Sizing a buried event-driven sensor means predicting how often the world will change under it, then guaranteeing the radio burst that reports the change at -45 C, years after installation. This paper works through the method: the chemistry comparison, the detection-and-event energy budget, the cold-pulse sizing, and the decision between a sealed primary battery and a rechargeable nickel-metal hydride module for high-turnover or harvested sites.
The animated scorecard rates primary Li-SOCl2/Li-MnO2, a NiMH pulse reservoir or rechargeable module, a supercapacitor and rechargeable Li-ion on standby energy, event-radio pulse current, discharge at -45 C, cycle tolerance of event bursts, sealed-housing safety and cost. Primary lithium wins long standby for low-turnover bays but passivates and sags in extreme cold; the supercapacitor pulses but holds negligible standby energy; Li-ion stores energy but needs a BMS and dislikes both deep cold and a pavement-baked enclosure; NiMH pulses strongly at low temperature, cycles through repeated events and is aqueous and safe.
The robust choices are a primary-lithium-plus-small-NiMH-reservoir design for sealed low-turnover sensors, and a rechargeable NiMH module (with harvesting or scheduled service) for high-turnover or accessible installations.
Build the budget from four contributions. Detection energy is the magnetometer (or radar) sample current times sample duration times samples per day, set by the duty cycle needed not to miss a fast park. Event energy is the radio transmit-and-receive current times duration times the expected events per day, including retries. Heartbeat energy is the periodic status uplink. Service energy is the BLE commissioning allowance. Sum to an equivalent continuous current and add self-discharge and an end-of-life margin.
The second animated figure is the energy waterfall: the detection floor, the event-radio contribution that grows with turnover, the heartbeat, BLE, self-discharge (accelerated by pavement heat) and the end-of-life/cold derating, ending at required capacity. Showing detection separately from events is what lets a designer predict life at both a quiet resident bay and a busy deck.
The worst case is an event uplink at -45 C at end of life: the LoRa/NB-IoT PA draws its transmit current while a passivated, aged primary cell's voltage sags. Size the pulse reservoir so the rail stays above the radio minimum through the uplink and its receive window and retries, at the cold extreme. NiMH's low internal resistance and flat plateau deliver that pulse where a primary cell alone may collapse, and unlike a supercapacitor the NiMH reservoir also carries enough energy for a multi-frame event with acknowledgement.
Welded tabs and matched cells are essential in a sensor that survives 15-tonne loading and years of vibration; contact resistance from a spring would grow over the mechanical abuse and undermine the cold pulse.
Choose a rechargeable NiMH module when the event count pushes primary-battery life below the service interval, when an above-ground or surface-mounted sensor can carry a small harvesting element, or when the housing is accessible for periodic recharge or pack swap. Size the harvesting source from realistic available energy (not idealised sun), and size the NiMH for a moderate routine depth of discharge and adequate reserve, with a charge manager that locks out charging in deep cold and tapers above 45 C.
Low-self-discharge NiMH tolerates the irregular, partial charge that harvesting produces and holds its standby reserve through quiet periods - a better match than a lithium chemistry that demands precise management inside a low-cost, sealed, mechanically abused sensor.
The pack is part of the load-bearing, thermal-cycled assembly: choose cells rated for the -45 to +85 C envelope (derating cold capacity and limiting hot charging), weld and pot against vibration and 15-tonne shock, provide a fuse and NTC, and direct any cell venting away from the antenna and electronics. For a flooded roadbed or chamber, conformal-coat the pack and avoid crevices that trap condensation. In a smart manhole, optimise for decade standby with a tiny pulse reservoir rather than a large rechargeable store.
These mechanical details decide field survival as much as the electrical calculation - a battery that meets its Ah target on paper but opens a weld under a truck wheel is a failed sensor.
Record the detection duty, event-rate assumption (and its high-turnover sensitivity), radio profile, temperature and mechanical rating, and the recharge/harvest decision. A sealed, buried, low-turnover sensor remains a primary-lithium design with a small pulse reservoir; a busy or harvested or accessible sensor is the NiMH module's territory. Paper C validates the design against IP68/IK10, wide temperature and the cell evidence.
Weijiang Power supplies sealed nickel-metal hydride cells and rechargeable buffer modules for wireless parking-detection and smart-manhole sensors. Tell us your detection sampling duty, events per day, radio technology, mechanical and temperature rating and whether energy harvesting is used, and our engineers will design a welded, cold-capable NiMH module or replaceable pack with charge management and protection. See formats on the products page.
