Standby (Fire Alarm) Battery Calculator

A standby battery is an electrochemical device that provides reserve (backup) power for a primary power source such as consumer mains power for fire safety systems and equipment. Various Australian Standards require the adequate reserve power to enable the system or equipment to continue to operate for extended periods of time.
Russ Porteous
CEO, Firewize

This battery capacity calculator has been developed to provide users with a simple method of determining the minimum standby battery capacity for a fire detection and alarm system and emergency warning system based on the formula found in Australian Standards AS 1670.1, AS 1670.4 and AS 1851:2012.

C20 = 1.25 * ((IQ * TQ) + FC(IA * TA))

where:

C20 = battery capacity in Ah at 20 h discharge rate
IQ = total quiescent current
TQ = quiescent standby power source time
FC = capacity de-rating factor
IA = total current in alarm state
TA = alarm load standby power source time
L = compensation factor for expected battery deterioration.

Where the load (IQ or TQ) fluctuates, the worst case average over the required period shall be used.

The capacity of the battery shall be such that, in the event of failure of the primary power source, the batteries shall be capable of maintaining the system in normal working [quiescent load current (IQ)] condition for the required standby time (TQ), after which sufficient capacity shall remain to operate two worst case alarm zones [alarm load current (IA)] for the required time.

When calculating battery capacity, allowance shall be made for the expected loss of capacity over the useful life of the battery. A new battery shall be at least 125% of the calculated capacity requirements, based on a loss of 20% of its capacity over the useful life of the battery.

Stationary Battery Requirements (Fire Detection and Alarm Systems)

The quiescent load of the Fire Detection and Alarm System ("FDAS") shall include any connectable devices and all fault and disablement indication loads.

The worst case of the following loads shall not exceed the power supply equipment rating:

  1. The total load of the FDCIE with five detection devices in alarm state in each of the two detection zones including all loads that will operate in the fire alarm condition, such as occupant warning systems if sharing the PSE, or the quiescent load of the FDAS including battery charging, whichever is greater.
  2. Where the system is a fire suppression system, and the Special Hazards Control and Indicating Equipment ("SHCIE") serves a single suppression risk, the load shall be calculated in accordance with Item (a) above. Where the SHCIE serves multiple risks, then the load shall be calculated on the two highest load risks in an activated state, or 20 percent of the total connected systems in an activated state whichever is the greater, unless varied by the suppression system design.

The standby power source shall consist of rechargeable stationary batteries in accordance with the relevant part of Australian Standard AS 4029 or as listed in the Fire Detection Control and Indicating Equipment ("CIE") conformity documentation referred to in Clause 2.1.1 of AS1670.1 as suitable and compatible with the CIE and PSE.

The capacity of the standby power source shall be such that in the event of failure of the power supply equipment ("PSE") main power source the standby power source shall be capable of maintaining the system in normal working (quiescent) condition for at least 72 hours, after which sufficient capacity shall remain to provide power for the main power source load determined in Clause 3.15.4 for 30 min.

Where the PSE produces a power supply failure signal and this signal is continuously monitored externally either on site or remotely, the minimum standby requirement may be reduced to 24 h.

The power supply failure signal is when the power supply voltage falls to the minimum operating voltage of the FDAS, or the final voltage of standby power source battery whichever is the greater.

Stationary Battery Requirements (emergency warning and intercom systems)

For an emergency warning and intercom system, the capacity of the standby power source shall be such that in the event of failure of the main power source the standby power source shall be capable of maintaining the system in normal working (quiescent) condition for at least 72 hours, after which sufficient capacity shall remain to provide power for the worst case load as determined for the main power source for 30 minutes.

If the power supply equipment ("PSE") produces a power supply failure signal when the power supply voltage falls to the minimum operating voltage of the system, or the final voltage of standby power source battery, whichever is the greater, and this condition is continuously monitored on site or externally, the minimum requirement is then reduced to from 72 hours to 24 hours.