prepare such a diagram, all loads (expressed in either current or power) expected during the cycle are
tabulated along with their anticipated inception and shutdown times. The total time span of the duty cycle is
determined by the requirements of the installation.
4.2.6 Defined loads
Loads whose inception and shutdown times are known are plotted on the diagram as they would occur. If the
inception time is known, but the shutdown time is indefinite, it should be assumed that the load continues
through the remainder of the duty cycle. Similarly, if the shutdown time is known, but the inception is not, it
should be assumed that the load begins when the duty profile begins.
4.2.7 Random loads
Loads that occur at random should be shown at the most critical time of the duty cycle in order to simulate the
worst-case load on the battery. These are noncontinuous or momentary loads as described in 4.2.2 and 4.2.3.
To determine the most critical time, it is necessary to size the battery without the random load(s) and to identify
the section of the duty cycle that controls battery size. Then the random load(s) should be superimposed on the
end of that controlling section as shown in Figure 1 (see 6.4.4).
15
IEEE Std 485-2020
NOTE—This example is worked out in detail in Annex A. There it is found that the first 120 min is the controlling portion
of the duty cycle. Therefore, the random load is located on the duty cycle so that the random load ends at the end of the
120th min. This is indicated by the dashed lines. See Annex F for an additional discussion on treatment of random loads.
Figure 1—Diagram of a duty cycle
4.2.8 Duty cycle example
Figure 1 is a diagram of a duty cycle made up of the following hypothetical loads expressed in amperes:
L1 40 A for 3 h, continuous load

L2 280 A for the 1st min, momentary load, actually 5 s starting current to load L3
L3 60 A from the 1st min through the 120th min, noncontinuous load
L4 100 A from the 30th min through the 120th min, noncontinuous load
L5 80 A from the 30th min through the 60th min, noncontinuous load
L6 80 A for the last minute, momentary load, actually a known sequence of: 40 A for the first 5 s, 80 A for the next
10 s, 30 A for the next 20 s
L7 100 A for 1 min, random load (Actually this consists of four 25 A momentary loads that can occur at any time
within the duty cycle. Therefore, the assumption is that they all occur simultaneously.)
When the duty cycle includes constant power and constant current loads, it is usually more convenient to
convert the constant power load values to constant current values for sizing calculations (see Annex D).
5. Cell selection

This clause summarizes some factors that should be considered in selecting a cell design for a particular
application. Various cell designs have different charge, discharge, and aging characteristics. Refer to
IEEE Std 1184-2006, Annex C of this document, and vendor literature for discussions of cell characteristics.
The following factors should be considered in the selection of the cell (or multi-cell unit):
a) Physical characteristics, such as dimensions and weight of the cells, container material, intercell
connectors, and terminals
b) Planned life of the installation and expected service life of the cell
16
IEEE Std 485-2020
c) Frequency and depth of discharge

d) Type of discharge (high-rate, long-duration, mixed loads)
e) Ambient temperature (Note that sustained high ambient temperatures result in reduced battery life.
See IEEE Std 484 and IEEE Std 1187.)
f) Charging characteristics
g) Maintenance requirements
h) Cell orientation requirements
i) Ventilation requirements
j) Seismic characteristics
k) Spill management
6. Determining battery size

6.1 General
Several basic factors govern the size (number of cells and rated capacity) of the battery, including the
maximum system voltage, the minimum system voltage, correction factors, and the duty cycle. Because a
battery is usually composed of a number of identical cells connected in series, the voltage of the battery is the
voltage of a cell multiplied by the number of cells in series. The ampere-hour capacity of a battery is the same
as the ampere-hour capacity of a single cell, which depends upon the dimensions and number of plates.
If cells of sufficiently large capacity are not available, then two or more strings (equal numbers of series-
connected cells) should be connected in parallel to obtain the necessary capacity. The capacity of such a
battery is the sum of the capacities of the strings. Consult the manufacturer for any limitation on paralleling.
Examples of conditions that can change the available capacity of the battery are as follows:
— The available capacity of the battery decreases as its temperature decreases.

— The available capacity decreases as the discharge rate increases.
— The minimum specified cell voltage at any time during the battery discharge cycle limits the available
capacity of the battery.
6.2 Number of cells

6.2.1 General
The maximum and minimum allowable system voltage determines the number of cells in the battery. It has
been common practice to use 12, 24, 60, 120, or 240 cells for nominal system voltages of 24 V, 48 V, 125 V,
250 V, or 480 V, respectively. In some cases, it is desirable to vary from this practice to match the battery to
system voltage limitations more closely. It should be noted that the use of the widest possible voltage window,
within the confines of individual load requirements, results in the most economical battery. Furthermore, the
use of the largest number of cells allows the lowest minimum cell voltage and, therefore, the smallest size cell
for the duty cycle. The application of the following principles is illustrated in A.2 of Annex A.
Furthermore, the use of the largest number of cells allows the lowest minimum cell voltage and therefore, the
smallest size cell for the duty cycle.
17
IEEE Std 485-2020
6.2.2 Calculating number of cells and minimum cell voltage
When the battery voltage is not allowed to exceed a given maximum system voltage, the number of cells
is limited by the cell voltage required for satisfactory charging or equalizing. The system (load equipment)
operating voltage range determines the number of cells that can be used and takes into consideration the
charging or equalizing voltage, as in the following equation:
maximum system voltage

= number of cells
cell voltage required for equalizing
Example: Assume 2.33 V/cell is required for equalize charging and that the maximum allowable system
voltage is 140 V or 135 V. Then,
140 V
= 60.09 cells (use 60 cells)
2.33
135 V
= 57.94 cells (use 58 cells)
2.33
If the number of cells is rounded up, the charging voltage should then be recalculated and verified for adequacy
of operation.
The minimum battery voltage equals the minimum system voltage plus the cable voltage drop. All voltage
drops should be considered. For example, unusually long cable connections or connectors with greater
resistance values than used to rate the battery may require an adjustment to the minimum battery voltage. The
minimum battery voltage is then used to calculate the allowable minimum cell voltage.
minimum battery voltage

= minimum cell voltag e
number of cells
In an application with a wide voltage window, particularly when long discharge times are required, the
minimum cell voltage recommended by the manufacturer for a given discharge time may be a factor. If so,
reduce the number of cells in the preceding calculation so that the minimum cell voltage per cell does not fall
below the recommended value.
Example: Assume that the minimum battery voltage for the example is 105 V. Then
105 V
= 1.75 V/cell
60 cells
105 V
= 1.81 V/cell
58 cells
This minimum cell voltage is then used in the sizing calculation.
6.3 Additional considerations

