Fire Alarm Battery Calculator
International Standards for NFPA 72 (US) & BS5839 (UK/Europe)
NFPA 72 Requirements
The National Fire Protection Association code requires fire alarm systems to have secondary (battery) power with 24 hours of standby operation followed by 5 minutes of alarm operation (15 minutes for voice evacuation systems).
BS5839-1 Requirements
British Standard BS5839-1 requires fire alarm systems to have secondary (battery) power with 24 hours of standby operation followed by 30 minutes of alarm operation. Category P systems may require up to 72 hours for unoccupied buildings.
Required Battery Capacity = (Standby Current × Standby Time + Alarm Current × Alarm Time) × Safety Factor
Required Battery Capacity = 1.25 × (Standby Current × Standby Time + Derating Factor × Alarm Current × Alarm Time)
Fire Alarm Battery Sizing and Requirements
Battery backup is a critical component of any fire alarm system, ensuring operation during power outages when the need for fire detection may be at its highest. Proper battery sizing is essential for code compliance and system reliability.
Key Battery Sizing Factors
- Standby current draw of all connected devices
- Alarm current draw when all notification devices are active
- Required standby duration (24 hours standard, 4-6 hours with generator)
- Required alarm duration (5-30 minutes depending on system type)
- Safety factors to account for battery aging and environmental conditions
Code Requirements for Fire Alarm Batteries
NFPA 72 (US Standard)
- Standard Standby: 24 hours
- Alarm Duration: 5 minutes (standard) or 15 minutes (voice evacuation)
- With Generator: 4 hours standby
- Safety Factor: 25% (1.25×)
- Testing: Annual capacity testing required
BS5839 (UK/European Standard)
- Standard Standby: 24 hours
- Alarm Duration: 30 minutes
- Unoccupied Buildings: Up to 72 hours standby
- With Generator: 6 hours standby
- Factors: 1.25 aging + 1.75 derating
Battery Capacity Calculation Methods
Common Battery Sizes for Fire Alarm Systems
| Battery Size | Typical System Application | Approximate Dimensions (mm) | Weight (kg) |
|---|---|---|---|
| 2.1 Ah | Very small conventional panels | 178 × 34 × 64 | 0.9 |
| 7 Ah | Small to medium conventional panels | 151 × 65 × 94 | 2.6 |
| 12 Ah | Medium panels or systems with multiple devices | 151 × 98 × 94 | 3.8 |
| 18 Ah | Larger conventional/medium addressable systems | 181 × 77 × 167 | 5.6 |
| 26-33 Ah | Large addressable systems with many devices | 195 × 130 × 168 | 9-11 |
| 40-55 Ah | Large networked systems or voice evacuation | 229 × 138 × 208 | 14-18 |
Note: For 24V systems, two 12V batteries with the same Ah rating are typically connected in series.
Factors Affecting Battery Performance
Battery Aging
Sealed lead-acid batteries (SLA) typically lose 3-5% capacity per year under normal conditions. The safety factor of 1.25 (25% extra) ensures adequate capacity through the expected service life.
Most batteries should be replaced every 3-5 years per manufacturer recommendations, or when they fail to hold at least 80% of rated capacity.
Temperature Effects
Cold temperatures significantly reduce available battery capacity. For every 1°C below 20°C (68°F), capacity decreases by approximately 1%.
At 0°C (32°F), a battery may only deliver 80% of its rated capacity. In unconditioned spaces, this must be factored into sizing.
Discharge Rate Effects
Batteries provide less capacity when discharged quickly (high current) compared to slow discharge. This is why BS5839 includes a derating factor of 1.75 for alarm currents.
The impact is more significant with very high alarm currents relative to battery size.
Recharge Requirements
NFPA 72 requires batteries to be recharged to 70% capacity within 12 hours after a complete discharge, and to 100% within 48 hours.
The fire alarm panel’s charging circuit must be capable of handling the installed battery size.
Important Sizing Considerations
- Always select the next standard size up from your calculated minimum requirement.
- Ensure the fire alarm control panel’s charging circuit can handle the selected battery size.
- For extreme environments (very cold or hot areas), additional capacity may be necessary.
- Document your calculations for approval by authorities having jurisdiction (AHJ).
- Batteries should be tested annually, with load testing to verify capacity per code requirements.
Common Calculation Examples
| System Type | Standby Current | Alarm Current | Calculation | Required Size |
|---|---|---|---|---|
| Small Office (NFPA) | 0.25 A | 0.75 A | (0.25A × 24h) + (0.75A × 0.083h) × 1.25 | 7.6 Ah → 12 Ah |
| Retail Store (BS5839) | 0.35 A | 1.5 A | 1.25 × [(0.35A × 24h) + 1.75(1.5A × 0.5h)] | 15.3 Ah → 18 Ah |
| School with Generator (NFPA) | 0.8 A | 2.5 A | (0.8A × 4h) + (2.5A × 0.083h) × 1.25 | 4.2 Ah → 7 Ah |
| Hospital Voice Evac (NFPA) | 1.2 A | 5.0 A | (1.2A × 24h) + (5.0A × 0.25h) × 1.25 | 37.5 Ah → 40 Ah |
Testing and Maintenance Requirements
Both NFPA 72 and BS5839 require regular testing and maintenance of fire alarm batteries to ensure reliable operation when needed:
Monthly Visual Inspection
- Check for physical damage or deformation
- Inspect terminals for corrosion or loose connections
- Verify proper mounting and no signs of leakage
- Confirm charging system operation (voltage reading)
Annual Load Testing
- Disconnect AC power and verify system operation under battery power
- Measure voltage under load conditions
- Confirm battery can support system for required duration
- Document test results in system logbook
Battery Replacement Criteria
Batteries should be replaced when any of these conditions are met:
- When they fail load tests (unable to maintain system operation)
- When they reach the manufacturer’s recommended replacement date
- When they reach 80% or less of their rated capacity
- Typically every 3-5 years, depending on environmental conditions
- If there is physical damage, corrosion, or leakage