1. What is Fire Alarm Battery Capacity?

Fire alarm battery capacity calculation determines the minimum battery size required to power a fire alarm system during both standby and alarm conditions, with appropriate safety margins.

2. How Does the Calculator Work?

The calculator uses the standard formula:

Where:

• \( Standby A \) — Standby current draw in amps
• \( Alarm A \) — Alarm current draw in amps
• \( 24 h \) — Required standby duration (24 hours)
• \( \frac{5}{60} h \) — Required alarm duration (5 minutes converted to hours)
• \( Derating Factor \) — Safety margin (typically 1.2)

Explanation: The formula calculates total energy requirement for both standby and alarm modes, then applies a safety factor to account for battery aging and temperature effects.

3. Importance of Proper Battery Sizing

Details: Correct battery sizing ensures fire alarm systems remain operational during power outages and can sound alarms when needed. Undersized batteries may fail prematurely, while oversized batteries are unnecessarily costly.

4. Using the Calculator

Tips: Enter standby current (normal operation current), alarm current (current during alarm activation), and derating factor (typically 1.2). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is the 24-hour requirement based on?
A: NFPA 72 requires fire alarm systems to operate for 24 hours in standby plus 5 minutes in alarm during power outages.

Q2: Why use a derating factor of 1.2?
A: The 20% margin accounts for battery aging, temperature effects, and manufacturing variations.

Q3: Should I round up the calculated capacity?
A: Yes, always select the next standard battery size above the calculated value.

Q4: Does this apply to all battery types?
A: The calculation works for lead-acid batteries. Lithium batteries may have different derating factors.

Q5: How often should batteries be tested?
A: NFPA 72 recommends monthly visual inspections and annual load testing.