1. What is the Fire Alarm Battery Calculation?

The fire alarm battery calculation determines the minimum battery capacity required to power a fire alarm system during both normal standby operation and alarm conditions, with an appropriate safety factor.

2. How Does the Calculator Work?

The calculator uses the following equation:

Where:

• System Quiescent Current — Normal standby current draw (amps)
• Alarm Current — Current draw during alarm condition (amps)
• 24 h — Required standby duration (hours)
• 5 min — Required alarm duration (minutes)
• Supervisory Factor — Safety margin (typically 1.2-1.5)

Explanation: The calculation accounts for both 24 hours of standby operation plus 5 minutes of alarm operation, multiplied by a safety factor to ensure reliability.

3. Importance of Battery Sizing

Details: Proper battery sizing is critical for fire alarm system reliability. Undersized batteries may fail during emergencies, while oversized batteries increase cost unnecessarily.

4. Using the Calculator

Tips: Enter the system's quiescent current and alarm current in amps, along with an appropriate supervisory factor (typically 1.2). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical supervisory factor?
A: Most codes require a factor between 1.2 and 1.5 to account for battery aging and temperature effects.

Q2: Why 24 hours standby and 5 minutes alarm?
A: These are common code requirements (NFPA 72 in the US) for fire alarm system battery capacity.

Q3: Should I use measured or manufacturer's current values?
A: For most accurate results, use measured values. Manufacturer specs can be used for design calculations.

Q4: How does temperature affect battery capacity?
A: Cold temperatures reduce battery capacity. In cold environments, a larger supervisory factor may be needed.

Q5: What battery types are used in fire alarms?
A: Sealed lead-acid (SLA) batteries are most common due to their reliability and maintenance-free operation.