Fire alarm battery size calculator
Work out the standby battery capacity a fire alarm panel needs, using the calculation in BS 5839-1:2025. Choose the standby period for your category of system, enter the loads from the manufacturer’s data, and we’ll give you the minimum size, rounded up to a standard battery.
Nova Security Systems S.E. Ltd
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01303 720282 · info@novasec.co.uk · novasec.co.uk
Fire alarm standby battery calculation
| Site or project | |
|---|---|
| Prepared by | |
| Date | |
| Standby (quiescent) load | |
| Alarm load | |
| Standby period (T1) | |
| Time in alarm (T2) |
Calculation to BS 5839-1
Cmin = 1.25 × (T1 × I1 + 1.75 × T2 × I2)
For a 24 V panel with two 12 V batteries in series, fit two batteries of this size. Check the panel’s maximum battery size and charger rating.
Important
This calculation is an estimate for guidance only, based solely on the figures entered, which Nova Security Systems S.E. Ltd has not checked. It is not a design, a design certificate or professional advice, and must not be relied on as one. The system designer must confirm the battery capacity using the manufacturers’ data and the current edition of BS 5839-1 or BS 5839-6, and check the panel’s maximum battery size and charger rating. To the fullest extent permitted by law, Nova Security Systems S.E. Ltd accepts no responsibility or liability for any loss, damage, cost or claim arising from the use of, or reliance on, this calculation.
Full terms: novasec.co.uk/terms/
How it’s calculated
BS 5839-1 gives the minimum battery capacity as:
Cmin = 1.25 × (T1 × I1 + 1.75 × T2 × I2)
- T1: the standby period in hours: 24, 6 or up to 72, depending on the category (see below)
- I1: the standby (quiescent) load in amps
- T2: the time in alarm in hours: 0.5 (30 minutes) for BS 5839-1
- I2: the alarm load in amps
- 1.75: a de-rating factor, because batteries give less capacity when discharged quickly in alarm
- 1.25: an allowance for the battery losing capacity as it ages
Worked example
A standby load of 222 mA and an alarm load of 222 mA, with 24 hours’ standby and 30 minutes in alarm:
1.25 × (24 × 0.222 + 1.75 × 0.5 × 0.222) = 1.25 × (5.328 + 0.194) = 6.90 Ah, so you need 7 Ah batteries or larger.
How long the batteries must last
There isn’t one answer. The standby period depends on the category of system and on how quickly someone would notice a mains failure, so don’t automatically use 24 hours, or 72 hours, for every system.
| System | Standby | Then in alarm | Clause |
|---|---|---|---|
| BS 5839-1 Category M or L, most buildings | 24 h | 30 min | BS 5839-1:2025, 24.3.5 a) |
| BS 5839-1 Category M or L, with an automatically started standby generator serving the fire alarm | 6 h | 30 min | 24.3.5 b) |
| BS 5839-1 Category P, most buildings | Longest unoccupied period + 24 h, up to 72 h | 30 min | 24.3.5 c) |
| BS 5839-1 Category P, continuously staffed or inspected so a power fault is noticed within 6 h, or power faults sent automatically to an ARC that tells a keyholder | 24 h | 30 min | 24.3.5 d) |
| BS 5839-6 Grade A | 72 h | 15 min | BS 5839-6:2019, 15.2 c) |
| BS 5839-6 Grade A, never left unattended, or fault signals sent to an ARC, with the fault fixed within the standby period | 24 h | 15 min | 15.2 c) |
| BS 5839-6 Grade C | 72 h | 4 min | 15.3 e) |
| BS 5839-6 Grade D | 72 h | 4 min | 15.4 e) |
Where 48 hours comes from
Neither standard has a fixed 48-hour period, but the Category P rule can produce one. The standby should cover the longest time the building is likely to be empty, plus 24 hours, up to a maximum of 72 hours:
- Empty for up to 8 hours overnight: 8 + 24 = 32 h
- Empty for 24 hours: 24 + 24 = 48 h
- Empty for 48 hours or more, such as a weekend: 72 h (the maximum)
For Category P, if the building may be empty for longer than the batteries last and fire signals already go to an alarm receiving centre, power supply faults should go to the ARC as well, so a keyholder is told straight away.
