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Battery Capacity Formula:
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The Conventional Fire Alarm Battery Calculator helps determine the required battery capacity (in ampere-hours) for fire alarm systems based on standby current and alarm current draw. This is essential for compliance with NFPA 72 requirements.
The calculator uses the following formula:
Where:
Explanation: The equation calculates the total energy needed for 24 hours of standby plus 5 minutes of alarm operation with a safety margin.
Details: Proper battery sizing ensures the fire alarm system will operate during power outages and meet code requirements. Undersized batteries may fail during emergencies, while oversized batteries may be unnecessarily expensive.
Tips: Enter the standby current (typically measured in milliamps) and alarm current in amps. These values can usually be found in the fire alarm panel specifications or measured with a multimeter.
Q1: Why is there a 1.25 multiplier in the formula?
A: The 1.25 multiplier (25% extra capacity) is a safety factor accounting for battery aging, temperature effects, and manufacturing variations.
Q2: What battery types are typically used?
A: Sealed lead-acid (SLA) batteries are most common, with 12V or 24V configurations depending on the system.
Q3: How often should batteries be replaced?
A: NFPA recommends replacement every 3-5 years, or when capacity drops below 80% of rated value.
Q4: What if my system has multiple alarm devices?
A: The alarm current should include the total current draw of all devices that might activate simultaneously.
Q5: Does this calculator apply to addressable systems?
A: While the principle is similar, addressable systems may have different calculation methods - consult the manufacturer's documentation.