In my last two blog posts, we learned about the Voltage Drop and Wire Size calculators in LifeSafety Power's Excel-based FlexCalculator Suite .  If you missed them, those posts can be found here:

Part 1 - Voltage Drop
Part 2 - Wire Size

This week, we will cover the Battery Size calculator.  This calculator will tell you the minimum battery size required for a given current and standby time.  Note that this calculator is only for lead-acid/gel cell type batteries.  The FlexCalculator Suite can be found on the LifeSafety Power website at Support>Calculators, or can be directly downloaded by clicking HERE.

Once the suite is downloaded, open the file.  A main menu will appear with buttons for each of the calculator pages.  For this exercise, click the "Battery Size" button.  The Battery Size Calculator page will open.

BatSize Overview

In the following sections, we will explain the interface then show two separate examples - the first for a basic access control application, and the second for a fire application.

About The Calculation
The Battery Size Calculator in the FlexCalculator Suite uses Peukert's Equation to accurately calculate the battery size based on rate of discharge.  This will give slightly different, but more accurate, results than the Amps x Hours calculation used in typical battery calculations.  This is because a battery's amphour capacity is rated at a 20 hour discharge rate.  If you discharge an 8AH battery over 20 hours, it will give you 8AH.  If you discharge that same 8AH battery over 48 hours it will give you MORE than 8AH.  Conversely, if you discharge the battery faster than 20 hours, it will give you far LESS than 8AH.  For more information on this, see our in-depth white paper on backup batteries available at Learning Center>Articles on LifeSafety Power's website or by clicking HERE.

Standby Load
This is where the standby load currents are entered.  The fields in this section are:

BatSize Standby

DC1 - This is the total standby load connected to the DC1 output terminals on the FPO Power Supply

DC2 - This is the total standby load connected to the DC2 output terminals on the FPO Power Supply.

Accessory Boards - The total standby load connected to any accessory boards connected to the FPO Power Supply.

Total Standby Load - This is a calculated field giving the sum of the DC1, DC2, and Accessory Board currents.   This value cannot be changed.

Note that breaking the currents out into the individual fields is not required.  If you know your total standby load is 2A, you may simply enter 2A into the DC1 field and leave the others blank.

Alarm Load
This is where the alarm load currents are entered.  If there is no alarm time, leave these fields blank or zero.  The fields in this section are:

BatSize Alarm

DC1 - This is the total alarm load connected to the DC1 output terminals on the FPO Power Supply

DC2 - This is the total alarm load connected to the DC2 output terminals on the FPO Power Supply.

Accessory Boards - The total alarm load connected to any accessory boards connected to the FPO Power Supply.

Total Alarm Load - This is a calculated field giving the sum of the DC1, DC2, and Accessory Board currents.   This value cannot be changed.

Note that breaking the currents out into the individual fields is not required.  If you know your total alarm load is 4A, you may simply enter 4A into the DC1 field and leave the others blank.

Required Backup Time
These fields are where the standby and alarm times are entered.  The fields include:

BatSize Backup

Standby (Hours/Minutes/Total) - These fields are where the total standby time is entered.  Enter the requirements into the Hours and Minutes fields.

Alarm (Hours/Minutes/Total) - These fields are where the total alarm time is entered.  Enter the requirements into the Hours and Minutes fields.  If there is no alarm time, leave these fields blank or zero.

Results

The results for the calculation appear in the Results section.  To get the results, click outside of the last field you entered information into, or click the "Calculate" button.  The results given are:

BatSize Result

Standby AH - This is the calculated minimum number of AH required to cover the standby portion of the requirement.  This is a calculation and cannot be changed.

Alarm AH - This is the calculated minimum number of AH required to cover the alarm portion of the requirement.  If there is no alarm requirement, this field will be zero.  This is a calculation and cannot be changed.

Minimum Total AH - This is the sum of the Standby and Alarm AH requirement above.  This is the minimum required battery size, based on a fresh set of batteries.  It does not account for the natural reduction in AH which occurs as the battery ages.  This is a calculation and cannot be changed.

Safety Factor - This is a safety factor to account for the reduction in AH capacity of a battery set as it ages.  Typically, 20% is a good safety factor to use for battery sets on a 3-5 year replacement rotation.  This field may be changed to the Safety Factor required for your application.

Recommended Battery - This is the recommended battery set based on the minimum battery size adjusted by the safety factor.  Round up to the nearest standard battery size.  For example, if the Recommended Battery size is 10AH, round up to a 12AH battery set.  This is a calculation and cannot be changed.

Example 1 - Access Control
In this example, we will be calculating the required battery set for a typical 4 door access control system with a required standby time of 4 hours.  The power supply installed is an FPO75-C4E1.  The loading requirements are:

  • Two access control boards powered directly from the FPO power supply's DC1 output, drawing 750mA each, for a total of 1.5A.
  • Four maglocks powered from the C4 board, each drawing 235mA, for a total draw of 940mA.

To calculate the required battery size for this example, enter the following:

BatSize Example 1

Standby Load Section
DC1 - Enter 1.5 Amps here for the 1.5A total draw of the access panels connected to DC1.
DC2 - Leave blank, since nothing is connected to DC2.
Accessory Boards - Enter 0.940 Amps for the 940mA total draw of the locks on the C4 board.

Alarm Load Section
Leave this section blank, since there is no alarm load in an access control application.

Required Backup Time Section
Enter 4 Hours in the Standby Time fields.  Leave the Alarm Time fields blank.

Results
In this application, the recommended battery size is 15AH with the 20% safety factor.  In this case, two 8AH battery sets in parallel would fit the requirement, giving 16AH total.

Example 2 - Fire System
In this example, we will be calculating the required battery set for a typical fire alarm system with a required standby time of 24 hours and an alarm time of 5 minutes.  The power supply installed is an FPO250-N24E2.  The loading requirements are:

  • Eight door holders powered from the DC2 Output, drawing 150mA each for a total draw of 1.2A.  All door holders will release on an alarm condition.
  • Ten horn-strobes powered from the N24 board, each drawing 250mA, for a total draw of 2.5A.

To calculate the required battery size for this example, enter the following:

BatSize Example 2

Standby Load Section
DC1 - Leave blank, since nothing is connected to DC1.
DC2 - Enter 1.2 Amps here for the 1.2A total draw of the door holders connected to the DC2 output.
Accessory Boards - Leave blank since there is no output from the N24 board in standby condition.

Alarm Load Section
DC1 - Leave blank, since nothing is connected to DC1.
DC2 - Leave blank, since the door holders will be unpowered during the alarm condition.
Accessory Boards - Enter 2.5A here for the 2.5A total draw of the horn-strobes under alarm condition.

Required Backup Time Section
Enter 24 Hours in the Standby Time fields.  Enter 5 Minutes into the Alarm Time Fields.

Results
In this application, the recommended battery size is 34AH with the 20% safety factor.  In this case, a 35AH battery would fit the requirement.

The next post in this series will cover the Standby Time calculator, which calculates the standby time of a battery with a given battery size and current draw.  As always, if you need assistance our This e-mail address is being protected from spambots. You need JavaScript enabled to view it   department is always here to help.

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