Paper A established that a ward UPS is a short-bridge device: its job is to keep the nurse-call master, charting workstations and local network hardware alive for minutes, not hours. Paper B turns that duty into numbers. It works the ampere-hour budget from the protected load, compares nickel-metal hydride (NiMH) with valve-regulated lead-acid (VRLA) and lithium on the dimensions that actually dominate float service, and shows where a welded NiMH string is the rational choice for an OEM building a compact ward-level backup module.
A defensible battery size is never the load energy divided by nominal voltage alone. Four deratings sit between the textbook calculation and a pack that still meets its bridge time on a warm day four years after installation. First, conversion efficiency: a line-interactive or double-conversion stage draws 10-20 percent more DC energy than the AC load receives. Second, depth of discharge: a pack that is routinely emptied ages fast, so the design holds a depth-of-discharge ceiling (around 0.8 for NiMH, tighter for VRLA if long float life is wanted). Third, end-of-life capacity: every chemistry loses capacity with age and float stress, and a safety-rated bridge must still be delivered at the declared end of service life, conventionally with 0.8 of nameplate capacity remaining. Fourth, temperature and high-rate correction: cold reduces available capacity and a short high-current discharge delivers less than the slow-rate ampere-hour figure suggests. The animated walk-through below applies each derating in turn for a representative 120 W, 15-minute ward bridge at 24 V; values are illustrative and should be replaced with a measured load list.
The derating chain typically inflates the nominal pack by 70-90 percent over the raw energy calculation. That is not conservatism for its own sake; it is the difference between a unit that passes its factory discharge test when new and one that still passes its annual test in year four after thousands of days at float on a warm corridor. The same arithmetic explains why capacity is only one of three sizing outputs the designer must record: the inverter's sustained and peak wattage (set by the laser-printer or PoE burst), the worst-case DC-link voltage sag during that burst (which sets cell count and internal resistance), and the thermal envelope around the pack (which sets charge-voltage compensation and expected float life). A pack that meets ampere-hours but sags too far under a printer inrush will reboot the very switch it is meant to protect.
Three chemistries compete for a small ward UPS, and the ranking changes with the duty. VRLA is cheap, quiet and universally understood, with mature recycling chains; against that, it is heavy, its float life is acutely temperature-sensitive, it must be kept upright (or use absorbed-glass-mat construction), and deep or frequent discharges shorten it sharply. Lithium-ion packs are light and long-cycling, but they require a battery management system, protection devices and a qualification burden whose fixed cost is hard to amortise across a 15-minute bridge. NiMH brings intrinsic aqueous chemistry with no thermal-runaway path, high pulse-current capability per gram, no memory effect under modern charge management, tolerance of being held ready indefinitely when float voltage is temperature-compensated, and a cadmium-free materials profile that simplifies RoHS documentation. The animated scorecard below rates the three chemistries on the five criteria that govern this specific application; it is a qualitative engineering rating, not a measured benchmark.
NiMH in a readiness application is not charged the way a consumer battery is. Continuous overcharge is the dominant aging driver for nickel systems, because the recombination reaction that makes the cell sealed releases heat, and heat accelerates the very degradation it comes from. A ward-UPS charge design therefore uses one of two disciplined regimes: a temperature-compensated trickle held well below the current that heats the pack, with charge voltage reduced as ambient rises, or a scheduled top-up scheme that charges to full and then rests, re-topping on a timer. A thermistor bonded to the cell string lets the controller cut back in a ceiling void that reaches 40-45 °C on summer afternoons. The -dV/dt full-charge termination used for rapid cycling is replaced in float service by temperature-rise and timer safeguards. When these rules are followed, NiMH holds a stable readiness state for years without the sulfation failure mode that quietly kills VRLA blocks left at partial state of charge.
A 24 V-class bridge is typically built from 20 series NiMH cells (1.2 V nominal each), with parallel strings where the ampere-hour target requires them; cells within a string are welded, matched for capacity and internal resistance at production, and fitted with a thermal cut-out and, on larger packs, cell-level fusing. Redundancy at ward level is usually achieved by splitting critical and convenience loads on the same inverter rather than duplicating the pack, so the nurse-call and alarm circuits are the last to shed. Modular packaging - a slide-out cartridge with keyed connector - lets facilities exchange an aged pack in under a minute during a maintenance round, which matters more to a hospital than a marginal per-cell cost saving. Paper C covers the validation regime that proves the bridge, the float regime and the exchange design before the module is declared compliant.
Weijiang Power supplies matched NiMH cells and welded 20-40 cell strings for ward UPS and embedded backup modules, with resistance binning for printer-inrush pulses, NTC thermistor integration for temperature-compensated float, and IEC 61951-2, IEC 62133-1 and UN 38.3 dossiers. Share your protected load list, bridge-time target, inverter DC voltage window and mounting ambient and we will return a sized pack, a recommended float regime and a year-four capacity projection so the bridge is guaranteed at end of life, not only on day one.
