Gas-meter battery design is a constrained optimisation: maximise a decade of background energy, guarantee a safety-critical valve pulse at end of life and in the cold, and stay inside the intrinsic-safety energy ceiling the whole time. This paper works through the method - the chemistry choice, the average-current budget, the separate valve-pulse calculation, the hybrid power path and the ATEX protection design - showing where sealed nickel-metal hydride is the deliberate pulse partner to a primary lithium energy cell.
The animated scorecard rates Li-SOCl2, Li-MnO2, a NiMH pulse reservoir, a supercapacitor and rechargeable Li-ion on background energy, valve-pulse current, cold-voltage stability, intrinsic-safety simplicity, cycle tolerance of daily events and cost. The primary lithiums win energy; the supercapacitor pulses but holds little energy and is bulky for a valve turn; rechargeable lithium pulses and stores but brings BMS and thermal questions into an ATEX enclosure; the NiMH reservoir combines strong pulse current, useful energy, simple trickle charging and an aqueous, thermally benign profile.
The robust architecture is therefore a primary lithium energy cell plus a current-limited NiMH (or, for the very smallest peaks, supercapacitor) pulse reservoir, with the division of labour explicit and documented for the ATEX file.
As with water meters, long-term life is an average-current sum: sleep current across the full period, plus measurement wakes, plus radio attach-and-transmit energy per report times reports per day, all converted to an equivalent microampere current and grown by self-discharge and an end-of-life margin. The first part of the energy waterfall shows that sleep still dominates the decade budget but the radio grows quickly with upload frequency.
Separating this budget from the pulse design is essential: the primary cell is sized for energy, and trying to size it for the valve peak as well leads to an oversized, intrinsically harder-to-protect energy store.
The safety shut-off sets the reservoir requirement. Multiply the motor running current (and its brief stall current) by the full travel time, add the energy to re-open, and require that the reservoir can deliver this at the minimum operating voltage at -25 C and at end of life - because the valve must close precisely when an aged battery meets a cold winter night. Add a margin for a retry if the valve meets resistance.
The second animated figure combines the background budget and the valve energy into two outputs: the primary-cell capacity for the decade, and the reservoir energy for the guaranteed pulse. A NiMH reservoir covers the valve comfortably in a small volume because its flat plateau and low resistance hold motor voltage through the travel.
The hybrid path current-limits the primary cell's charge of the reservoir, feeds the radio and valve from the low-impedance reservoir, and places defined protection - series fuse or PTC, limiting resistor, redundant current control - so that under any single fault the energy released into the explosive atmosphere stays below the permitted ignition curve for the meter's gas group and temperature class. Every protection component and its failure mode belongs in the ATEX documentation.
NiMH's predictable internal resistance and lack of a flammable organic electrolyte simplify the fault analysis compared with a rechargeable lithium reservoir, and the pack can be welded and potted to resist the meter's sealed, condensing, wide-temperature life.
Battery capability at -25 C and after a decade is the real specification. Primary lithium voltage sags and passivates in the cold; a NiMH reservoir, while also reduced in the cold, maintains a lower-impedance pulse path when kept charged, and it does not passivate. Design the reservoir so the worst-case valve pulse is delivered with margin at the lowest temperature and the most aged state, and have the firmware verify reservoir readiness between valve operations, raising a service warning if the guaranteed close can no longer be made.
This 'proof of closure' logic - charge the reservoir, test its state, guarantee the action - is the safety argument a gas network operator is entitled to see.
Record the reporting and valve assumptions, the current budget worksheet, the valve-pulse calculation with cold and end-of-life margins, the intrinsic-safety protection and fault analysis, and the chemistry division of roles. Where a meter is fully sealed with no service access for a decade, keep primary lithium as the energy store and NiMH as the protected pulse layer; in a serviceable or replaceable index module, NiMH can carry a larger share and be recharged during maintenance.
Paper C validates the design against EN 1359 and OIML R137 metrology, MID, the ATEX standards and the IEC/UN cell evidence.
Weijiang Power manufactures sealed nickel-metal hydride cells and pulse-assist modules for smart gas meters and safety shut-off actuators. Send us your radio profile, valve motor current and travel time, ATEX category and temperature class, and our engineers will design an intrinsically safe, welded NiMH pulse reservoir or service module matched to the primary lithium cell. See formats on the products page.