6.3.1 General
Before proceeding to calculate the cell capacity required for a particular installation, the designer should
consider factors that influence cell size.
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IEEE Std 485-2020
6.3.2 Temperature correction factor
The available capacity of a cell is affected by its operating temperature. If the lowest expected electrolyte
temperature is below the rated battery temperature, select a cell large enough to have the required capacity
available at the lowest expected temperature. If the lowest expected electrolyte temperature is above the rated
battery temperature, it is a conservative practice to select a cell size to match the required capacity at the
standard temperature and to recognize the resulting increase in available capacity as part of the overall design
margin. Table 1 lists cell size correction factors for various temperatures for lead-acid cells with nominal 1.215
specific gravity. For unlisted temperatures within the range of Table 1, interpolate between adjacent values and
round off to two decimal places. Consult the manufacturer for factors for the specific battery being evaluated.
NOTE—The standard US temperature for rating cell capacity is 25 °C (77 °F). Some European manufactures use 20 °C
(68 °F).
Table 1—Cell size correction factors for temperature for vented and VRLA cells
Electrolyte Electrolyte Temperature Electrolyte Electrolyte Temperature
temperature temperature correction temperature temperature correction
(°C) (°F) factor (°C) (°F) factor
4.4 40 1.300 26.1 79 0.987
7.2 45 1.250 26.7 80 0.980
10.0 50 1.190 27.2 81 0.976
12.8 55 1.150 27.8 82 0.972
15.6 60 1.110 28.3 83 0.968
18.3 65 1.080 28.9 84 0.964
18.9 66 1.072 29.4 85 0.960
19.4 67 1.064 30.0 86 0.956
20.0 68 1.056 30.6 87 0.952
20.6 69 1.048 31.1 88 0.948
21.1 70 1.040 31.6 89 0.944
21.7 71 1.034 32.2 90 0.940
22.2 72 1.029 35.0 95 0.930
22.8 73 1.023 37.8 100 0.910
23.4 74 1.017 40.6 105 0.890
23.9 75 1.011 43.3 110 0.880
24.5 76 1.006 46.1 115 0.870
25.0 77 1.000 48.9 120 0.860
25.6 78 0.994
NOTE—This table is based on lead-acid nominal 1.215 specific gravity cells rated at 25 °C (77 °F). For cells
with other specific gravities or rated temperatures, refer to the manufacturer.
6.3.3 Design margin
It is prudent to provide a capacity margin to allow for unforeseen additions to the dc system and less-than-
optimum operating conditions of the battery due to improper maintenance, recent discharge, ambient
temperatures lower than anticipated, or a combination of these factors. A method of providing this design
margin is to add 10% to 15% to the cell size determined by calculations. If the various loads are expected to
grow at different rates, it is more accurate to apply the expected growth rate to each load for a given time and to
develop a duty cycle from the results.
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IEEE Std 485-2020
The cell size calculated for a specific application seldom matches a commercially available cell exactly, and it
is normal procedure to select the next higher capacity cell. The additional capacity obtained can be considered
part of the design margin.
Note that margins are also discussed in clause 6.3.1.5 and clause 6.3.3 of IEEE Std 323™ [B2]. However, those
margins are applied during qualification and are not related to the design margins described in this clause.
6.3.4 Aging factor
As a rule, for long-duration discharges of a vented lead-acid battery, the capacity slowly declines throughout
most of the battery’s life, but begins to decrease rapidly in the latter stages, with the “knee” of the life versus
capacity curve occurring when the remaining capacity is reduced to approximately 80% of rated capacity. This
characteristic is well documented for discharges at the 1 h rate or longer.
Because of that, IEEE Std 450 and IEEE Std 1188 recommend that a battery be replaced when its actual
capacity drops to 80% of its rated capacity. Therefore, to maintain the battery’s capability of meeting its design
loads throughout its service life, the battery’s rated capacity should be at least 125% (1.25 aging factor) of the
load expected at the end of its service life.
For high-rate, short-duration discharges of vented lead-acid batteries and all discharges of VRLA batteries,
there are too many variables to state definitively where the “knee” occurs. Therefore, it is reasonable to expect
its short-duration performance to drop significantly below 80% of its rating before it reaches the “knee” at
that rate so a larger aging factor may be appropriate. Consult with the battery manufacturer for additional
information and recommendations.
Exceptions to this rule exist. For example, some manufacturers recommend that vented batteries with Planté,
and modified Planté be replaced when their measured capacity drops below 100% of their rated capacity (1.00
aging factor). These designs maintain a fairly constant capacity throughout their life.
6.3.5 Initial capacity
Batteries may have less than rated capacity when delivered. Unless 100% capacity upon delivery is specified,
batteries may be delivered with capacities as low as 90% of rating. This should rise to rated capacity in normal
service after several charge–discharge cycles or after several years of float operation.
If the designer has provided a 1.25 aging factor, there is no need for the battery to have full rated capacity upon
delivery because the capacity normally available from a new battery is above the duty cycle requirement.
When a 1.00 aging factor is used, the designer should verify that the initial capacity upon delivery is at least
100% or that there is sufficient margin in the sizing calculation to accommodate a lower initial capacity.
Example: If the cells have 90% initial capacity and the margin is greater than 11%, then no additional
compensation for initial capacity is required.
6.4 Cell size

This subclause describes and explains a proven method of calculating the cell capacity necessary for
satisfactory performance on a given duty cycle. The application of this method to a specific duty cycle, using
an optional preprinted worksheet to simplify the calculations, is demonstrated in A.3 of Annex A. Instructions
for the proper use of the worksheet are given in 6.5.
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IEEE Std 485-2020
6.4.1 Initial calculation
Equation (1) (see 6.4.2) requires the use of a capacity rating factor Ct (see 6.4.3) that is based on the discharge
characteristics of a particular plate type and size. Thus, the initial calculation is based on a trial selection of
positive plate type and capacity. Depending on the results of this initial calculation, it may be desirable to
repeat the calculation for other types or sizes of plates to obtain the optimum cell type and size for the particular
application. In addition, it may be desirable to repeat the calculation to take into account any differences in
performance per plate within a given series of cells. Use the capacity from the first calculation as a guide for
selecting additional types to size.
6.4.2 Sizing methodology
The cell selected for a specific duty cycle shall have enough capacity to carry the combined loads during the
duty cycle. To determine the required cell size, it is necessary to calculate, from an analysis of each section of
the duty cycle (see Figure 2), the maximum capacity required by the combined load demands (current versus
time) of the various sections. The first section analyzed is the first period of the duty cycle. Using the capacity
rating factor (see 6.4.3) for the given cell type, a cell size is calculated that is capable of supplying the required
current for the duration of the first period. For the second section, the capacity is calculated assuming that the
current A1, required for the first period, continued through the second period; this capacity is then adjusted for
the change in current ( A2 - A1 ) during the second period. In the same manner, the capacity is calculated for
each subsequent section of the duty cycle. This iterative process is continued until all sections of the duty cycle
have been considered. The calculation of the capacity FS required by each section S , where S can be any
integer from 1 to N , is expressed mathematically in Equation (1). FS is expressed as watt-hours, ampere-
hours, or number of positive plates, depending upon which Ct is used (see 6.4.3).
P= S AP − A( P−1)
Fs = ∑ (1)
P =1 Ct
Figure 2—Generalized duty cycle
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IEEE Std 485-2020
The maximum capacity (max FS ) calculated determines the uncorrected cell size that can be expressed by the
general Equation (2):
S=N S=N P= S AP − A( P−1)

F = max FS = max
S =1 S =1
∑
P =1 Ct
(2)
where
F is the cell size (uncorrected for temperature, aging, and design margin)
S is the section of the duty cycle being analyzed. [Section S contains the first S periods of the duty
cycle (e.g., section S5 contains periods S1 through S5 ). See Figure 2 for a graphical representation
of “section.”]
N is the number of periods in the duty cycle
P is the period being analyzed
AP are the amperes required for period P
t is the time in minutes from the beginning of period P through the end of Section S
Ct is the capacity rating factor (see 6.4.3) for a given cell type, at the t minute discharge rate, at 25 °C
(77 °F), to a definite minimum cell voltage
FS is the capacity required by each section
If the current for period P +1 is greater than the current for period P , then section S = P +1 requires a larger
cell than section S = P . Consequently, the calculations for section S = P can be omitted.
6.4.3 Capacity rating factor
There are two terms for expressing the capacity rating factor Ct of a given cell type in cell sizing calculations.
Rt is the number of amperes that each positive plate can supply for t min at 25 °C (77 °F) to a definite
minimum cell voltage. Therefore, Ct = Rt and
S=N S=N P= S
AP − AP−1
F = max FS = max
S =1 S =1
∑P =1 Rt
(3)
K t is the ratio of rated ampere-hour capacity [at a standard time rate at 25 °C (77 °F) and to a standard
minimum cell voltage] of a cell to the amperes that can be supplied by that cell for t minutes at 25 °C (77 °F)
and to a given minimum cell voltage. Therefore, Ct = 1 K t and Equation (3) can be rewritten as follows:
S=N S=N P=S

