When the mains fails, life-safety systems must not. Fire alarms, emergency lighting and smoke ventilation are designed on a simple assumption: a power cut is not “unlikely”, it’s inevitable and it may happen at the exact moment you most need these systems to work.
UK fire statistics underline the stakes: Fire and Rescue Services in England attended 40,351 building fires in the year ending September 2025, and overall incidents totalled 642,170.
That is why UK standards don’t treat standby batteries as optional extras. They are part of the safety architecture:
· Fire detection & alarm power supplies are governed by EN 54-4 (product requirements), with standby duration and battery sizing set by BS 5839-1:2025 (system design and maintenance code).
· Emergency lighting central supplies are governed by BS EN 50171:2021, commonly designed around 1–3 hours autonomy (often 3 hours) with defined recharge expectations.
· Smoke ventilation power supplies (SHEVS/AOV) are governed by BS EN 12101-10:2005, with typical expectations around 72 hours standby plus a defined “operate under load” period.
Just as importantly, UK fire law places a clear duty on the Responsible Person to keep these provisions maintained and in efficient working order.
Why backup batteries are a life-safety component, not an accessory
A backup battery is not there to “keep things running if the lights flicker”. It is there to preserve the fire strategy, escape lighting, timely warning, smoke clearance - when the building is under stress.
In real incidents, power can fail for ordinary reasons (local network faults) or extraordinary ones: fire damage, firefighting water ingress, accidental isolation, or a fault upstream. The UK’s safety framework therefore expects dual supply and clear maintenance responsibilities. Under the Regulatory Reform (Fire Safety) Order, where necessary to safeguard relevant persons, equipment provided for fire safety must be kept under a suitable maintenance system and maintained in efficient working order.
Fire alarms: EN 54-4 power supplies and BS 5839-1:2025 standby duration
What EN 54-4 actually covers (and why it matters to batteries)
EN 54-4 is the European product standard for fire alarm power supply equipment. In practice, it drives the features you want when your system is on battery: regulated output, monitored faults, charger supervision, and battery health monitoring.
A good example of how strict this gets is the requirement to monitor and report battery resistance faults, because internal resistance is directly related to whether the batteries can deliver rated current when needed. A power supply installation guide referencing EN 54-4 compliance explicitly notes this requirement and explains why battery age, temperature and storage conditions matter.
EN 54-4-aligned designs also commonly include deep-discharge protection and frequent battery tests. For instance, one EN 54-4 PSU manual describes battery circuit fault signalling, undercharged battery conditions, and automated battery tests.
What BS 5839-1:2025 adds: standby time + sizing method
Where EN 54-4 sets the product behaviour, BS 5839-1:2025 is the UK’s core code of practice for non-domestic fire detection and fire alarm systems (design, install, commission and maintain).
BS 5839-1 systems are typically designed so that, if the mains fails, the batteries can sustain the system for 24 hours standby followed by 30 minutes in full alarm. Where a building is not continuously manned, guidance commonly increases standby expectation up to 72 hours before the alarm period.
BS 5839-1:2025 also formalises the battery calculation method in a normative annex (Annex E) and clarifies/aligns the way alarm time is included in the calculation framework.
A key practical point, often missed until it bites, comes from training guidance aligned to BS 5839-1 principles: the calculation includes an ageing allowance (commonly expressed as 1.25) and notes the standard’s expectation that lead-acid standby batteries should last at least 4 years (assuming correct selection and conditions).
Manufacturer spotlight: Kentec EN 54-4 power supplies
In EN 54-4 power supplies, the battery charger is not a basic trickle-charger. Kentec provides EN 54-4 certified power supplies that provide a stable (regulated) 24V output even on standby batteries, with microcontroller-managed charging and temperature compensation to maximise battery life.
Emergency lighting: BS EN 50171:2021 requirements and runtime expectations
BS EN 50171:2021 specifies general requirements for central safety power supply systems providing independent energy to essential safety equipment. This matters in larger sites using central battery systems/static inverters rather than self-contained luminaires.
How long must emergency lighting last?
Runtime is strongly linked to evacuation strategy. BS 5266-1 states 3 hours is appropriate where evacuation is not immediate or early re-occupation may be required; 1 hour may be acceptable where evacuation is immediate and the building won’t be reoccupied until batteries are recharged.
Maintenance expectations reflect that life-safety role. The Fire Protection Association advises monthly testing plus an annual full-duration test in line with BS 5266-style routines. BAFE similarly summarises routine checks, including monthly functional tests and at least annual full rated-duration discharge tests, with appropriate logging and interim safety measures if failures are found.
Smoke ventilation and AOV systems: BS EN 12101-10:2005 in plain English
Smoke ventilation systems, whether Natural Smoke and Heat Exhaust Ventilators (NSHEV), stairwell AOVs, or mechanical smoke extract, exist to keep escape routes tenable and support firefighting operations. If they lose power, the building can lose its smoke control strategy.
BS EN 12101-10:2005 is the standard focused on smoke and heat control system power supplies. Here are some of the standard’s practical expectations:
· For non “fail-safe” systems, a secondary power source (for example a battery or motor-generator) is necessary and must be permanently available, tested and maintained.
· Where a battery is used, it must continuously supply the maximum standby current and, at the end of standby, supply a defined short-duration current for 180 seconds.
· To allow for failure of equipment or primary power supply, the battery should be capable of maintaining operation for at least 72 hours, unless there is immediate notification of failure by local/remote supervision.
In other words: smoke ventilation backup power is commonly designed for hours and days, not minutes, because smoke control may need to remain ready after a fault, especially where repair response is not immediate.
If you’re replacing standby batteries as part of a service cycle, upgrading a PSU, or commissioning a new panel, browse Inbuild UK’s Batteries & Power Supplies collection, featuring options from recognised manufacturers including Kentec and Yuasa.
FAQ
How often should standby batteries be replaced?
It depends on the battery type, temperature, charging regime and duty cycle. Industry training aligned to BS 5839-1 notes an expectation that standby batteries should last at least 4 years under suitable conditions. High ambient temperatures can shorten life significantly.
Can I fit any “same size” battery into a fire alarm PSU?
EN 54-4 power supplies may monitor battery resistance and will fault if the battery cannot deliver required current. Battery quality, age, storage history and temperature all affect resistance.
Why do smoke vent systems often specify much longer standby times?
BS EN 12101-10 guidance commonly expects a battery to maintain the system for at least 72 hours, plus a defined period of operation under load, unless immediate fault notification/supervision exists.
What’s the simplest way to stay compliant day-to-day?
Documented routine testing and prompt defect management. For emergency lighting, monthly functional tests and annual full-duration tests with logbook records are frequently highlighted in UK guidance. For fire alarms, ensure servicing is carried out at appropriate intervals and battery capacity remains verified.
