A central battery system is a safety power supply that feeds all connected emergency and exit sign luminaires in a building from one shared battery housed in a separate room. It consists of battery, charger, changeover device, monitoring and circuit distribution, and is covered by the product standard EN 50171, which distinguishes between CPS (central battery) and output-limited LPS (group battery). Typical consequences are a structurally protected battery room and fire-resistant cabling to the luminaires, depending on national building law.
What is a central battery system?
A central battery system is a central safety power supply for emergency lighting: instead of each luminaire having its own battery, one shared battery supplies all connected luminaires via a dedicated wiring network. Such installations are also referred to as emergency lighting systems or central power supply systems.
EN 50171 is the product standard for central safety power supply systems and distinguishes two variants:
| Variant | Designation in EN 50171 | Characteristic | Typical use |
|---|---|---|---|
| Central battery system | CPS (central power supply system) | no output limit | entire buildings or large parts with many luminaires |
| Group battery system | LPS (low power supply system) | output-limited | smaller buildings or defined areas |
The group battery is therefore the smaller variant of the same principle. The comparison with luminaires that carry their own battery is in self-contained vs central battery; how such a luminaire works is explained in self-contained emergency luminaire.
What are the components of a central battery system?
| Component | Function |
|---|---|
| Battery | Stores the energy for the full rated duration of all connected luminaires, often lead-acid batteries in a cabinet or rack |
| Charger | Keeps the battery charged in normal operation and recharges it after a discharge |
| Changeover device | Switches automatically from mains to battery supply on mains failure and back when the mains returns |
| Monitoring | Monitors mains, battery, charger, circuits and, in many systems, every single luminaire; reports faults centrally |
| Circuit distribution | Distributes power to the final circuits, often via sub-stations in the fire compartments |
There are also control functions: the system can operate luminaires in maintained or non-maintained mode, and mains monitors in the general lighting sub-distribution boards ensure that emergency lighting also switches on when only one area loses its supply. Many systems can be linked to the building management system.
How does a central battery system work?
In normal operation the luminaires are supplied from the mains and the charger keeps the battery full. If the normal supply fails, the system switches to battery operation and all connected luminaires are fed via the emergency supply network. When the mains returns, the system switches back and the charger recharges the battery.
The luminaires themselves contain no battery. In an emergency they therefore depend on the battery, the central unit and the cables up to the luminaire all working. This is where the particular building requirements come from.
What building requirements apply?
Two factors shape the design and cost of a central battery system:
- Battery room or electrical operating room: the system is usually installed in a dedicated room that must be structurally separated, fire-protected and adequately ventilated. In Germany, for example, the requirements come from the federal states' regulations on electrical operating rooms (EltBauVO). Ventilation is especially important with lead-acid batteries, because gases can form during charging.
- Circuit integrity of the cabling: the cables from the unit to the luminaires must remain functional for a certain time in a fire. Requirements are set nationally. In Germany, the Model Wiring Installations Directive (MLAR), as adopted by the federal states, is the reference; circuit integrity is typically achieved with cable systems of a function-retention class such as E30, depending on building law, cable routing and fire compartments.
Which requirements apply in a specific case depends on the building permit and the fire safety strategy. An overview of the rules is given in emergency lighting standards.
How is a central battery system tested?
The European basis for testing is EN 50172. In Germany, for example, DIN EN 50172 (VDE 0108-100):2024-10 replaces the former pre-standard DIN VDE V 0108-100-1 (transition period until 27 May 2027) and provides for:
| Interval | Test |
|---|---|
| Daily (depending on the defined testing basis) | Reading the operating and fault indicators on the unit |
| Monthly | Function test: switch to battery operation, all luminaires light up, return to normal mode, indicators OK |
| Annually | Duration test for the full rated duration |
| Every 5 years | Measurement of illuminance in emergency mode |
Existing systems may still be subject to older requirements, for example weekly checks under the former German pre-standard, where the building permit, inspection report or fire safety strategy refers to them. Define the testing basis and record it in the logbook. All intervals and who carries them out are listed in emergency lighting testing intervals.