F = max FS = max
S =1 S =1
∑  A
P =1
P − A( P−1)  K t (4)
Rt is not equal to 1 K t because of the different units applied to each factor. However, Rt is proportional to
1 K t . The values are obtained from battery manufacturers for each positive plate design and various minimum
cell voltages. A similar factor, Pt , expressed as watts per positive plate, can be used for calculations involving
batteries with only constant-power loads.
Batteries experience a voltage dip during the early stage of discharge, following which the voltage shows some
recovery. The designer should verify that this effect (known as the coup de fouet) has been taken into account
in the manufacturer’s published capacity rating factor. For additional discussion of coup de fouet, see E.2.3.
A battery discharge curve is often used to obtain Rt values and can also provide other useful information about
specific battery characteristics. An example of a battery discharge characteristic curve is provided in Annex E
along with information on how it is developed and other useful information that can be found on these curves.
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IEEE Std 485-2020
6.4.4 Sizing to include random loads
When equipment loads that occur at random are included as part of the duty cycle, it is necessary to calculate
the cell size required for the duty cycle without the random load(s) and then add to this the cell size required for
the random load(s) only. See Annex F for additional discussions regarding random loads.
6.4.5 Number of plates per cell
When used with the factor Rt (amperes per positive plate), the general equation expresses the cell size as the
number of positive plates. In the manufacturer’s literature, the cell size may be listed as the total number of
positive and negative plates. The conversion from number of positive plates to the total number of plates is as
follows: total number of plates = 1 + (2 × number of positive plates).
6.5 Cell sizing worksheet

A worksheet, given in Figure 3, has been designed to simplify the manual application of the procedure
described in 6.47. Examples of its use are found in Annex A. Instructions for the proper use of the worksheet
are as follows:
a) Fill in necessary information in the heading of the chart. The temperature and voltage recorded are
those used in the calculations. The voltage used is the minimum battery voltage divided by the number
of cells in the battery.
b) Fill in the amperes and the minutes in Columns (2) and (4) as indicated by the section heading
notations.
c) Calculate and record the changes in amperes as indicated in Column (3). Record whether the changes
are positive or negative.
d) Calculate and record the times from the start of each period to the end of the section as indicated in
Column (5).
e) Record in Column (6) the capacity factors (Rt or Kt, from the manufacturer’s literature) for each
discharge time calculated in Column (5).
f) Calculate and record the cell size for each period as indicated in Column (7). Note the separate
subcolumns for positive and negative values.
g) Calculate and record in Column (7) the subtotals and totals for each section as indicated.
h) Record the maximum section size [the largest total from Column (7) on Line (8), the random section
size on Line (9), and the uncorrected size on Lines (10) and (11)].
i) Select the correction factor from Table 1 or from the manufacturer’s published data for the temperature
shown in the main heading and record it on Line (12).
j) Enter the design margin on Line (13) and the aging factor on Line (14). Combine Lines (11), (12),
(13), and (14) as indicated and record the result on Line (15).
k) When Line (15) is in terms of ampere-hours and does not match the capacity of a commercially
available cell, the next larger cell is required. When Line (15) shows a fractional number of positive
plates, use the next larger integer. Show the result on Line (16).
l) From the value on Line (16), the equation in 6.4.5, and the manufacturer’s literature, determine the
commercial designation of the required cell and record it on Line (17).
m) Verify that the factors and discharge rates used are appropriate for the selected size. If not, sizing
should be performed again using the factors and discharge rates for the selected cell size.
7
Annex G contains a full-size worksheet that users of this recommended practice may freely reproduce so that it can be used for their
intended purposes.
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IEEE Std 485-2020
Figure 3—Cell sizing worksheet
24
IEEE Std 485-2020
7. Cell voltage/time profile calculation

When the battery sizing procedure and methods described above are used for the specified duty cycle and the
cell size selected, the average cell voltage will not drop below the specified minimum (e.g., 1.67 V, 1.75 V,
or 1.81 V) at any point in the duty cycle. It is therefore not normally necessary to calculate the cell or battery
terminal voltage because it remains above the predetermined allowable minimum through the discharge
period. If the need for such information should arise, Annex B describes one method of calculating the voltage
at various points in the duty cycle utilizing the typical discharge characteristic curves published by the battery
manufacturer.
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IEEE Std 485-2020
Annex A
(informative)
Battery and cell sizing examples

A.1 General
The following examples use the same duty cycle discussed in 4.2.8 with a lowest expected electrolyte
temperature of 18.3 °C (65 °F). Subclause A.2 provides several examples of calculations selecting the number
of cells to be used in the battery and shows how the number of cells affects the required cell capacity. Subclause
A.3 shows how the cell sizing worksheet can be used to calculate the required cell size.
A.2 Required number of cells

A.2.1 Example number 1
The following example describes two cases where system design affects the number of cells. For both cases,
the voltage window is 105 V to 140 V, and the battery manufacturer recommends a float voltage of 2.25 V/cell
and equalizing at 2.33 V/cell.
Case 1
The battery and charger are continuously connected to the loads.
140 V
Number of cells = = 60.09 (use 60 cells)
2.33 V/cell
105 V
End-of-discharge voltage = = 1.75 V cell
60 cells
This case is worked out in detail in Figure A.1 and results in a corrected size of 1010.4 Ah at the 8-h rate using
1.75 minimum cell voltage with the K t rating factor.
Case 2
The battery and charger are isolated from the loads during equalizing.
140 V
2.25 V/cell
105 V
62 cells
A cell sizing worksheet is not provided for this example, but calculations show that the corrected cell size
is 944 Ah at the 8-h rate. The reduction in the end-of discharge voltage results in about a 7% reduction in
corrected cell capacity with only a 3% increase in the number of cells. The equalizing voltage while isolated
would be 144.5 V.
Although the example illustrates the effect of using more cells in a string, it is not recommended to size a
battery below 1.75 V per cell for discharge times longer than 1 h.
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IEEE Std 485-2020
Figure A.1—Sample worksheet using Kt
27
IEEE Std 485-2020
A.2.2 Example number 2

Same conditions as Example 1, Case 1, except that the dc system voltage limits are now 105 V to 135 V.
135 V
2.33 V/cell
105 V
58 cells
A cell sizing worksheet is not provided for this example, but calculations show that the corrected cell size is
1186 Ah at the 8-h rate. In comparison to Example 1, the increase in the minimum cell voltage results in a 17%
increase in the corrected cell size with only a 3% reduction in the number of cells required.
A.3 Required cell capacity

Table A.1 can be constructed from the duty cycle diagram in Figure A.2. This table is of value in filling out the
cell sizing worksheet.
Table A.1—Sample cell sizing data

Period Loads Total amperes Duration (min)
1 L1 + L2 320 1
2 L1 + L3 100 29
3 L1 + L3 + L4 + L5 280 30
4 L1 + L3 + L4 200 60
5 L1 40 59
6 L1 + L6 120 1
R L7 100 1
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IEEE Std 485-2020
Figure A.2—Duty cycle diagram
Figure A.3 is a hypothetical composite rating curve for the XYZ cell manufactured by the ABC Company.
The graph gives values for both types of capacity rating factors for discharges started at 25 °C (77 °F) and
terminated when the average cell voltage reaches 1.81 V, 1.75 V, or 1.69 V. Figure A.4 shows the way in which
the cell sizing worksheet and the R t rating factor would be used to size the XYZ cell for the Figure 1 duty cycle.
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IEEE Std 485-2020
Figure A.3—Hypothetical composite rating curve for XYZ cell manufactured by ABC
Company
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IEEE Std 485-2020
Figure A.4—Sample worksheet using Rt capacity factor
31
IEEE Std 485-2020
Annex B
(informative)
Calculating cell voltage during discharge

