Battery Sizing 

By on April 8th, 2012

This article gives an introduction to IEEE 485 method for the selection and calculation of battery capacity.

Definitions

  • battery duty cycle - the load (including duration) the battery is expected to supply
  • cell size - rated capacity of the battery
  • equalizing charge - prolonged charge, at a rate higher than the normal float voltage
  • full float operation - operation with the batteries and load connected  in parallel
  • period - time during which load is expected to be constant during sizing calculations
  • rated capacity - capacity of the battery cell (usually for a given discharge rate and end of cell voltage)
  • valve-regulated lead-acid (VRLA) cell - sealed lead-acid cell (with the exception of a valve that opens when the internal pressure exceeds the external pressure)
  • vented battery - battery in which the products of electrolysis and evaporation are allowed to escape freely to the atmosphere

Battery Selection

The selection of the physical [[batteries|battery]] (cells) is dependant on several factors:

  • type of battery (sealed, vented, lead acid, NiCad, etc.)
  • expected life of the battery
  • usage of the batter (number of charge/discharge cycles)
  • dimensions and weight of the battery
  • construction materials
  • connectors and terminals
  • ambient environment and conditions
  • maintenance requirements
  • seismic characteristics

Ampere-hour and Watts/cell

The Ah or Ampere-hour capacity is the current a battery can provide over a specified period of time.  For example 100Ah @ C10 rate to end of discharge of 1.75 V/cell means the battery can provide 10 Amps for 10 hours to an end of discharge voltage of 1.75 V per cell.

Different battery manufacturers will use different Cxx rates depending on the market or application at which their batteries are targeted. Typical rates used are C3, C5, C8, C10 and C20. Because of this it is important, when comparing batteries from different manufacturers.

Ah is used for sizing batteries based on constant [[electric current|current]] methods and watts/cell on constant [[Electrical Power|power]] methods.

IEEE 485 Lead Acid Batteries for Stationary Applications

This standard details methods for defining the dc loads and for sizing a lead-acid battery to supply those loads in full float operation. A brief  description of the method presented by the standard follow.  For a full and accurate description, refer to the full standard.

Load Definition

Loads are classified as:

  • continuous - loads continually present
  • non-continuous - loads lasting for a specific period
  • momentary - loads lasting for less than 1 minute
Typical Loads
Continuous
Non-continuous
Momentary
Lighting

Continuous Motors
Converters
Indicating Lights
UPS
Control Systems

Emergency motors

Fire protection systems
Valve operations ( > 1 min)

Switchgear operations

Valve operations ( < 1 min)
Isolating switch operations
Field flashing of generators
Motor starting currents
Inrush currents

Note: commonly momentary loads are assumed to last for 1 minute during battery sizing calculations.

Duty Cycle Diagram

IEEE 485 Std. Recommended Practice for Sizing Lead Acid Batteries for Stationary Applications - Typical Duty Cycle The standard recommends a duty cycle be drawn showing the anticipated loads (in [[Ampere]] or power) for the required duration of battery backup time.

IEEE 485 Std. Recommended Practice for Sizing Lead Acid Batteries for Stationary Applications - Typical Duty Cycle

Considerations

  • loads and times where known should be shown
  • random loads should be shown at the most critical times

Calculation of Battery Size

Number of Cells and Cell Voltage - the number of cells is estimated based on the maximum battery voltage and float charge voltage:

myElectrical Equation

The minimum battery voltage is the minimum system voltage (including voltage drops across cables).  Given the minimum cell voltage the minimum cell voltage is given by:

myElectrical Equation

Temperature Correction - at temperature decreases the capacity of a cell decreases (and vise verse as the temperature increases).  Manufacturers quote cell capacity at a given temperature and appropriate correction factors should be used for other temperatures.

Aging Factor - battery performance is relatively stable through out its life, dropping of rapidly towards the end.  To ensure the battery can meet the design requirements throughout its life the standard suggestions the initial capacity should be 125% of the design capacity.

