Estimate standby battery capacity using quiescent load, alarm load, standby time and alarm duration.
Need Fire Alarm Support?This fire alarm battery calculator helps estimate the minimum standby battery capacity required for a fire alarm control panel or associated power supply unit.
Enter the system quiescent current, alarm current, standby period and alarm duration. The calculator will estimate the required amp hour capacity and suggest the next common battery size.
Normal standby current drawn by the system when not in alarm.
Current drawn when alarm outputs, sounders, beacons and relevant devices are active.
Common values include 24 hours, 36 hours or 72 hours depending on the system and site requirements.
30 minutes is commonly used for fire alarm evacuation load calculations.
Default 1.25 allows for battery ageing and capacity reduction over time.
Default 1.75 allows for reduced battery efficiency under higher alarm load.
For a 24V panel using two 12V batteries in series, the Ah rating does not double. Two 12V 7Ah batteries in series = 24V 7Ah.
Optional. Enter the maximum battery size allowed by the panel manufacturer if known.
The calculator estimates the required battery capacity using standby load, alarm load, standby period, alarm duration, an ageing factor and an alarm de-rating factor.
The calculation used is:
Where:
When sizing fire alarm batteries, the current figures should include the full load connected to the relevant panel or power supply. This may include detection circuits, sounder circuits, beacons, interfaces, communicators, remote indicators, network cards and any critical signalling equipment.
Always check whether additional power supply units have their own batteries. External PSUs used for sounders, signalling or critical path equipment should normally be assessed separately and matched to the required standby period.
The calculated battery capacity should be rounded up to the next suitable battery size, but the selected battery must also be compatible with the control panel charger, enclosure space, manufacturer limits and installation conditions.
Two 12V batteries in series increase voltage, not amp hour capacity. Two 12V 7Ah batteries give 24V 7Ah, not 14Ah.
Alarm load must include active sounders, beacons, outputs and any equipment powered during alarm condition.
Sounder PSUs, door release supplies, communicators and other critical equipment may need separate battery calculations.
If the calculated battery is larger than the panel manufacturer allows, a different approved power arrangement may be required.
Commissioning support for fire alarm systems, including testing, cause and effect checks and handover support.
View commissioning servicesSupport with loop faults, earth faults, PSU faults, device failures, false alarms and panel issues.
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View all servicesQuiescent current is the current drawn by the fire alarm system during normal standby operation when there is no active alarm condition.
Alarm current is the current drawn when the fire alarm system is in alarm and relevant sounders, visual alarm devices, outputs and interfaces are active.
The ageing factor allows for battery capacity reduction over time. A default value of 1.25 is commonly used to provide a margin for battery ageing.
The alarm de-rating factor allows for reduced effective battery performance under higher alarm load conditions. This calculator uses a default value of 1.75.
Only if the battery is suitable for the panel, charger, enclosure and manufacturer’s instructions. A larger battery may not be acceptable if the panel cannot charge it correctly.
No. This calculator provides an estimate only. Fire alarm battery sizing should be confirmed by a competent person using the project requirements, manufacturer information and relevant standards.
If you need support with fire alarm commissioning, battery calculations, power supply faults, false alarms or system upgrades, PM Controls can help across Basildon, Essex and London.
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