Consultant’s Guide for Fire Detection & Alarm Systems for Buildings

Calculation of Standby Battery Capacity 

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For systems designed in accordance with BS 5839-1, compliance with the code requires that the battery capacity of  valve regulated lead acid batteries should be calculated in accordance with the following formula:

The formula in question is: 

CMIN = 1.25 (T1 I 1 + DI 2 /2) 


CMIN = minimum capacity of the battery when new at the 20 hour discharge rate and at 20ºC (in ampere-hours). 

T1 = total battery standby period in hours. 
I 1 = total battery standby load in amperes. 
I 2= total battery alarm load in amperes. 
D = a de-rating factor. 
1.25 is a factor to allow for battery ageing. 

The de-rating factor is intended to take into account the fact that the effective capacity of a battery depends on the rate at which it is discharged. Battery capacity is normally quoted at the 20 hour discharge rate. Thus, a 20 amperes hour battery would be capable of providing one amp for 20 hours. However, it would not be capable of providing 20 amperes for one hour. The de-rating is needed in cases  in which the alarm current is sufficiently high to reduce the effective capacity below its nominal value. 

Where CMIN/20 will be equal to or greater than I 2, it can be assumed that D = 1. When CMIN/20 is less than I 2, the value of D should either be based on the battery manufacturer’s data or should be 1.75. 

Example 1: Category M or Category L System 
Consider premises that are unoccupied from 6.00 pm Friday until 9.00 am Monday. Assume the normal operating current of the system is 350mA and the maximum alarm load is 2.0A. 

The capacity of the standby batteries would be: 

1.25(24x0.35+1.75 x 2 ÷2)=12.7 

The next highest available capacity size should be used. If, however, the circuit serving the fire alarm system is served by an automatically started standby generator, the capacity can be reduced to: 

1.25 (6 x 0.35 + 1.75 x 2 ÷ 2) = 4.8Ah

Example 2: Category P System 
As the premises are unoccupied for 63 hours, a battery having capacity to operate the system for 72 hours is required. 

Accordingly, the required battery capacity would be: 

1.25 (72 x 0.35 + 1.75 x 2 ÷ 2) = 33.7Ah. 

Again, the next highest available size should be used.

This article was extracted from " Consultant’s Guide for Fire Detection & Alarm Systems for Buildings" by Tyco 

Download the Guide Here

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