An RMU battery is sized from three competing demands at once: the daily energy of multi-channel measurement and reporting, the ampere-class pulse of a GPS fix and a satellite transmission, and the multi-day reserve needed to survive a cloudy, cold stretch on a remote right-of-way. This paper works through the method - the chemistry comparison, the state-by-state energy budget, the pulse and solar-autonomy sizing, and the wide-temperature protection design that lets a nickel-metal hydride buffer protect the protector.
The animated scorecard rates primary lithium, a solar-charged NiMH buffer, a supercapacitor and rechargeable Li-ion on daily-cycle tolerance, GPS/satellite pulse current, solar charge acceptance, cold discharge, sealed-enclosure safety and cost. Primary lithium suits a low-reporting, non-solar unit but is pulse-weak and passivates in the cold; the supercapacitor handles the pulse but cannot hold days of reserve; Li-ion stores energy but needs a BMS and carries thermal risk in an unattended weatherproof box; NiMH accepts irregular solar charge, pulses hard for satellite, cycles daily for years and is aqueous and safe.
The defensible architectures are a large primary-lithium pack (with a small pulse reservoir) for non-solar, infrequently reporting sites, and a solar panel plus NiMH buffer for any site requiring frequent synchronised surveys, satellite contacts or continuous logging.
Sum charge per cycle across every state: sleep current across the period, multi-channel analog excitation and conversion per measurement times measurements per day, GPS fix current times fix-acquisition time (allowing for weak-signal, longer fixes), interruption-relay drive, and radio attach-plus-transmit time times contacts per day - satellite contacts weighted heavily. Convert to daily energy and add charging losses, self-discharge and an end-of-life margin.
The second animated figure is the energy waterfall: measurement, GPS synchronisation, cellular or satellite radio, relay, conversion losses, the cloudy-day autonomy reserve, and end-of-life/cold derating, ending at the buffer capacity. Showing GPS and satellite separately is important because they, not the analog measurement, usually dominate an RMU's energy.
The worst single event is a GPS-synchronised interruption that coincides with a satellite alarm: GPS receiver active, relay driven, multi-channel instant-off sampling, then an ampere-class transmit. Size cells and welded tabs for that combined peak at the lowest temperature and at end of life, requiring the rail to stay above the satellite modem's minimum - satellite PA shutdown from voltage sag is the classic remote-RMU failure.
NiMH's flat plateau and low resistance hold the modem rail through the burst; a supercapacitor could parallel the pack for the very peak but cannot replace the multi-day energy reserve, which is why NiMH (rather than a capacitor bank) is the primary buffer.
Size the panel from realistic effective sun-hours at the pipeline's latitude and season - winter, not July - so it can refill the daily load plus losses and recover the autonomy reserve after a cloudy spell. Size the NiMH buffer for the chosen autonomy days (commonly three to five) at a moderate routine depth of discharge to protect daily-cycle life. A charge manager tapers or stops charging near 45 C and prevents overcharge, and a low-temperature charge lockout protects the cells in freezing dawn conditions.
Low-self-discharge NiMH tolerates the resulting partial, irregular charge pattern and holds its reserve without the precise charge management a lithium pack demands in a low-cost, sealed remote enclosure.
Pipeline rights-of-way run from deserts to frozen plains, so design for a wide span: derate cold capacity, limit cold charging, and protect against summer heat inside a sealed weatherproof enclosure. Welded cells, a fuse, an NTC, conformal coating and a vent-friendly layout survive condensation and thermal cycling; for the harshest sites, choose wide-temperature-rated cells and keep the pack shaded from direct sun within the enclosure.
For mains-powered rectifier posts the same pack becomes a float-ready DC-UPS held at a gentle maintenance current (at or below C/20), ready to ride out an AC outage and keep the CP parameters logging and alarming throughout.
Record the channel and measurement schedule, GPS synchronisation frequency, radio type and peak current, solar autonomy days and temperature derating. A truly inaccessible, non-solar site with monthly reporting may remain a primary-lithium design; any site needing frequent synchronised surveys, satellite bursts or continuous trends is better served by solar plus a NiMH buffer. Paper C validates the design against the CP standards, environmental tests and cell evidence.
Weijiang Power manufactures sealed nickel-metal hydride cells and solar-compatible buffer packs for cathodic-protection remote monitoring units and pipeline telemetry. Send us your analog channel count, cellular or satellite radio profile, GPS-synchronisation requirement, solar-panel size and remote-site temperature range, and our engineers will design a welded, wide-temperature NiMH buffer with charge management and protection. See formats on the products page.
