Every fire alarm panel on site has a battery calculation behind it, and a surprising number of them are wrong. Not wildly wrong — wrong in the way that only shows up eighteen months later, when the mains drops out on a Friday evening and the panel is in low-voltage fault by Saturday morning. The arithmetic itself is straightforward. What makes it go wrong is the quality of the numbers going in, and the fact that nobody re-runs it after the system has been extended twice.
This is a working guide to doing the calculation properly, recording it so the next engineer can check it, and spotting the mistakes that are common in the field.
Who this is for
Fire alarm engineers, commissioning engineers and technical managers who size, verify or sign off standby batteries on commercial systems. It assumes you can read a manufacturer's current-consumption table and understand the difference between a quiescent and an alarm condition.
Experience level
Competent engineer. If you are new to standby sizing, work through a calculation alongside someone who does them regularly before you rely on your own figure for a handover.
What the standby supply is actually for
The standby supply exists so that a mains failure does not become a detection failure. It has to keep the system in full working order, monitoring and reporting, for a defined period without mains, and then still have enough left to drive the alarm devices for a defined alarm period at the end of that time.
That last part is the bit people forget. The alarm load is not calculated from a fully charged battery — it is calculated from a battery that has already been slowly discharging for the whole standby period. A battery that comfortably runs the sounders on day one may not do it after a long standby discharge.
How long the standby period should be depends on the installation: how quickly a mains failure will be noticed and attended, whether faults are transmitted to a monitoring centre, and what the designer specified. Those durations, and the alarm period that goes with them, are set out in the current edition of BS 5839-1 and should be taken from it rather than assumed.
The two loads you are sizing for
Every calculation reduces to two figures.
Quiescent load. The steady current the system draws when nothing is happening: the panel itself, each loop card, every detector, call point, module and interface, plus anything else fed from the same supply — repeat panels, door holders on a monitored output, network cards, a communicator.
Alarm load. The current drawn with the system in alarm: sounders, beacons, voice alarm amplifiers if they are on this supply, relays energised by the cause and effect, and the panel itself in its alarm state.
Both figures should come from the manufacturer's published data for the exact equipment installed. Two points worth being deliberate about: loop device currents vary between protocols and even between generations of the same detector, and beacons in particular can draw far more than engineers expect because their published figures are often peak rather than average.
Working through the calculation
Step 1 — fix the periods. Establish the standby duration and the alarm duration the design calls for, from the specification and the current standard. Write down where the figures came from.
Step 2 — total the quiescent current. Add up every device and every card. Count what is actually installed today, not what was on the original drawing.
Step 3 — total the alarm current. Add the alarm-condition draw of everything that operates. If the cause and effect is phased, be clear whether you are sizing for the worst-case simultaneous operation — normally you should be.
Step 4 — combine them. Multiply the quiescent current by the standby period and the alarm current by the alarm period, then add the two together. That gives the raw ampere-hour demand.
Step 5 — apply a capacity margin. Increase the raw figure to allow for the loss of usable capacity as the battery ages, and for reduced performance at low temperature. Panels in unheated risers and external cabinets deserve more thought here than panels in a warm reception area.
Step 6 — select a real battery. Round up to a standard capacity that physically fits the enclosure and matches the terminal and link arrangement. A battery that only fits with the lid forced is a battery someone will replace with the wrong one later.
Check the charger, not just the battery
Power supply equipment to BS EN 54-4 has a maximum battery capacity it is designed to recharge within the required time. Fitting a battery above that limit is a common and quietly damaging mistake: after a mains failure the battery never gets fully back up, so the real standby capability is lower than the paperwork claims and the fault is invisible until the next outage.
If the calculation lands above what the panel's supply can charge, the answer is a supplementary power supply feeding the load locally — not a bigger battery in the same cabinet.
Where the calculation usually goes wrong
- Stale device counts. The system has been extended and the calculation has not been re-run. This is the single most common cause.
- Peak figures used as continuous, or continuous used as peak. Read which one the data sheet is giving you.
- Ancillary loads left out. Door holders, repeat panels, interfaces, network cards and communicators all draw current and all are easy to miss.
- No margin. The raw figure is treated as the answer, so the system meets its standby period only when the battery is new.
- Charger limit ignored. Covered above, and worth checking on every visit where batteries have been changed by someone else.
- Inputs not recorded. A result with no working shown cannot be checked, defended or updated.
What to record
The number on its own is close to useless. Record the working:
- Device counts by type, and the source of each current figure.
- The standby and alarm periods used, and why.
- The quiescent and alarm totals.
- The margin applied.
- The battery capacity selected, the type, and the date fitted.
- The panel's rated maximum chargeable capacity.
Under the Regulatory Reform (Fire Safety) Order 2005 the responsible person has to keep the fire safety measures in efficient working order, and the system documentation is how that is demonstrated. A calculation with its inputs visible is documentation. A bare ampere-hour figure is not.
Relevant standards
BS 5839-1 is the code of practice covering the design, installation, commissioning and maintenance of fire detection and fire alarm systems in non-domestic premises, and it is where the standby and alarm durations for a given system come from. BS EN 54-4 covers the power supply equipment itself, including its charging performance. The Regulatory Reform (Fire Safety) Order 2005 sets the underlying legal duty on the responsible person; the standards are the recognised means of meeting it, not statutes in their own right. Always work from the current editions.
Safety warning
Sealed lead-acid batteries store enough energy to cause a serious burn or start a fire if the terminals are shorted. Remove watches and rings, use insulated tools, disconnect the negative link first and reconnect it last, and never place a battery on top of a live terminal block while you rearrange the cabinet. Batteries showing swelling, leakage or heat should be removed from service and disposed of through the correct waste route rather than refitted.
When not to rely on a calculator alone
A calculator gives you an answer for the numbers you typed in. It cannot tell you that the beacon figure you used was a peak value, that the drawing is two extensions out of date, or that the enclosure will not take the battery you have specified. Treat the tool as arithmetic support and keep the engineering judgement — and the responsibility — with the competent person doing the work.
Related documentation
Read this alongside the guidance on fire alarm power supplies and standby batteries, on certification and handover documentation, and on testing and maintenance requirements. The battery calculation belongs in the site file with the cause and effect and the as-fitted drawings, not loose in an email.
Professional disclaimer
This page is engineering support, not a substitute for competent design, the manufacturer's instructions, the applicable standards or the responsible person's procedures. Specific durations, capacities and thresholds must be taken from the current edition of BS 5839-1, BS EN 54-4 and the equipment manufacturer's documentation for the system in front of you.