Battery Standby Calculator

BS 5839‑1 requires a fire alarm system's standby battery to hold enough charge to keep the system in normal quiescent operation for a set standby period, then still run the system at full alarm condition (all sounders, beacons and outputs active) for a further 30 minutes at the end of that period — the logic being a fault could occur just before an evacuation is needed at the end of the standby window.

The two standard standby periods are 24 hours, used where the premises has a rapid response arrangement in place (staff on site, or a maintenance contract that guarantees next‑day attendance), and 72 hours, used where that isn't guaranteed.

A shorter 24‑hour standby can also apply on an otherwise‑unattended premises, where the system's fault signals are transmitted to an ARC (Alarm Receiving Centre) that notifies a keyholder on receipt of a fault, rather than the fault being discovered only when someone next attends site. This requires the ARC connection to meet the relevant monitoring standard, with an active keyholder‑notification process in place — always confirm the exact clause against your current copy of BS 5839‑1 before relying on it for a specific job. See our ARC monitoring page for what's involved, or contact us directly.

The calculation used here is:

Required capacity (Ah) = 1.25 × [ (Standby hours × Standby current) + (0.5 × Alarm current) ] ÷ 1000

The 1.25 multiplier is a standard ageing/derating margin, accounting for batteries losing capacity over their service life, so the battery still meets the standby and alarm requirement as it ages. The result is then rounded up to the nearest standard battery size available (7, 12, 17, 24, 38 or 65 Ah).