Design Margin - to cater for unexpected circumstances (increased loads, poor maintenance, recent discharge, etc.) it is common to allow a design margin of 10% to 15%.

IEEE 485 Std. Recommended Practice for Sizing Lead Acid Batteries for Stationary Applications - Typical Duty CycleSizing Methodology - the required capacity of the cell FS is given by:

myElectrical Equation

Where S can be any integer from 1 to N depending on the section being calculated and FS is expressed in watt-hours or ampere-hours depending on which Ct is used.

The required uncorrected cell size F, is then given by:

myElectrical Equation

where:

  • F       - is the uncorrected (temperature, aging and design margin) cell size
  • S       - is the section of duty cycle being studied (containing all previous sections)
  • N       - is the number of periods in the duty cycle
  • P       - is the period being analysed
  • AP     - the amperes required for period P
  • t        - the time in minutes from the beginning of period P through the end of Section S
  • Ct      - is the capacity rating factor (for a given cell type, at the t minute discharge rate, at 25 °C, to a definite minimum cell voltage
  • FS     - is the capacity required by each section

Capacity rating factor

There are two ways of expressing capacity:

Term Rt

The term Rt is the number of amperes each plate can supply for t minutes, at 25oC to a defined minimum cell voltage.

myElectrical Equation

giving:

myElectrical Equation

Term Kt

The term Kt is the ratio of ampere-hour capacity, at a standard time rate, at 25oC and to a defined minimum voltage which can be delivered for t minutes.

myElectrical Equation

giving:

myElectrical Equation

Rt is not equal to 1/Kt because each factor is expressed in different units.

See Also

References

  • [1] IEEE Std. 485 'IEEE Recommended Practice for Sizing lead-Acid Batteries for Stationary Applications, Institute of Electrical and Electronics Engineers


Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author


comments powered by Disqus



Latest Questions:

  1. Lightning Protection calculation Free Software
  2. How many motors, or of what kind, do I need to get the output to be 5 Volts under load?
  3. Three winding transformer impedance
  4. Short Circuit Fault Current (comprehensive)
  5. help for electricity distribution (HTA) through underground cables (6 circuits)
  6. CT Secondary Rating
  7. Embedded system design
  8. PV system design - advice

most popular notesMost Popular Notes:







