Fire Alarm Battery Calculations

Battery Capacity Formula:

\[ \text{Battery Capacity (Ah)} = [(\text{Standby Current (A)} \times 24 \text{ h}) + (\text{Alarm Current (A)} \times 0.5 \text{ h})] \times 1.25 \]

Required Battery Capacity:

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1. What is Fire Alarm Battery Calculation?

The fire alarm battery calculation determines the minimum battery capacity required to power a fire alarm system for 24 hours in standby plus 30 minutes in alarm, with a 25% safety margin, as specified in BS 5839-1.

2. How Does the Calculator Work?

The calculator uses the standard formula:

\[ \text{Battery Capacity (Ah)} = [(\text{Standby Current} \times 24) + (\text{Alarm Current} \times 0.5)] \times 1.25 \]

Where:

Standby Current — Current drawn by system in normal operation (amps)
Alarm Current — Current drawn during alarm condition (amps)
24 — Required standby duration in hours
0.5 — Required alarm duration in hours (30 minutes)
1.25 — 25% safety margin factor

Explanation: The calculation ensures the battery can power the system for the required durations with additional capacity for aging and temperature effects.

3. Importance of Proper Battery Sizing

Details: Correct battery sizing is critical for fire alarm system reliability. Undersized batteries may fail during power outages, while oversized batteries increase cost and space requirements unnecessarily.

4. Using the Calculator

Tips: Enter the standby and alarm currents in amps. These values are typically available from the fire alarm panel specifications or can be measured. Both values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: Why is there a 25% safety margin?
A: The margin accounts for battery aging, temperature effects, and manufacturing tolerances to ensure reliable operation throughout the battery's service life.

Q2: What battery type should be used?
A: Sealed lead-acid batteries are most common, but the specific type should match the fire alarm manufacturer's recommendations.

Q3: How often should batteries be replaced?
A: Typically every 4-5 years, or as recommended by the manufacturer, unless performance testing indicates earlier replacement is needed.

Q4: Does this calculation apply to all fire alarm systems?
A: This is the standard method per BS 5839-1. Some specialized systems may have different requirements.

Q5: What if my system has multiple current draws?
A: Sum all standby current draws for the standby current value, and sum all alarm current draws for the alarm current value.