It is one thing to know that a Grade 2 panel needs twelve hours of standby and a Grade 3 panel twenty-four; it is another to convert that clause into a cell count, a capacity, a string voltage and a charge profile that still hold true eight years later. Paper B provides the engineering method. It builds the energy budget from the real load list, derates it for efficiency, depth of discharge and end-of-life fade, compares the candidate chemistries on the axes that actually matter in an alarm enclosure, and sets out the pack-architecture and charger decisions that separate a panel which passes its annual test from one that does not.
Every credible battery calculation starts from an itemised load table, not a single number guessed from the panel's label. List the controller sleep and active currents, each wired loop and its supervision current, every keypad and expander, the communicator in heartbeat and in full transmission, and the sounder or bell output in alarm. Multiply each current by the time it is drawn in the required autonomy scenario: for a Grade 3 Type A design that is 24 hours of normal operation, and for the common UK expression of Grade 2 it is 12 hours normal followed by 4 hours in alarm. The sum, in ampere-hours at the pack voltage, is the raw energy the cells must deliver.
Communicators deserve a separate line. An IP or cellular radio may draw only tens of milliamps on average but several hundred milliamps, or more, during a transmit burst; if the required scenario assumes repeated alarm transmissions during the outage, those bursts cannot be treated as instantaneous and free. Capturing them explicitly is what keeps the rail stable and the autonomy honest.
A new cell's nameplate ampere-hours measured at 20 degrees C is not the capacity available to the panel at end of life. The animated waterfall walks a worked example for a 12 V Grade 3 panel drawing 0.12 A for 24 hours and 0.35 A for four alarm hours. The raw demand is 2.88 Ah of quiescent energy plus 1.4 Ah of alarm energy, 4.28 Ah. Divide by conversion efficiency of roughly 0.90 to cover the OR-ing and regulation path, limit usable depth of discharge to about 0.80 so the string never sits deeply depleted, and reserve a further 0.80 factor for capacity fade at the declared end of life. The result - about 7.4 Ah of nameplate capacity - is roughly 1.7 times the raw number.
That multiplier is not pessimism; it is the difference between a pack that passes its type test on the bench and one that passes its graded annual test years later in a warm cupboard. Designers who quote nameplate capacity against raw demand are, in effect, spending their end-of-life and temperature margin on day one.
The animated scorecard rates sealed lead-acid, NiMH and lithium-ion on five axes specific to alarm backup: float readiness, seamless changeover, warm-enclosure life, intrinsic safety and cost value. Lead-acid scores on familiarity and first cost but is heavy, sulphates under the perpetual near-full state and weakens in cold spots. Lithium-ion scores on energy density and changeover but requires a protection and balancing circuit, carries more transport and regulatory weight, and offers little cost benefit where the enclosure is volume-tolerant.
NiMH scores strongly across the middle: a flat 1.2 V plateau and low impedance give a clean, fault-free transfer; the aqueous chemistry is intrinsically tolerant of abuse and short-circuit within its design envelope; there is no cadmium and no memory effect; and the cost per delivered standby-hour is competitive in the AA-to-sub-C sizes that dominate compact panels. Its one discipline - temperature-aware charging to avoid dry-out above about 45 degrees C - is a design rule, not a barrier, and is fully manageable in a panel PSU.
Panel electronics usually run at a nominal 12 V rail, which a NiMH string builds from ten series cells at 1.2 V nominal (a charged string near 13.5-14 V tracks the legacy lead-acid rail comfortably); 6 V and 24 V systems use five and twenty cells respectively. Within that string the cell size - AA, C or sub-C high-rate - sets both capacity and the ability to supply the alarm surge without voltage sag. A larger electrode also runs cooler under the maintenance charge, which extends float life.
Cell matching is non-negotiable in a series string: the weakest cell governs the knee, and a mismatched string reaches end-of-life early and risks reversal under deep discharge. Industrial-grade packs use cells from a single production lot sorted for capacity and internal resistance, welded nickel tabs rather than hand-sprung holders for vibration security, and an integrated thermal fuse or PTC for fault protection. The pack should also expose a thermistor so the panel PSU can throttle charge as the enclosure warms.
In a standby product the charger runs for the product's entire life, so its design decides the battery's life. EN 50131-6 requires the depleted pack to return to 80 percent within 72 hours, which a modest constant-current charge easily meets; the harder problem is what happens after full charge, during years of readiness. A long-term maintenance current at or below C/20, or a pulsed/temperature-compensated float, keeps the string full without the sustained overcharge that drives gas evolution and dry-out. A thermistor cut-back above about 40-45 degrees C, and an absolute temperature termination, protect the worst-case summer enclosure.
The charger must also coexist with the seamless-changeover requirement: charge current withdrawn at the instant of mains loss must not cause the rail to dip, and the return of mains must not look like a fault. A clean diode or MOSFET OR-ing path, with the battery always connected to the rail, achieves both.
For a panel manufacturer the pack is also a service item, so architecture should favour a defined, replaceable form factor, clear polarity and keyed connectors, and a capacity label that states the graded autonomy the pack supports. Welded packs with shrink-wrap and an insulating carrier survive installation and transport far better than loose cells, and a consistent cell supplier keeps the matched-lot discipline repeatable across production batches. Documenting the pack with IEC 61951-2 performance data, IEC 62133-1 sealed-nickel safety evidence and UN 38.3 transport data lets the panel's own EN 50131-6 declaration rest on traceable cell evidence rather than assertion.
Paper C closes the loop with the full test and certification trail: how the graded standby, changeover, recharge and endurance clauses are verified, and what documentation a notified-body or insurer expects to see.
Weijiang Power builds graded NiMH standby packs for alarm panel OEMs: matched welded strings in AA, C and sub-C for 12/24 V rails, nameplate capacity derated to clear EN 50131-6 standby at end of life, thermistor-ready designs for temperature-aware charging, and full IEC 61951-2, IEC 62133-1 and UN 38.3 documentation. Share your load list and target grade and we will return a sized pack, a derating calculation and a service-ready form factor.