96 hours
BS 5839-1 and BS 5839-6 don’t ask for 96 hours, and Category P is capped at 72. The option is there for specifications or insurers that ask for it.
Also worth knowing
- Calculated and recorded. The battery calculation belongs in the system’s documentation, so anyone servicing it can check the batteries are big enough. It’s part of the operation and maintenance information we hand over.
- A dedicated, labelled mains supply. The panel should be fed from its own circuit, with the isolating device labelled “FIRE ALARM. DO NOT SWITCH OFF”.
- Checked at every service. Battery condition and the charger are checked at each periodic inspection and service, normally every six months.
- BS 5839-6 systems (homes and HMOs) use the calculation in Annex G of that standard. This calculator uses the BS 5839-1 method, so treat it as a guide for those.
See BS 5839-1:2025: what changed and fire alarm categories explained.
Getting the loads right
Standby load
Add the panel’s quiescent current to the standby current of every detector, call point, sounder, interface and module on every loop or zone, from the manufacturers’ data sheets.
Alarm load
Add the panel’s alarm current to the alarm current of every sounder, beacon and voice sounder, plus anything else that draws more in alarm. Include every device that operates in a full evacuation.
Check the panel
Compare the answer with the panel’s maximum battery size and charger rating. If it’s too big, reduce the load or add a separate EN 54-4 power supply.
This calculator follows the BS 5839-1 method, but it doesn’t replace the system designer’s calculation using the actual manufacturers’ data. Panels that use a different method in their own software should be calculated their way as well.
Disclaimer. This calculation is an estimate for guidance only, based solely on the figures entered, which Nova Security Systems S.E. Ltd has not checked. It is not a design, a design certificate or professional advice, and must not be relied on as one. The system designer must confirm the battery capacity using the manufacturers’ data and the current edition of BS 5839-1 or BS 5839-6, and check the panel’s maximum battery size and charger rating. To the fullest extent permitted by law, Nova Security Systems S.E. Ltd accepts no responsibility or liability for any loss, damage, cost or claim arising from the use of, or reliance on, this calculation. See our website terms and disclaimer.
Questions
Is the standby period always 24 hours?
No. Most BS 5839-1 Category M and L systems need 24 hours, then 30 minutes in alarm, but it’s 6 hours where an automatically started standby generator serves the fire alarm, and up to 72 hours for Category P (property protection) systems in buildings left empty. BS 5839-6 Grades A, C and D in homes need 72 hours unless the conditions for 24 hours are met. Check the category or grade first.
Why multiply by 1.25?
A battery loses capacity as it ages. The 1.25 factor in BS 5839-1 means the battery can still carry the full standby and alarm load towards the end of its life, not just when it’s new.
Why is the alarm load multiplied by 1.75?
Batteries deliver less of their rated capacity when they’re discharged quickly. The alarm load is a heavy, fast discharge, so BS 5839-1 applies a de-rating factor of 1.75 to it.
My panel takes two batteries. Do I double the answer?
No. Most panels run at 24 V from two 12 V batteries in series, so the current flows through both. Fit two batteries, each of the capacity the calculator gives.
What if the result is bigger than my panel can take?
Check the panel’s maximum battery size and charger rating in the manufacturer’s data. If it can’t take the batteries you need, the usual answers are reducing the load, a larger panel, or a separate EN 54-4 power supply with its own batteries. We can help.
How often should fire alarm batteries be replaced?
Sealed lead-acid batteries in fire panels typically last around four to five years, but follow the manufacturer’s figure. They’re checked at every service visit and replaced sooner if a test shows they’ve lost capacity.
Need a system designed or batteries checked?
We design to BS 5839-1:2025 and check batteries at every service. Call 01303 720282 or send us a message.