B.1 General
When the battery sizing procedure and methods described in this recommended practice are used for the
specified duty cycle and the cell size selected, the average cell voltage does not drop below the specified
minimum at any point in the duty cycle. It is not, therefore, normally necessary to calculate the cell or battery
terminal voltage because it remains above the predetermined allowable minimum throughout the discharge
period. If the need for such information should arise, a method of calculating the voltage at various points in
the duty cycle, utilizing the battery manufacturer’s typical discharge characteristic curves, is described in B.2
and B.3.
B.2 Method
Two types of typical discharge characteristic curves in common use are “fan” curves and “S” curves, typical
examples of which are shown in Figure B.1 and Figure B.2, respectively. Although these two curves have
different coordinates and appearance, they show comparable data for two different cell types.
The method of calculation (using either curve) is an iterative process and consists of the following:
a) Keeping a cumulative total of the ampere-hours “removed” from the cell during each discharge
segment/period
b) Identifying the discharge current at that time
c) Using this information, along with the battery manufacturer's typical discharge characteristics, to
determine the cell terminal voltage at various points in the duty cycle
The voltage/time profile is then a plot of these cell voltages versus time for the entire duty cycle. To determine
the voltage just before and just after a step change in the discharge rate, keep the cumulative ampere-hours
constant and determine the voltage based on the two different (before and after) discharge rates. The battery
terminal voltage at each point is equal to the average cell voltage multiplied by the number of cells connected
in series.
Note that care should be taken in calculating currents for voltages of periods less than one minute in duration.
If multiple current levels exist within a minute (typically due to momentary loads), the lowest voltage occurs
when the current is greatest. When that current is decreased, the voltage does not increase instantaneously as
it responds more slowly due to the chemical response of the battery. In this case the low voltage value should
be shown as the voltage for the balance of the minute consistent with the sizing methodology for momentary
loads described in 4.2.3.
To determine the voltage profile for cells at other than 25 °C (77 °F) electrolyte temperature and/or less than
100% of rated capacity, multiply the current for each period during the duty cycle by the same correction
factors used to size a battery per this recommended practice. For example, to predict the performance of a cell
at 15.6 °C (60 °F) and 80% of its rated capacity, multiply each current value in the duty cycle by the temperature
correction factor and the aging factor to determine the cell voltage. Similarly, if the cell performance (in
amperes per positive plate) is not a constant value for all sizes of a given type of cell, the current for each
period during the duty cycle should be adjusted by using the appropriate correction factor as provided by the
battery manufacturer.
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IEEE Std 485-2020
Figure B.1—Discharge characteristics of ABC-type cell (Fan Curve)
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IEEE Std 485-2020
Figure B.2—Discharge characteristics of DEF-type cell (S curve)
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IEEE Std 485-2020
B.3 Sample calculations

B.3.1 Example number 1 (using “fan” curves)
A sample voltage calculation for an assumed cell type, duty cycle, and conditions follows:
Cell type: ABC, 10 positive plates

Duty cycle: 700 A for 1 min, then
500 A for 59 min, then
150 A for 180 min

Conditions: Electrolyte temperature: 25 °C (77 °F)

Cell capacity: 100% of rated capacity
The average cell voltage at the time where the voltage is to be determined is found on the “fan” curve at the
intersection of the following:
a) the cumulative total ampere-hours per positive plate

b) the ampere-hours per positive plate. When necessary, interpolate between adjacent “final volt” lines
Step 1
Using the ABC discharge characteristic curves (Figure B.1), determine cell volts during the first 1 min load:
700 A for 1 min = 11.67 Ah increment
11.67
11.67 Ah cumulative, or = 1.17 Ah positive plate cumulative
10
700 A
= 70 A/positive plate
10 positive plates
From the curve, a 70 A/positive plate intersects a 1.17 Ah/positive plate (at the abscissa line) at approximately
the 1.86 V/cell.
The initial (only) voltage can also be obtained by projecting a 70 A/positive plate vertically to the “initial
volts” line where 1.86 V is also read. Thus, the first load drops the battery voltage to the 1.86 V/cell for the
entire minute.
Step 2
Because the second load is less than the first load, the cell terminal voltage rises upon its initiation. To
determine the recovery voltage: 500 A for 0 additional minutes is a 0 Ah increment, 11.67 Ah cumulative, or
1.17 Ah/positive plate cumulative.
500 A
= 50 A/positive
10 positive plates
On the curve, 50 A/positive plate intersects a 1.17 Ah/positive plate (at the abscissa line) at approximately 1.89
V/cell.
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IEEE Std 485-2020
Step 3
To determine the voltage at the end of 30 total minutes: 500 A for 29 additional minutes is a 241.67 Ah
increment, 253.34 Ah cumulative, or 25.34 Ah/positive plate cumulative.
500 A
= 50 A/positive
10 positive plates
On the curve, a 50 A/positive plate intersects a 25.33 Ah/positive plate at approximately 1.88 V/cell.
Step 4 through Final
The preceding process is repeated for each step to determine the cell voltage at each point in the duty cycle.
The results for this example are tabulated in Table B.1 and the voltage/time profile is shown in Figure B.3.
36
Table B.1—Cell voltage over time using “fan” curve
(1) (2) (3) (4) (5) (6) (7) (8)
Load
Cumulative Cumulative amperes Intersection
Time Cumulative
Load discharge Incremental Ah per per of (6)
interval Ah
(A) time Ah removed positive positive and (7)
(min) removed
(min) plate (5)/10 plate (V/cell)
(1)/10
700 0 0 0 0 0 70 1.86
700 1 1 11.67 11.67 1.17 70 1.86
500 0 1 0 11.67 1.17 50 1.89
500 29 30 241.67 253.34 25.33 50 1.88
500 30 60 250 503.34 50.33 50 1.86
150 0 60 0 503.34 50.33 15 1.94
150 30 90 75.0 578.34 57.83 15 1.94
150 30 120 75.0 653.34 65.33 15 1.93
150 30 150 75.0 728.34 72.83 15 1.93
37
150 30 180 75.0 803.34 80.33 15 1.92
150 30 210 75.0 878.34 87.83 15 1.92
IEEE Std 485-2020
150 30 240 75.0 953.34 95.33 15 1.90

IEEE Std 485-2020
Figure B.3—Calculated voltage/time profile from “fan” curves
B.3.2 Example number 2 (using “S” curves)

A sample voltage calculation for an assumed cell type, duty cycle, and conditions follows:
Cell type: DEF-21, 10 positive plates and characteristics as shown in Figure B.2

Duty cycle: 1000 A for 1 min, then
700 A for 29 min, then
300 A for 180 min

Conditions: Electrolyte temperature: 25 °C (77 °F), TF = 1.00
80% capacity [1.25 aging factor (AF) = 1.25]
Design margin (DM) = 1.00
Cumulative correction factor = TF × AF × DM = 1.00 × 1.25 × 1.00 = 1.25
Calculations: Refer to Table B.2 and Figure B.2.
Column A: This is the time period in minutes. Example: 1 min to 30 min (29 min of discharge).

Column B: This is the time interval for this part of the calculation. Example: The time interval
is zero at the beginning of any period. The example only calculates the value at the
beginning and end of each period. Intermediate values can be calculated.

Column C: This is the duty cycle in amperes for the period. Example: 700 A for 1 min to 30 min.

38
Table B.2—Cell voltage over time using “S” curve
A B C D E F G H I J K
Time Time Duty Corrected A-min A-min/ A/positive Time to final A-min/ % Discharge V/ cell
period interval cycle amperes increment positive plate voltage in min positive plate (Col. F (from
(min) (A) (Col. C × cumulative plate (Col. D/ (from “S” curve) A/positive plate × cumulative/ “S” curve)
correction (Col. D × incremental positive time to final voltage Col. I) × 100
factors) Col. B) cumulative plates) (Col. G × Col. H)
(Col. E/
positive
plates)
cumulative cumulative
value value
0 0
0 1000 1250 125 — — 0 1.815
0 to 1 0 0
min 1250 125
1 1000 1250 125 50 6250 2 1.81
1250 125
0
0 700 875 0 1250 87.5 85 7437.5 1.68 1.865
1 to 30 125
min 25 375 2537
39
29 700 875 87.5 85 7437.5 35.8 1.82
26 625 2662
0
0 300 375 0 2662 37.5 270 10 125 26.3 1.91
IEEE Std 485-2020
30 to 26 625
210
min 67 500 6750
180 300 375 37.5 270 10 125 92.9 1.75
94 125 9412

IEEE Std 485-2020
Figure B.4—Calculated voltage/time profile from “S” curves
Column D: Corrected amperes to account for aging, temperature, and design margin.
Example: 700 × 1.25 = 875 A.

Column E: Ampere-minutes in both incremental and cumulative values. Example: The incremental ampere-
minutes for the 29 min interval is 875 × 29 = 25375. The cumulative ampere-minutes is the
previous cumulative total (1250) plus the incremental value (1250 + 25375 = 26625 A-min).

Column F: Ampere-minutes per positive plate in both incremental and cumulative values. Example: Take the
values from Col. E and divide by the number of positive plates. 25375/10 = 2537 incremen-tal ampere-
minutes per positive plate and 26625/10 = 2662 cumulative ampere-minutes per positive plate.

Column G: Amperes per positive plate, which is Col. D divided by the number of positive plates.
Example: 875/10 = 87.5 A per positive plate.

