Sizes fire alarm standby batteries to BS 5839-1 Annex D/E from a device/panel current
schedule: required minimum capacity, a recommended standard battery size, and an optional pass/fail
check against a candidate battery.
Tender-stage only. Verify final design against BS 5839-1 and manufacturer guidance.
For design guidance only. Always verify with a qualified engineer and the panel manufacturer's own battery-charging guidance.
About this fire alarm battery sizing calculator
This free fire alarm standby battery sizing calculator works out the sealed lead-acid (VRLA) capacity a fire detection and alarm panel needs to ride through a mains failure, following the standby-battery method in BS 5839-1 Annex D/E. Build a device and panel current schedule, and it returns the total quiescent and alarm current, the minimum required capacity, and the next standard battery up the ladder, with an optional pass/fail check against a candidate battery. It is aimed at UK fire alarm designers, installers and contractors sizing batteries at tender or commissioning stage. Everything runs in your browser. Nothing is uploaded.
How the standby battery capacity is calculated
The required capacity is Cmin = 1.25 × (I₁ × T₁ + D × I₂ × T₂), where I₁ is the total quiescent current, T₁ the standby period, I₂ the total alarm current and T₂ the fixed 30-minute (0.5 h) alarm period. The 1.25 is the ageing factor (roughly 5%/year capacity loss over four years) and D is the battery de-rating factor, usually 1.75. Currents are summed from your schedule (each row’s current × quantity), converted mA → A, and the result is rounded up to the nearest stocked VRLA size (a 7/12/18/24/38 Ah ladder). The standby period comes from the category duration matrix in BS 5839-1 clause 25.4, 24 h for Category L/M or staffed/ARC-monitored Category P, 6 h with an automatic standby generator, or 72 h for an unstaffed Category P.
Frequently asked questions
What standby period should I use?
BS 5839-1 clause 25.4 sets it by system category: 24 h for a Category L or M system (or a staffed / alarm-receiving-centre-monitored Category P), 6 h where an automatic standby generator is fitted, and 72 h for an unstaffed Category P with no ARC monitoring. The tool offers these presets and lets you enter a custom value.
Why the 1.25 and 1.75 factors?
The 1.25 ageing factor allows for the battery losing capacity over its service life (about 5% a year across four years), so a new battery is oversized enough to still cover the load when aged. The 1.75 de-rating factor D allows for the reduced effective capacity at the high discharge rate of the 30-minute alarm load. Both are the values used in the Annex D/E worked method.
Does this replace the panel manufacturer’s guidance?
No. It is a tender-stage sizing aid. Always confirm the quiescent and alarm currents from the actual device data, check the figure against the panel manufacturer’s own battery-charging guidance, and verify the final design with a competent fire alarm engineer.