A backup module is sized from the work it must finish, not from a generic 'UPS rating'. This paper gives the selection method: separate the milliseconds the power supply already handles from the seconds the application genuinely needs, choose the storage technology on power-versus-energy grounds, compute the ride-through energy task by task, and design the 24 V rail topology - blocking, charge management, undervoltage monitor and protection - so a nickel-metal hydride module connects cleanly and never back-feeds or drags the rail.
The animated scorecard places four technologies on the criteria that matter for control-rail backup: instantaneous pulse power, usable energy for a seconds-to-minutes shutdown, float readiness in a cabinet, cycle and calendar life, abuse safety and cost. The internal electrolytic capacitor has high power but negligible energy; a supercapacitor bank extends this to a few seconds of high power but is bulky and leaky for minute-scale energy; lithium-ion stores energy but adds a BMS and thermal burden in a sealed cabinet.
Sealed NiMH occupies the deliberate middle: enough pulse current for a fieldbus transmission, enough stored energy for tens of seconds of controlled shutdown, permanent float readiness and an intrinsically safe aqueous chemistry - the combination a PLC or DAQ rack actually needs.
Graceful shutdown is a sequence of short tasks, each with a current and a duration: finish the current scan and freeze outputs; write retentive registers, counters and the data log to non-volatile memory; close communication sockets and send the last-gasp frame including retries; drive any actuator to a safe position; hold the rail until the power supply's 'power-good' signal confirms loss. Summing current-times-time for every task, divided by converter efficiency, gives the net charge the module must deliver.
The second animated figure is the task-energy waterfall: CPU and I/O standing current over the shutdown window, the non-volatile-write pulse, the fieldbus transmit pulse with retries, a safe-state actuator pulse, converter losses, an end-of-life and low-temperature derating, and finally the design capacity. Showing each task separately prevents the classic error of sizing only for the transmit pulse and then running out before the file system finishes flushing.
Twenty series NiMH cells present a nominal 24 V that matches the control rail; the exact count and a small buck-boost converter are chosen so the rail stays inside the controller's 19.2-28.8 V working window from a fully charged pack down to the end voltage. A blocking diode or ideal-diode path prevents the module back-feeding the main supply, and a rail supervisor connects the backup only when the primary rail sags below a defined threshold, avoiding two sources fighting.
Charging comes from the healthy 24 V rail through a current-limited manager that tops the pack gently (a maintenance current at or below C/20) and cuts back above 45 C. Because outages are rare, the pack spends almost all its life fully ready, which suits low-self-discharge NiMH.
The last-gasp radio pulse and any safe-state actuator set the peak current, which can be many times the average shutdown current. Size the cells and tabs for that peak so the pack voltage does not sag below the DC-DC undervoltage lockout mid-transmission; undersized cells are the usual cause of a last-gasp frame that starts but never completes.
Low internal resistance and a flat 1.2 V plateau make NiMH forgiving here: a modest pack holds a stable input to the converter across the pulse and across the whole discharge, whereas a capacitor stack droops linearly and forces an oversized converter input range.
Apply the same derating discipline used across this series: an end-of-life capacity factor so the module still completes shutdown years after installation, and a low-temperature factor for unheated cabinets, where every chemistry loses some effective capacity. NiMH discharges reliably below freezing (unlike lithium, whose charge is restricted in the cold), but the cold still reduces available capacity and should be budgeted.
Add a series fuse, cell-level consistency matching, an NTC for charge compensation and a layout that keeps the pack away from hot drive components. The result is a module that is boringly reliable - precisely the requirement for a device whose only job is to be ready when everything else has just failed.
For a representative CPU plus I/O plus cellular gateway needing roughly twenty seconds to log, park and send three last-gasp retries, the waterfall lands on a compact NiMH module of a few hundred to a couple of thousand milliampere-hours rather than a capacitor bank or a full UPS. If the requirement instead stretches to many minutes of continued operation, the problem becomes a small UPS - covered in the companion small-UPS paper - and the topology changes accordingly.
Paper C closes the loop with the IEC 61131-2 and IEC 61000-4-11 dip and interruption tests, IEC 60204-1 machine-supply considerations and the IEC 62133-1 / UN 38.3 cell evidence behind the module.
Weijiang Power manufactures sealed nickel-metal hydride cells and compact 24 V backup modules that extend PLC and data-acquisition ride-through from milliseconds to an orderly shutdown. Send us your controller rail current, the hold-up or last-gasp time you need, the fieldbus protocol and cabinet temperature, and our engineers will design a welded NiMH module with charge management, blocking and protection matched to the 24 V rail. See module formats on the products page.