Column H: Time to final voltage is determined from the “S” curves. Example: Time to final
voltage is read at 87.5 A/positive plate on the x axis up to the “capacity to final voltage”
curve. The needed value is then read from the left y axis and is 85 min. The “capacity
to final voltage” curve is at the extreme right-hand side of the 1.67 V line.

Column I: This is ampere/positive plate times the time to final voltage.
Example: Multiply Col. G by Col. H (87.5 × 85 = 7437.5 A-min per positive plate).

Column J: Percent of discharge is the cumulative ampere-minutes/positive plate divided by ampere/positive plate
times time to final voltage.
Example: This is Col. F (lower value) divided by Col. I (2662/7437.5 = 0.358 or 35.8%).

Column K: Volts per cell is the expected cell voltage at the calculated point in time for the conditions specified
as determined from the “S” curves. Example: The value is determined by taking the discharge
rate in ampere/positive plate on the x axis and projecting up to the percent of discharge curve
and then reading the cell voltage at the right y axis (87.5 A and 35.8% discharge—interpolate
between the 20% and 40% curves and read the value at the right y axis of 1.82 V/cell).
The results for this example are tabulated in Figure B.2, and the voltage/time profile is shown in Figure B.4.
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Annex C
(informative)
Consideration of cell types

Lead-acid batteries share many similarities in construction and operation, but there are differences in design
that should be considered when selecting a battery to achieve the best possible fit for the application.
IEEE Std 1184-2006 is a comprehensive UPS guide and should be used as a reference for comparison of
different battery types. This annex is a supplement to IEEE Std 1184-2006.
The dc system designer should recognize that some lead-acid batteries are designed for low-rate, long-duration
loads and that other batteries are better for high-rate, short-duration loads so the selection of the battery type
is dependent on the duty cycle. Generally, some differences between the battery types would be number and
thickness of the plates, separator material and thickness, distance between the plates, and available sediment
space among other factors.
The dc system designer should be aware of pitfalls that could result from the selection of the wrong battery
type, such as the application of a battery designed for low-rate, long-duration loads that might not have a
1-min rate sufficient to allow the battery to operate needed momentary loads. Conversely, application of a
battery that is designed for high-rate, short-duration loads may have short-circuit capability that exceeds the
capability of the system and the installed protective devices.
Table C.1 is typical of a series of battery types from a single manufacturer. The type listed as C would be
representative of a battery for communication service, type S would be representative of a battery for
switchgear service, and type U would be representative of a battery for UPS service. As mentioned in 6.4.3,
K t is the ratio of ampere-hour capacity (at a standard time rate, at 25 °C and to a standard minimum cell
voltage) of a cell, to the amperes that can be supplied by that cell for t min at 25 °C and to a standard minimum
cell voltage. As such, a battery with a higher K t would be less efficient at higher rates.
Table C.1—Representative battery types

Type 80-h capacity 1-min rate Kt
C 1220 Ah 924 A 1.32
S 1120 Ah 1190 A 0.94
U 1168 Ah 2677 A 0.43
Each of the batteries listed in Table C.1 could be used in any application if the battery’s capability meets the
duty cycle. But the design engineer should be aware that significant differences exist in battery types and a
misapplication can result in the purchase of a larger battery than is really needed. Even more significant, a
battery could be specified that would not be able to supply all of the loads within the duty cycle.
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Annex D
(informative)
Constant power and constant resistance sizing

D.1 Overview
Much of today’s equipment requires constant power. Constant power loads differ from constant current in that
as the battery voltage decays, the current required increases, as shown in Figure D.1.
Figure D.1—Voltage versus time constant power load
For many battery models, constant power fan curves or tables are available from the manufacturer. The same
general principles apply for data collection and fan curve generation as for constant current fan curves or
tables. The main differences are as follows: the type of discharge performed to gather the required data and the
change of units from amperes per positive plate to watts per positive plate and ampere-hours per positive plate
to watt-hours per positive plate. Figure D.2 shows a typical constant power discharge characteristic curve.
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Figure D.2—Constant power discharge characteristic curve cell type: ABC
D.2 Examples
For the following examples, factors for aging, temperature, and design margin are neglected for simplicity.
D.2.1 Example number 1

Determine the battery required to provide 250 kW for 15 min with a battery terminal voltage window of 140 V
to 100 V by calculating the following:
a) Number of cells = 140 V/2.33 VPC = 60 cells

b) Minimum cell voltage = 100 V/60 cells = 1.67 VPC
c) Load per cell = 250 kW/60 cells = 4.167 kW per cell
Assume a charging requirement of 2.33 V per cell (VPC).
From the sample sizing chart (Table D.1), determine (under the columns for 15 min and 1.67 VPC) the kW per
cell capability that meets the 4.167 kW per cell requirement.
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Table D.1—Sample sizing chart—values shown in kW per cell

1.67 Final volts Minutes to final volts
ABC
5 10 12 14 15 16 18 20 30
cell type
ABC-17 2.787 2.409 2.275 2.163 2.120 2.069 1.981 1.895 1.559
ABC-19 3.135 2.709 2.560 2.434 2.386 2.327 2.229 2.132 1.754
ABC-21 3.484 3.010 2.844 2.704 2.650 2.585 2.477 2.369 1.949
ABC-23 3.784 3.276 3.110 2.945 2.886 2.816 2.697 2.297 2.132
ABC-25 4.072 3.535 3.357 3.182 3.117 3.042 2.914 2.789 2.307
ABC-27 4.354 3.781 3.590 3.413 3.344 3.262 3.126 2.989 2.484
ABC-29 4.666 4.044 3.839 3.650 3.576 3.489 3.343 3.197 2.657
ABC-31 4.974 4.302 4.085 3.883 3.805 3.712 3.556 3.401 2.827
ABC-33 5.278 4.560 4.330 4.116 4.034 3.935 3.771 3.606 2.997
ABC-35 5.581 4.811 4.568 4.342 4.255 4.051 3.977 3.803 3.161
ABC-37 5.878 5.057 4.802 4.564 4.473 4.364 3.181 3.998 3.322
ABC-39 6.173 5.305 5.037 4.788 4.692 4.577 4.386 4.195 3.486
The ABC-33 at 4.034 kW per cell is close, but the actual minimum cell size required is type ABC-35. The
rating for this cell is 4.255 kW per cell, which is greater than the 4.167 kW per cell requirement.
If individual cells of sufficiently large capacity for the specified load are not available, then two or more strings
of equal numbers of series connected cells should be connected in parallel to obtain the necessary capacity.
The capacity of such a battery is the sum of the capacities of the strings. (Additionally—although rare—
certain situations occur where it is more economical to provide parallel strings of multicell units instead of one
string of large, single-cell units.)
D.2.2 Example number 2

Instead of a 250-kW load, the requirement is for a 300-kW load. The required cell capability is now 300 kW/60
cells = 5.00 kW per cell, but from the sizing chart, the largest cell is only capable of 4.692 kW; therefore,
parallel battery strings shall be provided.
Because the largest ABC cell can provide 4.692 kW, another ABC-39 in parallel would double the capability
to 9.384 kW. While this would be more than adequate, this is far larger than the 5.00 kW per cell required. Two
parallel strings of a smaller cell is adequate.
A 5.00 kW per cell requirement requires a minimum of 2.50 kW per cell for each parallel string. From the
sizing charts, the ABC-21 at 2.65 kW per cell meets this requirement. Therefore, the minimum battery size
required is two parallel strings of 60 cells per string of type ABC-21 cells.
D.3 Conversion from constant power loads to constant current

Loads applied to the battery are normally categorized as constant power, constant resistance, or constant
current. The designer should review each system to verify that all possible loads and their variations have been
included.
The battery voltage decreases as the battery discharges (as does the voltage at the loads). The amount by
which the battery voltage decreases depends on the battery design and the load placed on the battery. For
constant power loads, the current increases with a voltage decrease. Inverters and dc/dc power supplies are
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IEEE Std 485-2020
usually constant power; they are internally regulated to maintain a constant output voltage as the input voltage
decreases. As a result, the dc input current increases as the input voltage decreases. If the constant power load
is remote from the battery, the voltage drop increases because of the cable resistance and the resulting input
current is higher. It is desirable to consider the increase in load current as battery voltage declines. This can be
calculated as follows:
P
I AVG =
EVAVG
where
I AVG is the average discharge current (A) for the discharge period
P is the discharge load (W)
VAVG is the average discharge voltage for the discharge period
Because the voltage profile for a particular battery is typically unknown, an alternative method for calculating
the current is simply to divide the power by the end voltage. This method results in a conservative estimate of
current (minimum volts, maximum amperes). Thus,
P
I MAX = .
EVMIN LOAD
where
I MAX is the discharge current at the end of the discharge period

