Paper A established the emergency luminaire's split life - years of hot float followed by a one-to-three-hour discharge that must not fail. Paper B turns that duty into a cell-selection decision: it compares nickel-metal hydride (NiMH) with nickel-cadmium (NiCd), valve-regulated lead-acid (VRLA) and lithium iron phosphate (LiFePO4), works the ampere-hour budget for the rated duration, and explains the charge design that lets NiMH reach the four-year service mark IEC 60598-2-22 expects.
NiCd defined emergency lighting for decades: it tolerates abuse, high temperature and overcharge, and delivers high current. Its problems are regulatory and electrochemical. Cadmium is a RoHS-restricted heavy metal whose emergency-lighting exemptions have narrowed, forcing manufacturers toward documented cadmium-free designs; NiCd also exhibits the memory effect that shrinks usable capacity after the repeated shallow discharges of monthly self-tests, and its energy density is the lowest of the four. VRLA is cheap at larger capacities but heavy, temperature-sensitive in float and - under recent Chinese emergency-system rules that ban lead-acid in self-contained fittings - increasingly excluded from this product class. LiFePO4 is long-lived and light but requires protection circuitry, brings its own capacity caps in regulated systems and raises bill-of-materials cost for a device that spends almost all its life idle. NiMH removes cadmium, eliminates memory effect, roughly doubles NiCd volumetric energy, keeps the intrinsic safety of an aqueous chemistry and needs no complex management system. The animated scorecard rates all four on the five criteria that decide float service; it is a qualitative engineering comparison.
The energy target is LED emergency power times rated duration divided by driver efficiency and nominal pack voltage, but a compliant pack is larger than that raw number for three reasons specific to emergency lighting. First, end-of-life capacity: IEC 60598-2-22 requires the fitting to deliver its rated duration not only when new but after the declared service life (commonly four years), so the design starts with roughly 20-25 percent headroom against capacity fade. Second, temperature: a cold corridor or stairwell in winter reduces available capacity, while the same cells must survive a hot ceiling in summer - the pack is sized for the cold case and charge-managed for the hot case. Third, the LED driver's minimum input voltage: as the pack discharges its voltage falls, and the pack must still hold the driver above dropout at the end of the rated duration at end of life. The animated walk-through applies the chain for a representative 3 W, three-hour, 3.6 V fitting; values are illustrative and a declaration uses measured driver efficiency and cell curves.
NiMH reaches four-year service only if the charger respects how the chemistry ages. The enemy is sustained overcharge: once a sealed nickel cell is full, continued current is consumed by oxygen recombination, which releases heat, and heat accelerates separator and electrolyte degradation in a self-reinforcing loop. Three disciplined regimes are used in emergency fittings: a temperature-compensated trickle whose current falls as enclosure temperature rises; a pulse or timed top-up that charges fully and then rests, re-topping periodically; and -dV/dt-terminated fast recharge after a real event or full self-test, followed by a low maintenance phase, because EN standards require recharge within 24 h. A thermistor bonded to the cell string lets the controller cut back when the ceiling fitting runs hot. With that design, NiMH float life at a controlled 35-40 °C enclosure reaches the four-year horizon reliably; without it, no chemistry selection can rescue the product.
Emergency LED loads of 1-6 W map neatly onto three to six series NiMH cells (3.6-7.2 V nominal) in AA, C or sub-C formats, with capacity chosen from the duration ladder. Cells are welded into matched strings - capacity and internal-resistance binning matters because the weakest cell sets end-of-duration voltage - and fitted with thermal protection and, on higher-current designs, a resettable or fuse device. A practical advantage over NiCd emerges in the self-test regime: because NiMH does not develop voltage depression from repeated shallow cycles, monthly functional tests do not progressively shrink the apparent capacity that the annual full-duration test measures, removing the periodic conditioning cycles NiCd fleets require. Pack marking carries chemistry, nominal capacity, manufacture date and replacement guidance, which increasingly matters for the battery-removability and labelling expectations of European battery regulation.
Emergency batteries are wear items inside a fitting that lasts a decade or more, so a slide-out, keyed, tool-light battery compartment extends the luminaire's compliant life and aligns with producer-responsibility rules that favour removable, replaceable batteries. Selecting NiMH also simplifies the end-of-life story for European buyers: no cadmium, established nickel-recycling routes, and UN 3496 Class 9 transport rather than lithium controls. Paper C sets out the type-test evidence - rated-duration discharge, switchover, high-temperature endurance, self-test compatibility and cell safety - that converts these design choices into a certified luminaire.
Weijiang Power supplies high-temperature NiMH cells and matched welded strings for self-contained emergency luminaires across the 1-3 h duration ladder, with float-charge recommendations tuned to ceiling-ambient profiles, removable-pack design support and IEC 61951-2, IEC 62133-1 and UN 38.3 documentation. Send your emergency LED wattage, rated duration, worst-case enclosure temperature and target service life and we will size a cadmium-free pack that meets its rated duration at year four, not only at factory test.
