Fire Alarm Battery Sizing — Method
The secondary supply must run the system in normal (standby) condition for the required period, then sound the alarm for the specified time, with margin for battery ageing. Capacity Ah = (I_standby × standby hours + I_alarm × alarm hours) × ageing factor. Sum every device current at the panel: control panel quiescent, detectors, interfaces and any auxiliary loads for standby; sounders, beacons and relays for alarm.
Frequently Asked Questions
Ah = (total standby current × standby hours) + (total alarm current × alarm hours), then multiply by an ageing factor (typically 1.2–1.25). NFPA 72 commonly requires 24 h standby + 5 min alarm; size the next standard battery above the result.
For most protected-premises systems NFPA 72 requires 24 hours of standby followed by 5 minutes in alarm. Some systems (e.g. certain emergency communication systems) require 15 minutes or more. BS 5839-1 commonly specifies 24 or 72 hours standby plus 30 minutes alarm.
Sealed lead-acid battery capacity falls with age and temperature. A 1.2–1.25 factor (≈20–25%) ensures the battery still meets the standby + alarm demand near the end of its service life, before it is replaced.
From the manufacturer data sheets: panel quiescent current, plus each detector/module standby current for the standby total; and sounder/beacon/relay currents at full load for the alarm total. Add any auxiliary 24 V loads.
Yes — published Ah is usually the 20-hour (C/20) rate. The standby discharge is slow (≈C/24) so the C/20 figure is appropriate; the short, higher alarm current has a small effect on total Ah but verify against the battery's high-rate curve for large systems.
Fire Alarm Battery Sizing Explained
Every fire alarm system needs a secondary power supply that keeps it running if the mains fails. Codes set a two-part duty: a long standby period in the quiescent state, then a short period driving the alarm load at full output. The battery must deliver both, with margin for ageing.
Standby vs alarm load
The standby current is small but runs for many hours — panel electronics plus every detector and interface. The alarm current is large but brief — all sounders and beacons energised. Multiply each by its time, add them, and apply the ageing factor.
Choosing the battery
Pick the next standard sealed lead-acid capacity above the calculated Ah (7, 12, 18, 26, 40, 65 Ah…). Check that the panel charger can recharge it within the code time (typically 24 h to 80–100%), and de-rate for high ambient temperatures. For very large loads, two batteries in parallel or a higher charger rating may be required.
Related: see the voltage drop calculator for NAC/sounder circuit drop and the battery short-circuit calculator for DC fault current.