A ride-through source is easy to over-specify into cost and bulk, and just as easy to under-specify into a failed shutdown. Paper B provides the disciplined middle path: convert the protected load and required bridge time into an energy budget, derate it for inverter efficiency, depth of discharge, temperature and end-of-life fade, choose the chemistry and topology on the axes that actually govern a short high-rate bridge, and design the pack and charger so the same clean transfer is delivered on the hundredth outage as on the first.
The bridge energy budget starts from watts, not ampere-hours. List every protected load and its real power - the PLC or controller, I/O and communication modules, contactor coils, sensors, HMI or network gear - and include the momentary inrush of any device that must start during the bridge. Multiply the summed power by the required bridge time in hours to obtain watt-hours, divide by the DC bus voltage for ampere-hours, and then apply the efficiency of the path: an inverter stage may be around 0.85 efficient, a direct DC-UPS path rather better.
The animated waterfall works a representative 24 V instrument UPS carrying 60 W for ten minutes: 60 W times one-sixth of an hour is 10 Wh, or about 0.42 Ah at 24 V; dividing by 0.85 inverter efficiency, limiting depth of discharge, reserving an end-of-life margin and adding contingency brings the nameplate requirement to roughly 0.8 Ah. The multiplier between raw energy and installed capacity is the same discipline as in every standby duty, and just as often omitted.
Three deratings compound in a UPS and must not be conflated. Rate: a cell's nameplate capacity is measured at a modest reference current, while a bridge may discharge at C/2 or faster, where usable capacity is lower and the voltage knee arrives earlier. Temperature: cold raises internal impedance and reduces available capacity, heat accelerates fade; a cabinet-rated design uses the worst-case seasonal ambient. Age: capacity fades with cycles and calendar life, so the bridge must still complete at the declared end of life. Folding all three into the Paper-B waterfall - rather than applying a single optimistic 'safety factor' - is what makes the rated bridge time a guarantee rather than a day-one result.
Because the bridge is short, designers often add a defined contingency for a generator re-start or a second controlled-shutdown attempt; this is cheap in a small pack and prevents the worst failure mode, in which the UPS dies precisely while writing its protected data.
The animated scorecard adds the supercapacitor to the usual three chemistries and rates all on sub-cycle transfer, high-rate pulse, hot-cabinet life, cycling after repeated outages and cost value. Lead-acid is inexpensive but rate- and temperature-limited and degrades under shallow cycling. Lithium is strong on rate and weight but requires protection electronics and carries transport overhead. Supercapacitors are unmatched on cycle life and instantaneous transfer but hold only seconds of energy and leak down over days.
NiMH occupies the practical centre for minute-scale bridges: high-rate and low-impedance enough for a clean transfer and a stiff rail, tolerant of repeated shallow cycles, far better than lead-acid in the cold, intrinsically safe in its aqueous chemistry and free of lithium's shipping constraints. The clean engineering message is to match the store to the bridge length - supercapacitors for sub-second hold-up, NiMH for seconds-to-tens-of-minutes ride-through, larger chemistries for hour-scale autonomy.
IEC 62040-3's VFD, VI and VFI classes make different demands on the store. A VFD passive-standby unit needs a battery that accepts a sudden step load after a few-millisecond transfer; a VI line-interactive unit adds an AVR that handles sags without touching the battery, reserving it for genuine outages; a VFI double-conversion unit floats the battery on the DC bus continuously, where float-charge discipline is paramount. For DC-native instruments, a DC UPS under IEC 62040-5 backs the 24 V rail directly, avoiding inverter losses and suiting high-rate NiMH strings especially well.
Selecting the topology first clarifies whether the pack sees occasional step loads (VFD/VI) or a permanent float (VFI/DC-bus), which in turn sets the maintenance-charge regime and the expected thermal environment - the inputs the pack sizing needs.
For a bridge source the cell format should favour power: sub-C and other high-rate electrode designs deliver sustained current with lower sag than energy-optimised AA cells of the same nominal capacity. Series count sets the bus voltage - twenty cells for a nominal 24 V rail, ten for 12 V - and matched-lot selection keeps the knee uniform so no weak cell reverses under the high-rate discharge. Welded nickel tabs, a rigid carrier and integrated PTC or thermal fuse suit the vibration and fault-current environment of an industrial cabinet, while a thermistor lets the charger adapt to cabinet temperature.
The pack should also be a defined, replaceable unit labelled with the protected-load range and bridge time it supports, so a maintenance engineer can verify the backup as part of the instrument's periodic proof-test rather than discovering its condition during a real outage.
A ride-through battery is charged after every event and held ready between them, so its charger governs its realised life. A controlled constant-current charge returning the pack to full after an outage, followed by a gentle maintenance regime at or below C/20 - or a temperature-compensated float in a VFI/DC-bus design - avoids the sustained overcharge that dries an aqueous cell; thermistor cut-back protects a sealed summer cabinet. Recharge time should meet the application's need to be ready for the next outage, which in outage-prone grids can mean recovering most of the bridge capacity within a few hours.
Pairing that charger discipline with a high-rate, matched NiMH pack and the Paper-B derating produces a small UPS whose transfer is clean, whose bridge completes every time, and whose evidence trail - assembled in Paper C - is straightforward to document.
Weijiang Power builds high-rate NiMH bridge packs for small-UPS and DC-UPS OEMs: sub-C and C strings sized to your protected load and bridge time with full rate, temperature and end-of-life derating, low-impedance matched cells for clean VFD/VI/VFI transfer, thermistor-ready charging for sealed cabinets, and IEC 61951-2, IEC 62133-1 and UN 38.3 documentation. Send your load list, bridge intent (shutdown or generator) and bus voltage for a sized calculation.
