A battery that may only ever be used once - but must work perfectly years after installation - is the most demanding kind of energy reserve to design. This second paper converts the fail-safe load profile into a concrete nickel-metal hydride backup for an electric valve actuator. It works through stroke-energy sizing from the valve's break, running and seat torque and the geartrain efficiency, sizes the pack for the required number of strokes and the subsequent sleep period, adds a small capacitor to shoulder the motor's inrush while the NiMH string supplies the energy, specifies a continuous-readiness charge regime that keeps the pack healthy over years of float, and builds the state-of-health diagnostics that a functional-safety case under IEC 61508 requires. It also hardens the pack for the IP68, wide-temperature, vibration-rich reality of a field valve, and keeps the comparison with spring and supercapacitor fail-safe units candid so the designer chooses the right reserve for each valve's safety task.
Sizing begins with the valve's torque signature rather than a generic wattage: breakaway torque to unseat, the running torque across the stroke and the seating torque at the safe end position, each multiplied through the actuator's geartrain and motor efficiency to obtain the electrical energy a single trip demands. Multiplying by the required number of strokes and adding the control electronics' energy for the hold/sleep interval and the communications needed to report the trip sets the total reserve energy.
The pack is then sized with explicit margins for end-of-life capacity fade (reserving full trip capability at the planned replacement horizon), low temperature (thickened grease raises torque while battery impedance rises), and the voltage window the motor driver needs to deliver full torque. NiMH's flat plateau is helpful here: available torque stays consistent through most of the reserve rather than decaying, so the safety case can assume a stable motor supply almost to the end of the stored energy.
The series cell count is set so the pack's end-of-discharge voltage, under motor load and at low temperature, still clears the motor driver's minimum for full torque, while its fully charged voltage does not over-voltage the electronics. High-rate NiMH cells are chosen for low internal resistance because the motor draws a large current during the stroke and a high-impedance pack would sag below the driver's threshold mid-trip - the exact failure mode the reserve exists to prevent.
Cells are matched for capacity and impedance and joined with welded, low-resistance tabs; longer strings include cell-voltage monitoring so a diverging cell is flagged before it compromises trip capability. The pack is mechanically constrained against vibration and oriented for the valve's mounting, with thermal coupling between the cell cluster and the charger's temperature sensor.
A motor's starting inrush is shorter and larger than its running current, and asking the NiMH pack alone to supply that sharp edge forces an oversized, higher-impedance design. A cleaner architecture parallels the battery with a small capacitor bank sized for the inrush: the capacitor delivers the instantaneous edge while the NiMH string supplies the sustained stroke energy, recharging the capacitor between multiple strokes.
This hybrid mirrors the way the broader industry combines storage types - a spring or supercapacitor for an instantaneous action, a battery for energy - and lets each element do what it is best at. The capacitor also stabilises the rail against the motor's commutation noise, protecting the actuator's microcontroller and its safety communications during the trip, which matters when the actuator must report its safe position even as it moves.
The pack spends nearly all its life fully charged and waiting, so charge management determines whether it is actually capable years later. NiMH is held by a controlled readiness charge - a maintenance/trickle regime after a proper -delta-V or peak-detected full charge, with the cluster thermistor reducing current at high temperature - rather than a hard constant-voltage float that would accelerate ageing. The charger tops the pack back up after every self-test and after any real trip, and it can replenish at cool temperatures where lithium cannot be charged.
Because the reserve is rarely discharged, calendar ageing and time at high state of charge dominate over cycle wear; selecting cells with good standby characteristics and avoiding sustained high-temperature float (aided by siting the pack away from the actuator's hottest components) preserves the trip energy across the required service interval, with a planned replacement horizon that the diagnostics make visible.
Under IEC 61508 a safety function's reserve must be diagnosable, because an undetectable loss of trip capability is a dangerous undetected failure. The actuator's controller therefore trends pack voltage under a small diagnostic load, internal resistance and capacity estimate, runs scheduled partial-stroke tests that exercise the motor and battery without moving the valve out of tolerance, and raises a diagnostic alarm - and, in a redundant architecture, a safe-state action - if the reserve can no longer guarantee the trip.
NiMH supports this well: its internal resistance rises and its plateau shortens in measurable ways as it ages, giving early warning rather than sudden death, and it tolerates the frequent small diagnostic discharges and recharges. The diagnostic coverage achieved, the proof-test interval and the pack's failure-rate data feed the safety-integrity calculation, which is why a battery supplier able to provide reliability and ageing data is a genuine contributor to the SIL case rather than a commodity vendor.
Field valves live outdoors or in process areas, so the pack is built to the actuator's IP68 enclosure with conformal protection, corrosion-resistant interconnects and a wide operating-temperature range; vibration and thermal-shock validation reflect pipelines and tank farms. Connectors are keyed and latching so field replacement cannot be mis-wired, and the pack is labelled with its role, date code and replacement horizon.
The choice between spring, supercapacitor and NiMH remains application-specific and should be stated honestly: specify a mechanical spring where a single, fastest possible trip at the highest integrity level dominates and multi-stroke is unnecessary; specify a supercapacitor where one short trip cycles very frequently; specify NiMH backup where the safety procedure needs several controlled strokes, extended control during the outage, richer diagnostics and a compact, cold-tolerant, economical reserve - often the case for modulating process valves and remote unmanned stations. The validation programme then proves trip capability at worst case and supplies the evidence the certification paper sets out.
Weijiang Power designs and manufactures sealed nickel-metal hydride cells and matched industrial packs for remote, off-grid and safety-related equipment, and supports OEM partners with IEC 61951-2 performance files, IEC 62133-2 safety evidence, pulse-load characterisation, wide-temperature testing and charger/pack co-validation. Tell us your duty cycle, peak current, temperature envelope, autonomy target and the standards your product must meet, and our engineers will specify a cell-and-pack combination that protects runtime, reliability and service life. Review the range on the products page.
