An AMR collector or AMI data concentrator is normally mains powered, which hides a subtle requirement: the instant the mains disappears, the grid operator most wants to hear from it. A 'last gasp' - a power-failure message sent before the device goes silent - lets the head-end distinguish a true outage from a communications fault and locate the faulted section. A bare supercapacitor buys a single radio frame; a small nickel-metal hydride module extends that to multiple frames, retries and a store-and-forward window, turning a one-shot notification into a managed outage report.
In an AMI network, meters talk to collectors or data concentrators that aggregate readings and forward them to the head-end over cellular, Ethernet or RF mesh, commonly using the DLMS/COSEM protocol suite of IEC 62056 (with regional companions such as DL/T 698 in China). Meters and their ancillary equipment are type-tested under IEC 62052-11 and related standards, and their immunity to supply dips and interruptions follows IEC 61000-4-11.
Commercial practice shows the established pattern: a smart meter or collector detects mains loss - for example when the rail falls below roughly 80% of nominal for more than a mains cycle - and uses local storage to transmit a last-gasp frame. A representative grid meter, the Aclara SGM3000 series, uses a supercapacitor for short-term timing (a few days of clock keep-alive) and a non-rechargeable lithium backup for longer timing (up to a couple of years), illustrating how different storage durations are deliberately layered.
The first animated figure zooms into the first second after mains loss. The bulk capacitor and a supercapacitor bank - commonly 1-5 farads at 5.5 V - hold the rail long enough for the MCU to detect the event, assemble one frame and key the radio (around 200-500 ms at a transmit current near 250 mA). That is enough for a single best-effort message on a good link, and it is the industry's baseline because it is cheap and needs almost no maintenance.
It is also fragile: if the network is busy, the base station not responding, or the first frame collides, there is no stored energy for a retry, and the head-end never learns the device went dark. A single-frame gasp is a notification, not a reliable report.
The second figure compares what each storage tier delivers. The supercapacitor delivers sub-second, single-frame coverage; a non-rechargeable lithium cell can keep the clock and memory alive for years but is poorly suited to repeated high-current radio pulses; a rechargeable NiMH module delivers seconds to minutes of regulated power - enough to send the last gasp, wait for the acknowledgement, retry across several channels, notify downstream meters, and store context for a 'first gasp' when power returns.
That window is what converts a fire-and-forget pulse into a reliable, acknowledged outage report with retries and a clean power-restart sequence - a meaningful improvement in outage-management-system accuracy for a modest amount of storage.
The choice is the power-versus-energy axis seen across backup design. A supercapacitor is excellent for milliseconds and a few seconds of high power, but its stored energy is small, its voltage droops linearly with charge, and reaching a multi-second, multi-frame window means a bulky, costly bank. Non-rechargeable lithium stores years of energy but is not designed to be repeatedly recharged by the returning mains and is weak on high-current pulses.
Sealed NiMH sits in between: it recharges from the mains rail on every restoration, delivers the radio pulse at low impedance with a flat plateau, survives thousands of outage-charge cycles across a multi-decade deployment, and uses an aqueous chemistry that is safe in a sealed, sun-heated street cabinet with no thermal-management burden.
Concentrators and larger collectors aggregate many meters and often keep a local store-and-forward buffer. For these, the NiMH module acts as a miniature DC-UPS: on outage it finishes the current polling cycle, flushes the buffer and the event log to non-volatile memory, sends last gasps for itself and its downstream nodes, and holds the real-time clock. On restoration it sends a first gasp, reconciles missed intervals and resumes without data loss.
Because outages are (happily) infrequent but the device is mains-connected, the pack lives in an easy duty - kept topped up, discharged only on real events - where NiMH's low self-discharge and cycle robustness are exactly the required attributes.
The design brief records the mains rail and its loss-detection threshold, the radio technology and transmit current, the number of last-gasp frames and acknowledgement/retry window, the buffer-flush and clock-keep-alive needs, and the cabinet temperature range (street cabinets can swing very widely). Paper B turns these into a storage and chemistry calculation; Paper C maps the design onto the IEC 62056, IEC 62052-11 and IEC 61000-4-11 evidence.
Weijiang Power manufactures sealed nickel-metal hydride cells and DC-UPS / last-gasp modules for AMR collectors, data concentrators and smart-grid endpoints. Send us your mains rail, radio technology and transmit current, the number of last-gasp frames and retry window, and the enclosure temperature range, and our engineers will design a welded, wide-temperature NiMH module with charge management and protection. See modules on the products page.