Modern systems run function and duration tests automatically and log the results. Reading, reviewing and repairing defects remain the responsibility of the operator and their electrician. In high-risk or high-occupancy buildings, inspections by approved experts may be added depending on national law.
Practical tip: After the annual duration test the central battery is discharged and can no longer fully supply all luminaires at once in an emergency. Schedule the test outside occupied hours and allow enough time for recharging before the building is used again.
When does the battery need replacing?
The central battery is a wear part too. It loses capacity over the years, faster at elevated room temperatures. Service life depends on battery type, operating conditions and the manufacturer's specification. At the latest, replacement is due when the annual duration test shows that the rated duration can no longer be reliably reached.
Unlike self-contained luminaires, the entire battery of a system is replaced in one go. This can be planned, but it means a larger one-off investment and interim measures during the replacement. How batteries age and why it matters is explained in emergency light battery.
When does a central battery system make sense?
Central systems play to their strengths where many luminaires are to be monitored centrally and the building work is needed anyway:
- Large buildings with many luminaires: maintenance and battery replacement are concentrated in one place.
- High-risk or high-occupancy buildings: in hospitals, places of assembly or large retail premises, the fire safety strategy sometimes specifies central solutions or makes them the obvious choice.
- Unfavourable conditions at the luminaires: at very high or low temperatures the battery sits protected in a temperature-controlled room.
- New builds with a planned battery room: fire-resistant cabling and the room can be designed in from the start.
In small and medium-sized buildings, existing premises and where use changes frequently, self-contained luminaires are often more economical because the battery room and fire-resistant cabling are not needed.
What to do when a central battery system is ageing?
With older systems, several decisions often arise at once: the battery is at the end of its life, the central unit is technically outdated, spare parts are getting scarce or the luminaires still use fluorescent lamps. There are basically two routes:
| Route | What happens | What to watch |
|---|---|---|
| Renew battery or central unit | New battery or new central unit; existing wiring and battery room continue to be used | Condition of the wiring, compatibility with existing luminaires, room requirements |
| Switch to self-contained | Luminaires with their own battery, or emergency drivers in existing LED fittings, replace the central supply | New design to EN 1838, coordination with the fire safety strategy, removal or reuse of the old network |
Only a comparison over the entire service life shows which route is economical. NOTLUX clarifies this during a free on-site survey. Further guidance on upgrades is given in upgrading emergency lighting.
Frequently asked questions
What is a central battery system?
A central battery system is a safety power supply that feeds all connected emergency luminaires in a building from one shared battery. It is housed in a separate room and connected to the luminaires via a dedicated wiring network. The product standard is EN 50171.
What is the difference between CPS and LPS?
EN 50171 distinguishes central power supply systems without an output limit (CPS, central battery) from output-limited systems (LPS, group battery). The group battery is the smaller variant and is used for smaller buildings or defined areas.
Does a central battery system need its own room?
Usually, yes. The system is installed in a structurally separated, ventilated electrical operating room; the detailed requirements are set by national or regional building regulations. The building permit and fire safety strategy are decisive.
How often must a central battery system be tested?
Under EN 50172 monthly for function, annually for the full rated duration and, in Germany under DIN EN 50172:2024, every 5 years with an illuminance measurement. Depending on the defined testing basis, daily reading of the indicators is added; existing systems may still be subject to older requirements.
How long does the battery in a central battery system last?
That depends on battery type, room temperature and the manufacturer's specification. The annual duration test is what counts: if the system can no longer reliably reach its rated duration, the battery must be replaced.
Can a central battery system be replaced with self-contained luminaires?
Yes, this is a common option in upgrades, provided the building permit and fire safety strategy allow it. It requires a new design to EN 1838; existing LED luminaires can sometimes be integrated using emergency drivers.
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