1. What is the Fire Alarm Battery Time Calculation?

The Fire Alarm Battery Time calculation determines how long a fire alarm system's backup battery will last during a power outage, considering both standby and alarm current draws.

2. How Does the Calculator Work?

The calculator uses the following equation:

Where:

• Battery Capacity — Total amp-hour rating of the battery
• Standby Current — Current drawn during normal operation
• Standby Fraction — Fraction of time in standby mode (typically 1 for full-time standby)
• Alarm Current — Current drawn during alarm condition
• Alarm Fraction — Fraction of time in alarm mode

Explanation: The equation calculates total battery life by dividing the battery capacity by the weighted average current draw.

3. Importance of Backup Time Calculation

Details: Proper backup time calculation ensures fire alarm systems remain operational during power outages as required by building codes (typically 24 hours standby plus 5 minutes alarm).

4. Using the Calculator

Tips: Enter battery capacity in amp-hours, currents in amps, and fractions as decimals between 0 and 1. For continuous standby, use Standby Fraction = 1. For continuous alarm, use Alarm Fraction = 1.

5. Frequently Asked Questions (FAQ)

Q1: What is the typical backup time required by codes?
A: Most codes require 24 hours of standby operation plus 5 minutes of alarm operation.

Q2: How do I find the standby and alarm currents?
A: These values are typically listed in the fire alarm control panel specifications or can be measured with an ammeter.

Q3: Should I use battery capacity at 20-hour rate or 1-hour rate?
A: Use the 20-hour rate capacity for standby calculations as it's more representative of actual usage.

Q4: What factors can reduce actual backup time?
A: Battery age, temperature extremes, and higher than expected current draws can all reduce actual backup time.

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