UPS Battery Sizing 

By on

Various techniques exist to enable the correct selection of batteries for UPS applications. The procedure described below is one of the more common.  It is also possible to use the IEEE 485 method (see link at the bottom of the page).

Battery Types

Typically the following battery types are used in UPS systems:

  • Lead Acid/Plante Battery
  • Lead Acid/Antimony Battery
  • Lead Acid/Calcium Battery
  • Lead Acid/Calcium, Maintenance-free Liquid Electrolyte Battery
  • Lead Acid/Calcium, Maintenance-free Gelled Electrolyte, Sealed Battery
  • Lead Acid (Special Alloy), Suspended Electrolyte, Maintenance-free, Sealed Battery
  • Nickel Cadmium, Pocket Plate Liquid Electrolyte Battery

Battery Sizing

Example of UPS battery sizing

Select the battery model number and quantity (using the typical watts per cell table) for a 300 kVA UPS, 94% efficiency, power factor of 0.8, for a backup time of 15 minutes.

The UPS battery bus voltage is 480 V. The typical table is for 12 V batteries (six cells of 2 V each).

Quantity of batteries per bank = 480/12 = 40 batteries

Number of cells per bank = 40 x 6 = 240 cells

image42

Looking at the capacity, we see that the required watt/cell is too much for one bank. However, various options are available, for example if we decided to use three banks in parallel:

watt/cell (three banks in parallel) = 1063/3 = 354
- select a S12V370(F) battery

Total number of batteries required
= 40 (per bank) x 3 (banks) = 120

Manufacturers provide sizing information for their batteries. Generally this information assumes a room temperature of 25oC. Batteries which will be operated at different temperatures continuously should be calculated specifically for that temperature.

Batteries are generally sized using [Watts] per cell or on [Ampere] per cell.

Watts per cell method

Normally information supplied for lead acid batteries designed for short discharge times (5-120 minutes) is in the form of kilowatts per cell tabulated for various back-up times. The required [Watts] per cell are given by:

Image [4]

Where:
VA    = VA of the load
pf     = power factor
η      = efficiency of the UPS
N     = Number of cells
Al    = any addition load connected to the batteries (in VA) 

0830_BatteryWatts_2
Typical Performance Specification (Amperes per Cell @25 °C)

Amperes per cell

Average Battery Voltage

Battery voltage varies in use - staring high and then decreasing to it's end of discharge voltage.  Taking into account this variation, makes calculation more complicated. 

More often, an average voltage value is taken and calculations based on this. 

If unsure about what the average value to use, then the end of discharge voltage could be used (as this is on the safe side).

Long term discharge lead acid batteries and most nickel cadmium batteries are sized using charts expressed in available amps for specified periods of time. The required [Amperes] per cell is:

image38

Where:
VA    = VA of the load
pf     = power factor
η      = efficiency of the UPS inverter (dc to ac)
Vdc   = Average Discharge Voltage
Al     = any addition load connected to the batteries (in A)

 2474_BatteryAmps_2
Typical Performance Specification (Amperes per Cell @25 °C)

Battery Charger Sizing

In general, a short term discharge battery can be recharged to 85% capacity in 8-10 times the discharge time. A long term discharge battery can be recharged to 85% capacity in a minimum of 8 hours provided the charger is sized properly.

Assuming the UPS is float charging, the following charging current Ic  is required:

Given:

image40 
and
image41

image39

Where: 
Ic = Charging current
IB = Battery current required
Td = Battery discharge (run) time
k = Safety factor (typically 1.5)
Tr = Battery recharge time
Ii = Inverter current required
Ia = Any additional load in (A)  

See Also



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

myElectrical Engineering

comments powered by Disqus

  1. vinh's avatar vinh says:
    3/29/2013 5:50 AM

    it is OK in general but is better with examples,
    Especially "Vdc = Average Discharge Voltage" . What is average Disc volt?

    • Steven's avatar Steven says:
      3/31/2013 2:46 PM

      Thanks for the suggestions. I've added a quick example to the post - hopefully it will help.

  2. Vinh's avatar Vinh says:
    4/2/2013 11:22 AM

    Steven,
    Thanks for the example.
    One thing I'm not sure is why real power P (watt), instead of apparent power S (VA), is used to size the batteries.
    If P is used then where the Q power (VAR) comes from?
    Vinh

    • Steven's avatar Steven says:
      4/4/2013 4:52 AM

      In a d.c. system there is no reactive power. As the batteries are d.c. I think it just makes sense to use Watts for the calculation.

      Reactive power for the a.c. would have to be generated by the UPS.

  3. senthil kumar's avatar senthil kumar says:
    5/8/2013 1:42 PM

    How to do maintenance for battery long life.

  4. Notes's avatar Notes says:
    8/21/2013 3:21 PM

    Trackback from Notes

    A UPS is an uninterruptible power supply.  It is a device which maintains a continuous supply of electrical power, even in the event of failure of the mains (utility) supply.  A UPS is installed between the mains supply and the equipment to be protected... ...


Comments are closed for this post:
  • have a question or need help, please use our Questions Section
  • spotted an error or have additional info that you think should be in this post, feel free to Contact Us



Post Authorship

In 2011, with the introduction of it’s Panda search ranking algorithms, Google introduced tools for determining the original author of posts.  The intention...

Cables for MV Power Distribution - Earthed versus Unearthed Systems

Power cables can basically be classified into earthed and unearthed cables, where earthed and unearthed refer to the application for which the cable is...

Photovoltaic (PV) - Utility Power Grid Interface

Photovoltaic (PV) systems are typically more efficient when connected in parallel with a main power gird. During periods when the PV system generates energy...

Motor Starting - Introduction

Motor starting and its associated problems are well-known to many people who have worked on large industrial processes. However, these things are, of course...

Material Properties

Everything physical in electrical engineering from insulations to conductors revolves around materials. Here we are listing common materials along with...

The ac resistance of conductors

In a previous article I looked at the dc resistance of conductors and in this article we turn our attention to ac resistance. If you have not read the...

Resistors

Resistors are electronic components that oppose the flow of current.  Manufactured in various types and ranges they have a wide application to electronics...

Restricted Earth Fault Protection

The windings of many medium and small sized transformers are protected by restricted earth fault (REF) systems. The illustration shows the principal of...

Capacitors - Energy Storage Application

Capacitors have numerous applications in electrical and electronic applications.  This note examines the use of capacitors to store electrical energy....

UPS - Uninterruptible Power Supply

A UPS is an uninterruptible power supply.  It is a device which maintains a continuous supply of electrical power, even in the event of failure of the...

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