Calculator · BS 5839-1:2025
Fire alarm battery calculator
Enter the standby and alarm current from the panel's data and choose the standby period. You get the minimum battery capacity by the BS 5839-1:2025 method, with every step shown. It doesn't know your panel, so check the answer against the manufacturer's data.
Result
Enter the standby and alarm currents to see the result.
Working
Cmin = 1.25 × (T1 × I1 + D × I2 × T2)
- Cmin is the minimum capacity when new, in Ah (20-hour rate, 20 °C).
- 1.25 is the ageing factor: 5% a year over 4 years.
- D is the de-rating factor. The default is 1.75, and you can change it under Advanced.
- T1 is the standby period and T2 the alarm period, both in hours. I1 is the standby current and I2 the alarm current, both in amps.
Source: BS 5839-1:2025, clause 24 and Annex E.
For an intruder alarm panel, use the intruder alarm battery calculator. It uses the same method, with the intruder alarm standby hours.
BS 5839-1:2025 (clause 24.3.3) recommends marking batteries with their installation date. Writing it on the casing in permanent marker is acceptable.
For guidance only. Always check against the current edition of the standard and the manufacturer's data.
Built by the engineer behind Kerova, certificate software for fire and security firms.