P is the discharge load (W)
VMIN LOAD is the minimum battery voltage minus voltage drop
Example: For a 24 cell battery operating in a nominal 48 V system with a minimum battery voltage of 42 V and
a voltage drop from the battery to the load of 2 V, a constant power load of 5000 W discharges the battery at a
rate no greater than
5000 W
I MAX = = 125 A
40 V
It is also important to be able to work the equations from having load data in wattage. When equipment loads
are specified in watts but no constant power load bank is available, then conversion from watts to amperes
is necessary. This is done by means of an average voltage curve, as explained subsequently. Because watts
= volts × amps, it follows that average watts = average volts × average amperes. Because a constant power
load on a battery is unvarying, watts = average volts × average amperes. If the average voltage is known for a
particular discharge span and end voltage, the average current can be calculated.
Figure D.3 shows a typical constant current voltage versus time curve with the calculated average voltage
during the discharge. Using this graph, the average voltage for any final voltage can be ascertained for this
discharge. Listed in Table D.2 are the final volts, time to final volts, and calculated average volts for a 7-h
discharge.
Table D.2—7 h discharge data

Final volts Time to final volts Average volts
1.75 7.10 h 1.912
1.80 6.71 h 1.917
1.85 6.04 h 1.929
1.90 4.64 h 1.944
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Figure D.3—Typical voltage versus time curve with calculated average volts cell type: ABC
The average voltage is then calculated for every constant current discharge to the required final voltage. All
the average voltages and the time to the average volts are then plotted on a separate curve. Figure D.4 shows
average voltages for discharges from 1 min to 480 min to various final voltages. The data points are curve fit.
The finished curve appears as in Figure D.5.
Using the curve:
From the previous 250 kW example load, with a 15-min duration and a minimum voltage of 1.67 VPC,
the average voltage is determined to be 1.73 VPC from Figure D.5. The average discharge current is then
calculated:
watts (load on battery)

= average amps (discharge current)
number of cells × average volts
250 000 (load on battery)

= 2408.5 (average amps)
(60 × 1.73 VPC)
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Figure D.4—Cell type: ABC-33 average volts to final volts—curve fit
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Figure D.5—Cell type: ABC-33 average volts to final volts—final
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D.4 Conversion from constant resistance loads to constant current

For constant resistance loads, current decreases as the voltage decreases. DC motor starting, emergency
lighting, relays, contactors, and indicating lights are usually constant resistance. A constant resistance load is
conservatively estimated as a constant current load as follows:
VOC W
I MAX = or I MAX = R
R VOC
where
VOC is the battery open circuit voltage (typically 0.85 + nominal specific gravity)
R is the resistance
WR is the rated power value
As with constant power loads, the load current can be calculated using the average battery voltage. The system
voltage drop to the loads can also be considered.
However, if significant motor starting currents are required from the battery at the beginning of the cycle,
the battery voltage should be calculated from initial data using an estimate of the inrush current, and then
checking that the initial voltage supports that level of current, iterating the level of current and voltage until a
satisfactory solution is obtained.
D.5 Other considerations

D.5.1 UPS
UPS power ratings are quoted in volt-amperes (VA) and/or watts. The rating in watts is equal to the rating in
volt-amperes multiplied by the power factor. The battery load for sizing purposes is the UPS output rating in
watts divided by the efficiency of the inverter. The efficiency should be based on rated UPS output. Therefore,
UPS output power rating in watts = UPS output in volt-amperes × power factor nominal battery load = UPS
output power/inverter efficiency.
Temperature, aging, and design margin considerations should be addressed as described in 6.3.
D.5.2 DC motors
While motors are typically considered constant power loads, dc motors can be approximated as constant
current. Within the normal battery voltage range, the flux is fairly constant in the motor. Modeling a dc motor
as a constant current load is conservative if the voltage maintains the motor in saturation.
D.6 Summary
To size a battery properly for a constant power application, the following information is required:
a) The system voltage window. This allows a calculation of the number of cells and minimum cell
voltage. Refer to the example provided previously.
b) The load in watts, kilowatts, or amperes imposed on the battery.
c) The length of time the battery must provide the load without falling below the minimum voltage.
d) The minimum temperature, aging allowance, and design margin at which the battery is expected to
perform.
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Annex E
(informative)
Development and use of battery discharge curves

E.1 Overview
This annex describes the construction and use of battery discharge characteristic curves. The actual
methodology involves multiple discharges with multiple cells at a controlled temperature approximating
77 °F. The average voltage versus time profile for each discharge is determined and then derated by a statistical
measure, usually 2.6 or 3 sigma, to allow for manufacturing variability and thus assure 100% compliance to
the products’ nominal ratings. The product ratings are then verified by testing on factory production strings
against the proposed rates.
This process is described in greater detail in E.2 and E.3.
E.2 Discharge curve fundamentals

A discharge characteristic curve is used to size batteries, and to experienced users, it is the most important tool.
Before interpreting this curve data becomes second nature, the curve itself must be understood: how the data
were obtained, how it works, and ultimately, how to make it work for you.
The fundamentals about these curves, once learned, apply to any characteristic curve.
E.2.1 How the data was obtained

A typical characteristic curve (Figure E.1) has a myriad of straight lines radiating out from a common point,
with a series of curving diagonal lines crossing their path. Even the coordinates sound similar enough to be
confusing: amperes per positive plate on the horizontal (x axis) and ampere-hours per positive plate on the
vertical (y axis). At the top, there is something labeled an initial volts line, which seems to bear no relationship
at all to the others. To understand how to use the curve, the procedures about how a discharge characteristic
curve is derived and plotted should first be understood.
First, a battery was discharge tested at several rates. The cell voltage was periodically monitored so the
voltages can be plotted against time. An example of typical, plotted, test data is shown in Figure E.2.
Three important items of information from the test data are used in the construction of a discharge characteristic
curve: the current at which the cell is discharged, the voltage of the cell at various times throughout the
discharge, and the ampere-hours removed from the cell at various points in the discharge.
Next, data from the discharges is collated using a common reference value, so the information can be applied
to any cell using the same size plates as the ones tested. This common value is the positive plate. Because
the plates are connected in parallel within a cell, the rating of a cell is the rating of a positive plate times the
number of positive plates in a cell.
A cell consists of positive and negative plates with one more negative plate than positive plates. The current
is equally divided among the positive plates. For example, a battery with 33 plates has 16 positive plates and
17 negative plates. At a test rate of 320 A, the battery is discharged at 20 A per positive plate (320/16 = 20).
Similarly, the capacities to various voltages can be shown as ampere-hours per positive plate.
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Figure E.1—Typical discharge characteristic curve for AB battery
Figure E.2—Test data curve
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If a battery cell had but one positive plate, and over a particular time period was able to deliver 100 Ah, a cell
with two positive plates would deliver 200 Ah and so on. Plates of the same polarity are always connected in
parallel.
E.2.2 Plotting the data

Discharge rates
From the example test data in Figure E.2, if the cell voltage is 1.90 at 4.63 h into a 320 A discharge, 1482 Ah
have been removed from the cell (320 A × 4.63 h). Also, the following occurs:
— At 1.85 V, 1926 Ah are removed (320 A × 6.01 h)

By calculating the ampere-hours per positive plate removed at various voltages and listing them opposite the
equivalent amperes per positive plate value of the discharge rates, the discharge characteristics are ready for
plotting (see Table E.1).
Table E.1—Preliminary test data

