Paper A set out the EN 50171 central architecture and its long, flat discharge duty. Paper B turns a building's escape-light schedule into a battery-bank specification: it works the load and duration arithmetic, compares nickel-metal hydride (NiMH) module strings with the traditional valve-regulated lead-acid (VRLA) racks and with LiFePO4 and NiCd alternatives, and addresses the series-string management that a central bank requires.
Sizing begins with a tabulation of every final circuit: its luminaires, maintained and non-maintained watts, and length (which sets cable losses). Summing connected emergency watts, adding the simultaneous-start inrush factor when hundreds of LED drivers transfer together, multiplying by the duration tier the building classification demands, and dividing by bus voltage and inverter efficiency yields the raw ampere-hour figure. The derating chain then applies with particular force at system scale: depth-of-discharge ceiling (VRLA banks kept shallow to protect float life, NiMH able to cycle more deeply), end-of-life capacity at the declared service horizon, equipment-room temperature correction, and the voltage-window requirement that the bus stay inside driver input limits at end of discharge with one string out for maintenance. The animated walk-through applies the chain for a representative 800 W, 1.5 h, 48 V bank; values are illustrative and a real project uses its own schedule.
Central systems have historically used VRLA monoblocs: cheap per kilowatt-hour, familiar to installers, and available in large capacities. Their penalties are structural. They need a ventilated, acid-resistant battery room or cabinet because charging emits hydrogen and oxygen; they are heavy and must be installed upright; float life is temperature-sensitive, with roughly halved life per sustained 10 °C rise; deep discharges damage them, so banks are oversized for shallow cycling; and periodic impedance testing and block replacement is a recurring maintenance burden across a decade. Chinese emergency-system rules (GB 17945 family) also cap lead-acid and NiCd capacities in certain system classes and restrict lithium in B-type central systems, narrowing the chemistry menu and creating a genuine opening for nickel-metal hydride.
NiMH module strings bring four structural advantages to a central bank. First, no acid and negligible gassing: the aqueous alkaline chemistry needs no hydrogen ventilation and no acid-resistant room, which simplifies the equipment space and the fire case. Second, mounting freedom: sealed modules can be installed in any orientation and stacked in standard cabinets without upright constraints. Third, cycle and depth-of-discharge tolerance: the monthly and annual EN 62034 test discharges age NiMH far less than VRLA, so less oversizing is needed. Fourth, intrinsic safety with no thermal-runaway path and cadmium-free materials. Against that, NiMH carries a higher first cost per kilowatt-hour and requires disciplined series-string management; LiFePO4 offers excellent cycle life but needs full battery management and faces the central-system lithium restrictions in some regulated markets; NiCd is rugged but cadmium-restricted and memory-prone. The animated scorecard rates the four chemistries on five bank-scale criteria as a qualitative engineering comparison.
A 48 V-class bank needs roughly 40 series NiMH cells and higher-voltage systems many more, so string management replaces the per-cell simplicity of a self-contained fitting. Industrial practice uses welded modules of 10-20 cells with matched capacity and resistance bins, module-level voltage taps, a string current sensor and thermistors distributed through the cabinet; a charge controller runs a temperature-compensated float or scheduled top-up regime, because - exactly as at cell scale - sustained overcharge heat is the dominant aging driver. Parallel strings provide N+1 redundancy so the bank survives a maintenance outage while still meeting rated duration, and each string gets its own fuse or breaker so a fault cannot cascade. Cabinet thermal design keeps the hottest module within the declared ambient, which in turn protects the four-to-ten-year service projection.
The final architecture decision is granularity: one large bank, or split banks on separate chargers serving separate fire compartments. EN 50171 and EN 50172 favour separation so a single fire or fault cannot extinguish the whole escape system, and modular NiMH cabinets make that split straightforward without duplicated acid rooms. Whole-life costing - first cost, room and ventilation works, test labour, block replacement and end-of-life treatment - frequently closes much of the NiMH premium over VRLA, particularly in retrofit projects where no compliant battery room exists. Paper C sets out the EN 50171, EN 50172, EN 62034 and GB 17945 validation programme that proves the bank at system level.
Weijiang Power builds industrial NiMH modules and welded strings for central emergency power supply systems, with matched cells, module taps and thermistors for string management, N+1 parallel-string layouts and IEC 61951-2, IEC 62133-1 and UN 38.3 documentation. Send your final-circuit schedule, duration tier, bus voltage class, redundancy target and equipment-room conditions and we will size a gassing-free, orientation-free bank that holds the emergency bus for its full rated duration at declared end of life.
