Fire Alarm Battery Calculator: engineer's guide
Battery Ah = standby current times standby hours plus alarm current times alarm hours, divided by aging factor. NFPA 72 sets the durations.
Open the Fire Alarm Battery calculator →
What this calculator does
The calculator sums standby and alarm current draws, applies 24-hour standby and 5-min alarm defaults (per NFPA 72), and derates for aging.
Formulas used
- Battery Ah:
Ah = (Isb × tsb + Ial × tal) / Kage
Variables
- Isb, Ial = standby, alarm currents (amps)
- tsb, tal = standby, alarm hours (24 h, 5/60 h typical)
- Kage = 0.80 aging factor (0.75 for stricter panels)
Worked example
Scenario: 3-loop panel: Isb 0.586 A, Ial 7.33 A (48 horn/strobes).
- Ah = (0.586 × 24 + 7.33 × 0.0833) / 0.80
- Ah = (14.06 + 0.611) / 0.80 = 18.34
- Round up to standard 26 Ah battery
Common mistakes to avoid
- Forgetting aux 24 V loads (annunciators, aspirating, door holders)
- Using NAC booster loads on panel battery (they are separate)
- Skipping aging derate
- Wrong alarm duration (5 min NFPA, 15 min voice, 30 min BS 5839)
Frequently asked questions
Matched-pair batteries?
Yes — same manufacturer, capacity, and lot for two 12 V batteries in series.
Standby duration?
NFPA 72: 24 h protected premises, 60 h supervising station. BS 5839: 24 h L systems, 72 h P systems.
Standards and references
NFPA 72, BS 5839-1, EN 54-4
Related tools on this site
Open the calculator that pairs with this guide: Fire Alarm Battery Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.