Final Amperes per positive plate (16 positive plates)
volts 20 40 63.8 85
1.90 92.6 — — — Ampere-
1.85 120.4 78.0 36.3 — hours per
positive plate
1.80 134.6 100.8 68.8 36.6 to final volts
1.75 142.0 112.8 87.3 61.2
Discharge rate, which is expressed in amperes or amperes per positive plate, is distinct from discharge capacity,
which is expressed in ampere-hours or ampere-hours per positive plate. As evident from Figure E.2, the higher
the discharge rate, the less capacity is available before a particular voltage is reached. This is because the
higher the discharge current (rate), the lower the cell voltage is at the beginning of the discharge, and because
the conversion of chemical to electrical energy is less efficient, the end voltage is reached more quickly.
When the data in Table E.1 is transposed onto a graph (Figure E.3) with coordinates of amperes per positive
plate and ampere-hours per positive plate, the discharge capability of the cell to a particular final voltage is
characterized. While in this example only four data points are used, there is a way of interpolating where
the final volt lines intercept the horizontal axis (0 Ah per positive plate). From the test data, note the battery
delivers less and less capacity to a particular final voltage as the discharge current is increased. Logically
then, there should eventually come a point where the discharge current is so great the cell voltage would drop
immediately to the minimum voltage level. In other words, discharging the cell at this high current value
would yield practically 0 Ah, because the final voltage would be reached as soon as the discharge was initiated.
This is the initial voltage of a cell being discharged.
E.2.3 Initial volts

The initial voltage drop of a cell is primarily a function of its internal resistance and the discharge current.
The voltage drop occurs at the positive electrode and is observed as a voltage drop by the cell. This effect is
sometimes designated as the coup de fouet (stroke of the whip), which describes the initial fall and subsequent
recovery of the voltage. As the discharge continues, the voltage again falls until the discharge is terminated.
During momentary discharges, there is sometimes little difference between initial and final voltage.
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Figure E.3—Typical discharge characteristics to 1.75 V
When plotted as amperes versus initial volts, the points fall in a straight line. Four initial voltage points can be
obtained from the original volts versus time discharge data plots of Figure E.2. Figure E.4 more clearly shows
the coup de fouet at the various discharge rates. These points are recorded in Table E.2.
Table E.2—Initial voltage points

Ampere-hours per positive
Initial volts at discharge current
plate discharge current
20.0 1.944
40.0 1.901
63.75 1.859
85.0 1.815
To plot the initial voltage data points, use the horizontal axis or amperes per positive coordinate and install
another vertical axis or initial volts coordinate. Then, plot the initial voltages for the various loads and draw
a straight line through the points. The line can be extended beyond 85 A per positive, the lowest voltage point
determined by the example test, so it intersects the minimum final voltage needed on the curve. In this case, for
1.75 V, it intersects 122 A per positive plate (Figure E.3). Plotting the intercept on the horizontal axis provides
the final data point needed to construct the 1.75 final voltage line. Final voltage lines for 1.80, 1.85, and 1.90
are done the same way (Figure E.5).
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Figure E.4—Coup de fouet at various discharge rates for cell type ABC-33
Figure E.5—Final voltage lines
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E.2.4 Time lines

After the final volt lines are plotted, another set of lines are added to plot the relationship of the coordinates
(amperes versus ampere-hours). The time lines radiating from the origin on the graph are plotted according to
discharge rate in amperes multiplied by time. For example, any battery discharged at 20 A per positive plate
for 8 h has 160 Ah per positive plate removed. A line drawn from the origin, or zero, through the intersection of
20 A per positive plate and 160 Ah, is the eight-hour line. Any discharge characteristic line for a particular
voltage crossing the eight-hour time line indicates a performance capability in ampere-hours or amperes, as
shown by the value of these coordinates at the voltage line-time line intersect. The same rationale is used
for the remaining time lines (Figure E.5). After all the lines are plotted, this graph is superimposed upon the
voltage graph, resulting in the familiar characteristic curve (Figure E.6).
Figure E.6—Time lines
E.3 Using the characteristic curve

The characteristic curve (Figure E.7) allows the user to size batteries for any load or combination of loads for
any reserve time and to any final voltage. Also, the performance of existing batteries can be predicted and
voltage profiles for given loads or load duty cycles can be calculated. Note that the x axis values are the Rt
values that can be used in the standard sizing calculation described in 6.5.
Three simple examples are given in E.3.1 through E.3.3.
NOTE—For simplicity, these examples do not consider the margins (design, aging, or temperature) required by this
standard.
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Figure E.7—Completed discharge characteristic curve
E.3.1 Example 1
Suppose a prospective buyer has a requirement for a battery capable of carrying a load of 400 A for 1 h without
the battery voltage falling below 1.75 average volts per cell.
From the sample discharge characteristic curve (Figure E.8), you see the 1.75 V line intersects the 1 h time line
at 69.3 A per positive plate. If you divide 1 h capability (69.3 A per positive plate) into the required load (400
A), the answer is the number of positive plates required by the ABC series battery to which the curve applies.
In this example, 5.77 positive plates are required, but the next highest whole number of positive plates is
needed-in this case, six. A battery consisting of 13 plates (6 positive and 7 negative) is required.
E.3.2 Example 2
Suppose a user already has a 15 plate cell (7 positive plates) and wants to know how long it will carry 700 A
before reaching 1.75 V per cell.
Divide 700 A by the number of positive plates (7) which equals 100 A per positive plate.
Next, find where 100 A per positive plate intersects the 1.75 voltage line, and then note the corresponding
value of ampere-hours on the vertical axis—36 Ah per positive plate (Figure E.9).
Finally, divide 100 A per positive plate into 36 Ah per positive plate (amperes into ampere-hours equals hours)
to get 0.36 h, which is 21.6 min (0.36 h × 60 min/h). This is the reserve time with a 700 A load.
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Figure E.8—One hour sizing calculation
Figure E.9—100 A/positive plate load calculation
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E.3.3 Example 3
Nontypical reserve times and end voltages can be calculated. Suppose the cell size required for a 2.5 h reserve,
350 A load, and 1.83 minimum average cell voltage needs to be determined.
First, draw in a 2.5 h time line on the characteristic curve. Do this by choosing an ampere value on the horizontal
axis (for example, 40 A per positive plate). Multiply this by 2.5 h (40 A/positive plate × 2.5 h = 100 Ah/positive
plate). Draw a line from the origin through the point where 40 A/positive plate and 100 Ah per positive plate
intersect. This is the 2.5 h line (Figure E.10).
Figure E.10—Sizing calculation 3
Next, determine where a 1.83 final volts line would intersect the 2.5 h time line (interpolate between the 1.80
and 1.85 voltage lines shown) and find the corresponding amperes per positive plate value on the horizontal
axis (37.1 A per positive plate, as shown in Figure E.10).
NOTE—Voltages can be interpolated; time lines cannot and are drawn based on test data.
Now, determine the number of positive plates required. In this instance, 350 A divided by 37.1 A per positive
plate = 9.43 or 10 positive plates. The cell meeting this requirement is the ABC-21.
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Annex F
(informative)
Random loads
Subclause 4.2.7 of this recommended practice addresses random loads and their application in the battery
sizing process. The method described is for loads that actuate randomly anytime during a duty cycle or for
which the actual actuation time in the duty cycle is unknown. However, if more specific information regarding
the timing of a random load can be ascertained, it may result in a requirement for a smaller battery, which is
typically desirable for economic reasons. This is typically achieved by ascertaining enough information to
allow the random load to be reclassified as either a momentary load or a non-continuous load and placed into
the load profile appropriately.
Sometimes enough information can be determined to classify the load as random within a portion of the duty
cycle. For example if it is known that a specific load could only operate during the last hour of a duty cycle,
then the load could be added to only the most critical portion of the last hour. If this were the case for the
random load shown in the battery sizing example of Annex A, the result would be a required battery size of
XYZ-25 (11.15 plates required) instead of the XYZ-27 (12.64 positive plates required) as shown in Figure F.1
and Figure F.2.
Figure F.1—Random load in last hour
If the specific actuation time of a typical random load is not determinable (often because it is process based),
general operation information may provide enough information to allow the load to be considered in a period
of the duty cycle that is not the most severe. Often, it is easier to determine when a load will not actuate than
to determine when it may. Additional review and or analysis of the load and its operation within the system
it is operating is required but may yield significant benefit. The larger the magnitude of the random load, the
greater the potential benefit of selecting the most economical cell size.
As an example: For the random load described in the sizing example of Annex A, assume that it can be
determined that the load does not randomly actuate within the second hour of the duty cycle (between 60
min and 120 min). In this example, the random load could be classified as a momentary load at the end of the
1st minute, the end of the first hour, or at the end of the scenario (ending in the 180th min) and would yield
the same result. This change, as shown in Figure°F.1 through Figure°F.6, would result in a reduction in the
required cell size to an XYZ-25 (11.13 to 11.15 positive plates required) instead of the XYZ-27 (12.6 positive
plates required) as shown in Annex A.
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Figure F.2—Battery sizing for random load in last hour
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If multiple random loads are being considered, additional care is required to understand what, if any,
interactions exist between these loads. Multiple random loads can be combined within specific periods and
their cumulative impact reduced by having them spread through the profile. When investigating the actual
operation of loads classified as random loads, it may become clear that some of the loads cannot operate
simultaneously. These reasons include self-excluding conditions, relay propagation, or even related process
conditions (such as a two valves operating to open but Valve 1 operating on receipt of a “Tank 1 level high”
signal and Valve 2 operating on a “Tank 1 low level”). In these cases, it is determined that the modeling include
only one of the valves as a random load.
If operating times can be determined to be limited in some way (for example, Valve 1 can only occur within the
30 min and Valve 2 could only occur in the last hour) the loads could be inserted into the profile as momentary
loads at the limiting part of the profile during the specified period. If it is unclear which portion of the period
is limiting, then the sizing should be run without the random load to determine the limiting step. Once this is
determined, the sizing should be re-run with the random load added to the limiting step.
In the example shown in Annex A, it may be unclear if it would be more limiting to show Valve 1 as a load
during the first minute or at the end of the 30 min. Two cases of the sizing sheet should be performed to
determine at which time the load would be most limiting.
Figure F.3—Random load in first minute
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IEEE Std 485-2020
Figure F.4—Random load at end of first hour
62
IEEE Std 485-2020
Figure F.5—Battery sizing for random load in first minute
63
IEEE Std 485-2020
Figure F.6—Battery sizing for random load at end of first hour
64
IEEE Std 485-2020
Annex G
(informative)
Full-size worksheet
On the next page is a full-sized worksheet.8
8
Users of this recommended practice may freely reproduce the form in this annex so that it can be used for their intended purpose.
65
IEEE Std 485-2020
66
IEEE Std 485-2020
Annex H
(informative)
Bibliography
Bibliographical references are resources that provide additional or helpful material but do not need to be
understood or used to implement this standard. Reference to these resources is made for informational use
only.
[B1] Hoxie, E. A., “Some discharge characteristics of lead-acid batteries,” AIEE Transactions Part II:
Applications and Industry, vol. 73, no. 1, pp. 17–22, March 1954.
[B2] IEEE Std 323™-2003, IEEE Standard for Qualifying Class 1E Equipment for Nuclear Power Generating
Stations.
[B3] IEEE Std 535™-2006, IEEE Standard for Qualification of Class 1E Lead Storage Batteries for Nuclear
Power Generating Stations.
[B4] IEEE Std 627™-1980 (Reaff 1997), IEEE Standard for Design Qualification of Safety Systems
Equipment Used in Nuclear Power Generating Stations (withdrawn).9
[B5] IEEE Std 946™-2004, IEEE Recommended Practice for the Design of DC Auxiliary Power Systems for
Generating Stations.
[B6] IEEE Std 1578™-2007, IEEE Recommended Practice for Stationary Battery Electrolyte Spill
Containment and Management.
[B7] IEEE Std 1660™, Application and Management of Stationary Batteries Used in Cycling Service.
9
IEEE Std 627-1980 has been withdrawn; however, copies can be obtained from Global Engineering, 15 Inverness Way East,
Englewood, CO 80112-5704, USA, tel. (303) 792-2181 (http://global.ihs.com/).
67
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IEEE Recommended Practice

