Fire alarm · BS 5839-1
How to work out a fire alarm battery size
The battery formula from BS 5839-1 Annex E, explained step by step, with a worked example, and how to pick the next battery size up.
The formula
BS 5839-1 Annex E gives the minimum standby battery capacity as:
Cmin = 1.25 × (T1 × I1 + D × I2 × T2)
| Symbol | What it is |
|---|---|
| Cmin | The minimum battery capacity when new, in amp hours (Ah), at the 20-hour rate and 20 °C |
| 1.25 | The ageing factor, allowing for the battery losing capacity over its life |
| T1, I1 | The standby period in hours, and the standby current in amps (the whole system on battery, no alarm) |
| T2, I2 | The alarm period in hours, and the alarm current in amps (the whole system in alarm) |
| D | The de-rating factor, 1.75 unless the battery maker gives another figure |
The usual starting figures are 24 hours of standby and 30 minutes of alarm. The result is the same as under the 2017 method.
A worked example
A panel draws 250 mA on standby and 750 mA in alarm. Standby is 24 hours and the alarm period is 30 minutes (0.5 hours).
- Standby charge: 24 h × 0.25 A = 6 Ah
- Alarm charge: 1.75 × 0.75 A × 0.5 h = 0.656 Ah
- Total: 6 + 0.656 = 6.656 Ah
- With ageing: 1.25 × 6.656 = 8.32 Ah
That is the minimum. A battery has to be at least that big, so in practice you pick the next size up. With common sizes of 7, 12, 17, 24, 38 and 65 Ah, 8.32 Ah means a 12 Ah battery.
The free battery calculator does these steps and shows the working, so you can print it with the certificate.
Where the currents come from
Use the figures from the panel's data, not a guess. The standby current is everything the battery feeds while the mains is off. The alarm current is the same, plus what the sounders and any communicator draw when the alarm sounds. Check your panel's maximum battery size and charger rating as well, because a bigger battery is no use if the charger cannot recharge it.
Mark the date on the battery
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.
What about intruder alarms?
Our intruder alarm battery calculator uses the same formula with the intruder standby hours from the grade. That is an estimate, not a rule in PD 6662 or BS EN 50131. Check the panel maker's own figures for the job.
Common questions
What is the formula for fire alarm battery capacity?
Cmin = 1.25 × (T1 × I1 + D × I2 × T2), from BS 5839-1 Annex E. T1 and T2 are the standby and alarm periods in hours, I1 and I2 the standby and alarm currents in amps, 1.25 the ageing factor and D the de-rating factor, 1.75 unless the maker gives another figure.
How many hours of standby does a fire alarm battery need?
The usual default is 24 hours of standby and 30 minutes of alarm. Other situations change the standby period, so check the standard for the job.
What size battery do I need for 250 mA standby and 750 mA alarm?
8.32 Ah as a minimum, using 24 hours of standby and 30 minutes of alarm. The next common battery size up is 12 Ah.
What is the de-rating factor?
A battery gives less than its rated capacity when it is drained quickly. The formula applies a de-rating factor D to the alarm current to allow for that. It is 1.75 unless the battery maker gives another figure.
This guide is general information for UK fire & security companies, not legal or engineering advice. Always work to the current edition of the relevant standard and your own competent-person judgement.
See this on a real certificateKerova builds the register, readings and verdict into the certificate itself.
Book a Demo