11. Battery Capacity Calculation
INTRODUCTION
The standby power source capacity, or battery capacity, determines how long the system will
continue to operate in the event of the loss of the primary power source. It therefore becomes
necessary to calculate the battery and hence power supply / battery charger capacity required for
each installation.
The following calculator has been designed to determine the required capacity to meet the required
standard. Should an existing panel be expanded the required battery and power supply capacity
should be recalculated to ensure the panel continues to operate within the standard.
DESCRIPTION
Enter the number of units listed in the left hand column which go to make up the panel, complete the
multiplication to obtain the quiescent current then multiply by the standby and alarm hours required
by the standard.
POWER SUPPLY RATING
The minimum Power Supply Rating (4) is obtained by calculating the manufacturers recommended
battery charge current and (see Note) (1) then adding the quiescent current of the entire system (2)
and the alarm current (3).
1. Battery Capacity (AH) (determined from Calculator) = Amps
24 x 0.8
2. Add Quiescent Current of the System (Iq) = Amps
3. Add the extra current that is drawn when in alarm (Ia.) = Amps
4. Minimum Current Rating of Power Supply is = Amps
Note: 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)
condition for at least 24 h, after which sufficient capacity shall remain to operate two worst case
AZF’s and associated ACF’s for 30 min.
Note: 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.
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