IEEE Power and Energy Society

IEEE Std 485™-2020

IEEE Recommended Practice

Energy Storage and Stationary Battery Committee

Approved 6 May 2020

IEEE SA Standards Board

The Institute of Electrical and Electronics Engineers, Inc.

Copyright © 2020 by The Institute of Electrical and Electronics Engineers, Inc.

PDF: ISBN 978-1-5044-6703-2 STD24186

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Updating of IEEE Standards documents

James Midolo, Chair

Amber Aboulfaida Rufus Lawhorn Kenneth Sabo

Amber Aboulfaida James Graham Arthur Neubauer

Gary Hoffman, Chair

Ted Burse David J. Law Mehmet Ulema

Annex A (informative) Battery and cell sizing examples������������������������������������������������������������������������������� 26

Annex B (informative) Calculating cell voltage during discharge������������������������������������������������������������������ 32

Annex D (informative) Constant power and constant resistance sizing���������������������������������������������������������� 42

Annex E (informative) Development and use of battery discharge curves����������������������������������������������������� 50

Figure A.4—Sample worksheet using Rt capacity factor�������������������������������������������������������������������������������� 31

Figure B.1—Discharge characteristics of ABC-type cell (Fan Curve)����������������������������������������������������������� 33

Figure B.2—Discharge characteristics of DEF-type cell (S curve)���������������������������������������������������������������� 34

Figure B.3—Calculated voltage/time profile from “fan” curves�������������������������������������������������������������������� 38

Figure B.4—Calculated voltage/time profile from “S” curves����������������������������������������������������������������������� 40

Figure D.2—Constant power discharge characteristic curve cell type: ABC������������������������������������������������� 43

Figure D.4—Cell type: ABC-33 average volts to final volts—curve fit���������������������������������������������������������� 47

Figure D.5—Cell type: ABC-33 average volts to final volts—final��������������������������������������������������������������� 48

Figure E.1—Typical discharge characteristic curve for AB battery���������������������������������������������������������������� 51

Figure F.5—Battery sizing for random load in first minute���������������������������������������������������������������������������� 63

Figure F.6—Battery sizing for random load at end of first hour���������������������������������������������������������������������� 64

Table D.1—Sample sizing chart—values shown in kW per cell�������������������������������������������������������������������� 44

2. Normative references

IEEE Std 1881TM, IEEE Standard Glossary of Stationary Battery Terminology.

NOTE—The primary source of power is normally the battery charger or rectifier.6

4. Defining loads

4.2 Load classification

4.2.1 Continuous loads

4.2.2 Noncontinuous loads

a) Emergency pump motors

4.2.3 Momentary loads

4.2.4 Other considerations

4.2.5 Duty cycle diagram

4.2.6 Defined loads

4.2.7 Random loads

Figure 1—Diagram of a duty cycle

4.2.8 Duty cycle example

L1 40 A for 3 h, continuous load

5. Cell selection

c) Frequency and depth of discharge

6. Determining battery size

— The available capacity of the battery decreases as its temperature decreases.

6.2 Number of cells

6.2.2 Calculating number of cells and minimum cell voltage

maximum system voltage

Annex A (informative) Battery and cell sizing examples�� 26

Annex B (informative) Calculating cell voltage during discharge�� 32

Annex D (informative) Constant power and constant resistance sizing�� 42

Annex E (informative) Development and use of battery discharge curves�� 50

Figure A.4—Sample worksheet using Rt capacity factor�� 31

Figure B.1—Discharge characteristics of ABC-type cell (Fan Curve)�� 33

Figure B.2—Discharge characteristics of DEF-type cell (S curve)�� 34

Figure B.3—Calculated voltage/time profile from “fan” curves�� 38

Figure B.4—Calculated voltage/time profile from “S” curves�� 40

Figure D.2—Constant power discharge characteristic curve cell type: ABC�� 43

Figure D.4—Cell type: ABC-33 average volts to final volts—curve fit�� 47

Figure D.5—Cell type: ABC-33 average volts to final volts—final�� 48

Figure E.1—Typical discharge characteristic curve for AB battery�� 51

Figure F.5—Battery sizing for random load in first minute�� 63

Figure F.6—Battery sizing for random load at end of first hour�� 64

Table D.1—Sample sizing chart—values shown in kW per cell�� 44