newsletter logo

Our Newsletter

Receive updates on new posts by email
down arrow

Electric Power Distribution Engineering, Third Edition
Electric Power Distribution Engineering, ...
Turan Gonen
Hardcover - 1061 pages
$119.96
The Ultimate Solar Power Design Guide: Less Theory More Practice (The Missing Guide For Proven Simple Fast Sizing Of Solar Electricity Systems For Your Home or Business)
The Ultimate Solar Power Design Guide: Less ...
Lacho Pop MSE, Dimi ...
Kindle Edition - 221 pages
Renewable and Efficient Electric Power Systems
Renewable and Efficient Electric Power ...
Gilbert M. Masters
Hardcover - 712 pages
$99.68
Solar Electricity Handbook - 2015 Edition: A simple, practical guide to solar energy - designing and installing solar PV systems.
Solar Electricity Handbook - 2015 Edition: ...
Michael Boxwell
Paperback - 204 pages
$16.73
Power System Relaying
Power System Relaying
Stanley H. Horowitz, ...
Hardcover - 398 pages
$119.66
Energy Systems Engineering: Evaluation and Implementation, Second Edition
Energy Systems Engineering: Evaluation and ...
Francis Vanek, Louis ...
Hardcover - 672 pages
$60.99
Design of Smart Power Grid Renewable Energy Systems
Design of Smart Power Grid Renewable Energy ...
Ali Keyhani
Hardcover - 592 pages
$111.84
Standard Handbook for Electrical Engineers Sixteenth Edition
Standard Handbook for Electrical Engineers ...
H. Wayne Beaty, ...
Hardcover - 2144 pages
$107.84
Electric Power Substations Engineering, Third Edition (Electrical Engineering Handbook)
Electric Power Substations Engineering, ...
Hardcover - 536 pages
$134.96
Power Systems and Renewable Energy: Design, Operation, and Systems Analysis (Power Generation Collection)
Power Systems and Renewable Energy: Design, ...
Gary D. Price
Paperback - 192 pages
$56.95
Do It Yourself 12 Volt Solar Power, 2nd Edition (Simple Living)
Do It Yourself 12 Volt Solar Power, 2nd ...
Michael Daniek
Paperback - 128 pages
$12.86
Solar Energy Engineering, Second Edition: Processes and Systems
Solar Energy Engineering, Second Edition: ...
Soteris A. Kalogirou
Hardcover - 840 pages
$86.96
Power Generation and the Environment
Power Generation and the Environment
Anco S. Blazev
Hardcover - 1333 pages
$223.66
Photovoltaic Systems Engineering, Third Edition
Photovoltaic Systems Engineering, Third ...
Roger A. Messenger, ...
Hardcover - 528 pages
$106.58
Electric Energy: An Introduction, Third Edition (Power Electronics and Applications Series)
Electric Energy: An Introduction, Third ...
Mohamed A. El-Sharkawi
Hardcover - 606 pages
$100.35
Principles of Sustainable Energy Systems, Second Edition (Mechanical and Aerospace Engineering Series)
Principles of Sustainable Energy Systems, ...
Frank Kreith, Susan ...
Hardcover - 790 pages
$100.63
Renewable and Efficient Electric Power Systems
Renewable and Efficient Electric Power ...
Gilbert M. Masters
Hardcover - 680 pages
Convex Optimization of Power Systems
Convex Optimization of Power Systems
Joshua Adam Taylor
Hardcover - 209 pages
$90.77
Solar PV Engineering and Installation: Preparation for the NABCEP PV Installation Professional Certification
Solar PV Engineering and Installation: ...
Sean White
Paperback - 248 pages
$34.45
Power Electronics and Renewable Energy Systems: Proceedings of ICPERES 2014 (Lecture Notes in Electrical Engineering)
Power Electronics and Renewable Energy ...
Hardcover - 1607 pages
$399.00
The Homeowner's DIY Guide to Electrical Wiring
The Homeowner's DIY Guide to Electrical ...
David Herres
Paperback - 336 pages
$19.08
Large-Scale Solar Power Systems: Construction and Economics (Sustainability Science and Engineering)
Large-Scale Solar Power Systems: ...
Dr Peter Gevorkian
Paperback - 400 pages
$49.99
Renewable Energy in Power Systems
Renewable Energy in Power Systems
Leon Freris, David ...
Hardcover - 300 pages
$77.60
Submarine Power Cables: Design, Installation, Repair, Environmental Aspects (Power Systems)
Submarine Power Cables: Design, ...
Thomas Worzyk
Hardcover - 296 pages
$189.00
How to Solar Power Your Home Everything You Need to Know Explained Simply (Back-To-Basics Conserving)
How to Solar Power Your Home Everything You ...
Martha Maeda
Paperback - 336 pages
$12.02
Large-Scale Solar Power System Design (GreenSource Books): An Engineering Guide for Grid-Connected Solar Power Generation (McGraw-Hill's Greensource)
Large-Scale Solar Power System Design ...
Peter Gevorkian
Hardcover - 704 pages
$91.34
Power Conversion and Control of Wind Energy Systems
Power Conversion and Control of Wind Energy ...
Bin Wu, Yongqiang ...
Hardcover - 480 pages
$85.44
Solar Electricity Handbook - 2012 Edition: A Simple Practical Guide to Solar Energy - Designing and Installing Photovoltaic Solar Electric Systems
Solar Electricity Handbook - 2012 Edition: ...
Michael Boxwell
Paperback - 200 pages


Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note