Fire Alarm Battery Calculator
Example Data Table
| Item |
Quantity |
Standby Load |
Alarm Load |
Notes |
| Main control panel |
1 |
0.120 A |
0.220 A |
Use listed panel data. |
| Smoke detectors |
60 |
0.25 mA each |
0.40 mA each |
Use device datasheets. |
| Control modules |
12 |
0.70 mA each |
1.20 mA each |
Include all active modules. |
| NAC circuits |
4 |
0 A |
0.750 A each |
Use horn and strobe load. |
| Auxiliary load |
1 |
0.050 A |
0.090 A |
Include annunciators and interfaces. |
Formula Used
Standby Ah = Total standby current × Standby hours.
Alarm Ah = Total alarm current × Alarm minutes ÷ 60.
Raw Ah = Standby Ah + Alarm Ah.
Required Ah = Raw Ah × (1 + Derating% + Aging% + Reserve%) ÷ 100.
Required Wh = Required Ah × System voltage.
How to Use This Calculator
Enter the system voltage first. Then add the required standby time.
Enter the alarm duration in minutes. Use values from the fire alarm
control panel manual and device datasheets. Add detector, module,
relay, NAC, and auxiliary loads. Enter standby current and alarm
current carefully. Use amps for panel, NAC, and auxiliary loads.
Use milliamps for detectors, modules, and relays. Add derating,
aging, and reserve percentages. Enter the selected battery rating.
Press the calculate button. Review the minimum amp hour size.
Compare it with the selected battery. Download the result when needed.
Fire Alarm Battery Calculation Guide
A fire alarm system needs reliable standby power. The battery must
support normal monitoring loads. It must also support alarm loads
during an emergency. This calculator helps estimate that battery
size. It combines panel demand, initiating device demand, notification
appliance demand, relay demand, and auxiliary demand.
Why Battery Sizing Matters
A weak battery can create trouble signals. It can also reduce system
reliability during power failure. Fire alarm batteries are not chosen
by guesswork. They should be sized from listed current values. The
standby load runs for many hours. The alarm load runs for a shorter
period. Both loads affect the final amp hour rating.
Standby and Alarm Loads
Standby current includes the panel, detectors, modules, annunciators,
communicators, and other powered circuits. Alarm current includes
the active panel load, notification circuits, relays, control modules,
and auxiliary outputs. Horns and strobes often create the largest
alarm load. Always check manufacturer sheets. Do not rely on labels
alone. Use the maximum listed current when available.
Safety Factors and Derating
Real battery capacity changes with age. It also changes with
temperature and discharge rate. A design margin helps cover these
limits. This tool lets you add temperature derating, aging allowance,
and reserve capacity. These values make the result more practical.
Local rules and project specifications may require higher margins.
Reading the Result
The required amp hour value is the key result. The selected battery
should be equal to or larger than that value. The calculator also
estimates watt hours. It checks the charger recovery capacity from
charger current and recharge time. A charger warning means the
battery may be sized correctly, but recharge performance needs review.
Final designs should follow approved plans, local codes, and equipment
listings.
FAQs
What is fire alarm battery capacity?
It is the stored electrical capacity needed to run the fire alarm
system during power loss. It is normally expressed in amp hours.
What current values should I enter?
Use values from manufacturer datasheets. Enter standby current for
normal monitoring. Enter alarm current for active emergency operation.
Why is alarm time entered in minutes?
Alarm operation is usually shorter than standby operation. Minutes
make the input easier and reduce conversion mistakes.
Does the selected battery count change amp hours?
This tool compares the entered amp hour rating with the demand.
Series batteries usually increase voltage, not amp hour capacity.
Why add aging allowance?
Batteries lose capacity over time. Aging allowance helps keep the
design useful after months or years of service.
Why include temperature derating?
Temperature can reduce available battery capacity. Derating gives a
safer estimate when batteries may operate in poor conditions.
What does charger status mean?
It compares charger output over the recharge period with required
amp hours. A warning means charger data needs review.
Can this replace code review?
No. This tool supports design checks. Final approval should follow
local codes, project documents, and listed equipment instructions.