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.