A self-contained emergency luminaire is one of the most numerous safety devices in any building and one of the least observed: a fitting that sits on a ceiling or wall for years, drawing a trickle of charge current, and is called upon exactly once - in the moment the normal lighting fails. Its battery lives a life of extreme contrast between endless readiness and a single high-stakes discharge. Paper A dissects that duty, frames it against IEC 60598-2-22 for emergency luminaires and EN 1838 for escape lighting application, and explains what the profile demands of a nickel-metal hydride (NiMH) cell.
IEC 60598-2-22 (the fifth edition, IEC 60598-2-22:2021, adopted in Europe as EN IEC 60598-2-22) defines a self-contained luminaire as one in which the battery, charger, inverter or LED driver, lamp and control elements are all contained within or adjacent to the fitting. Maintained versions keep the lamp lit from the normal supply and switch source on failure; non-maintained versions light only in emergency; combined fittings do both. In normal life the charger holds the battery at readiness with a small controlled current while monitoring circuitry watches the mains. In emergency life, the instant the supply fails - within the standard's switchover limits, no more than 0.25 s for high-risk-task systems and generally within 0.5-5 s depending on system class - the battery feeds the LED array at the declared emergency flux for the full rated duration. The animated trace below shows years of flat float compressed into a long plateau, followed by the declining discharge curve of the emergency event.
European application standard EN 1838 and national fire codes express the safety requirement as rated duration: the minimum time the fitting must deliver its declared emergency output after mains failure. Common ratings are one hour for general evacuation, two or three hours where sleeping risk, delayed evacuation or medical or high-rise occupancy applies, and the historical 90-minute class remains widespread. Chinese practice under GB 51309-2018 and GB 17945 sets duration by building type - 0.5 h for ordinary buildings, 1.0 h for medical and elderly-care buildings and large public complexes, 1.5 h for buildings over 100 m - while US practice under UL 924 centres on 90 minutes. The standard also requires the emergency output to reach a substantial fraction of declared flux within seconds (typically at least 50 percent within five seconds) and to hold the required illuminance - at least 1 lx on the escape-route centre line under EN 1838 - for the whole duration. The animated chart below converts a fixed pack into available minutes across LED loads; it is an illustrative model showing why the LED wattage and the duration rating are two halves of one decision.
The harshest fact of emergency-light battery life is environmental. Recessed ceiling fittings sit above warm air that rises from occupied floors and beside the heat of the normal-light driver; enclosure ambients of 40-50 °C are routine even where the rated ambient is declared at a lower value. A chemistry held at full charge at that temperature ages on an accelerated clock. IEC 60598-2-22 recognises this with high-temperature endurance requirements, and it is why charge design is not optional: the maintenance current or voltage must be temperature-aware, and the cell chemistry must tolerate years of readiness without drying out. This is also the axis on which NiMH, properly charge-managed, competes strongly with legacy nickel-cadmium while avoiding cadmium, as Paper B develops.
An emergency light that fails unnoticed is worse than none. EN 62034 governs automatic test systems for battery-powered escape lighting, defining short functional tests at frequent intervals and annual full-duration tests, with status indication and logging. Every self-test is a small discharge-recharge cycle: the battery feeds the LED briefly (or for the full rated time annually), then recharges within the standard's recharge-time requirement (typically 24 h after a full event). Over a four-to-eight-year service life that adds hundreds of micro-cycles, and the battery must retain rated duration after all of them. The charger must also distinguish a genuine self-test from a real event and always return to full readiness.
Emergency luminaires have used NiCd for decades because it survives float and abuse; cadmium is, however, a restricted substance under the RoHS directive with only narrow, time-limited exemptions, and NiCd suffers memory effect that erodes apparent capacity after repeated shallow self-tests. VRLA is bulky and temperature-fragile for a compact ceiling fitting, and lithium adds protection electronics and regulatory weight. NiMH offers cadmium-free materials, no memory effect, higher energy density than NiCd in the same envelope, intrinsic aqueous safety and good high-rate delivery to an LED driver - provided the float regime is correctly designed. Paper B compares chemistries and sizes the cells; Paper C walks the IEC 60598-2-22 type-test trail.
Weijiang Power manufactures high-temperature-tolerant NiMH cells and welded packs for self-contained emergency luminaires: matched cells in AA, C and sub-C sizes for 1-3 h durations, charge profiles designed for 40-50 °C ceiling ambients, and IEC 61951-2, IEC 62133-1 and UN 38.3 documentation. Send your LED emergency wattage, rated duration, enclosure ambient and recharge-time requirement and we will size a pack that still meets its rated duration at the declared end of service life.
