Why Battery Backup Matters

NFPA 72 Section 10.6.7 requires every fire alarm system to have a secondary power supply — typically a sealed lead-acid (SLA) battery — capable of operating the system for 24 hours in standby followed by 5 minutes at full alarm load. For systems with emergency voice evacuation, the requirement jumps to 24 hours standby plus 15 minutes alarm. Undersized batteries are one of the most common code violations found during inspections.

Step 1: Calculate Standby Load

The standby load includes all devices that draw current continuously during normal (non-alarm) operation. The primary consumer is the FACP itself, but addressable devices on the SLC loop also draw quiescent current. Gather the standby current from each component's data sheet:

  • FACP standby current (typically 0.2–1.5 A depending on panel size)
  • SLC loop device quiescent current (addressable devices: 0.3–2.0 mA per device)
  • Remote annunciators and displays
  • Any continuously powered modules (relay modules, input modules)

Sum all standby currents to get the total standby load in amperes (A). Multiply by 24 hours to get ampere-hours (Ah) required for standby.

Step 2: Calculate Alarm Load

The alarm load includes all devices that activate during a full alarm condition: all notification appliances (horns, strobes, horn/strobes), activated relay modules, and any alarm-state panel draws. This is the maximum current the system will ever draw.

Collect the alarm current for each appliance from manufacturer data sheets. Note that strobes draw significantly more current than horns — a typical wall-mount horn/strobe draws 85–100 mA at 24VDC. If you have 40 devices, that alone is 3.4–4.0 A of alarm current.

Sum all alarm currents and multiply by the alarm duration: 5/60 hours (5 minutes) for standard systems, or 15/60 hours (15 minutes) for voice evacuation systems.

Step 3: Total Battery Capacity Required

Add the standby Ah and alarm Ah together. Then apply a 20% derating factor to account for battery aging and temperature effects: divide the sum by 0.8 (or multiply by 1.25).

Formula: Battery (Ah) = (Standby load × 24 h + Alarm load × alarm duration) × 1.25

Example: A system with 0.5 A standby load and 3.2 A alarm load for a standard system: (0.5 × 24) + (3.2 × 0.083) = 12.0 + 0.27 = 12.27 Ah × 1.25 = 15.3 Ah. Select the next available standard battery size — in this case a 17 Ah or 18 Ah SLA battery.

Charging Time Verification

NFPA 72 also requires the battery charger to recharge the battery to full capacity within 48 hours after a complete discharge. Most FACP manufacturers include a charger sized for their listed battery options. Always verify that the charger in your panel is rated for the battery capacity you select — installing a larger battery than the panel's charger can handle creates both a code violation and a fire hazard.

Dual Battery Banks

For large systems requiring more than the FACP's built-in charger can handle, external battery chargers and remote battery cabinets are permitted. These must be listed for the purpose and interconnected per the FACP manufacturer's instructions. The total battery capacity calculation method remains the same regardless of how many physical batteries are used.

Documentation

NFPA 72 requires that battery calculations be included in the fire alarm system documentation submitted to the AHJ. Most fire alarm design software includes a battery calculation report. If doing it manually, a clear spreadsheet showing each device, its standby and alarm current draw, and the total Ah calculation is acceptable for most AHJs.