An intruder and hold-up alarm control panel is a device whose whole purpose is to be trustworthy at the one moment an attacker tries to make it fail. Cutting the mains is the oldest attack in the book, which is why the European standards family EN 50131 treats the standby power supply not as an accessory but as a graded, tested subsystem. Paper A - the first of three on the alarm panel battery - dissects the electrical duty a panel imposes on its cells, frames it against EN 50131-1 system requirements and EN 50131-6 power-supply requirements, and explains why a nickel-metal hydride (NiMH) pack is well matched to long, warm, mostly-quiescent standby life.
In the armed, healthy state a modern hybrid panel is a study in low current: the controller sleeps between polls, wired PIR and magnetic-contact loops are supervised with tiny supervision currents, and the communicator wakes on a schedule to send a heartbeat over IP or cellular radio. The animated trace below compresses that life into a long quiescent plateau punctuated by periodic poll and heartbeat blips. The character of the duty changes abruptly on two events. Mains loss forces the battery to feed the entire supervised system - controller, every loop, keypads, sounders in standby and the communicator - rather than merely topping itself up. A genuine intrusion or a communications test then adds a short, much higher burst as the communicator transmits and the bell or siren output is driven. Sizing a pack against only the quiescent number, while ignoring the alarm burst, is the classic under-design error.
EN 50131-6 distinguishes Type A power supplies (the rechargeable standby source is an integral part of the control and indicating equipment) from Type B (a separate, dedicated power supply). It also recognises that the quiescent current of a system grows with every added device: a second keypad, a radio expander, a GSM communicator and an external siren each add a measurable standing load that must appear in the battery calculation rather than in the field engineer's margins.
EN 50131-1 assigns every intrusion system a security grade from 1 to 4 according to the knowledge and resources of the assumed intruder, and an environmental class from I (indoor, protected) to IV (outdoor, severe). Most commercial and higher-residential installations in Europe are Grade 2 in environmental Class II; banks, jewellers and critical sites move to Grade 3. EN 50131-6 turns that grading into a hard standby requirement. For a Type A supply the minimum standby duration is 12 hours for Grade 1 and Grade 2 and 24 hours for Grade 3 and Grade 4; Type B supplies are specified at 24 hours. UK and several national markets additionally express Grade 2 as twelve hours of normal operation followed by four hours in alarm, and wired Grade 3 configurations - particularly those whose communicators must keep reporting through an extended outage - are frequently engineered for 60 hours of autonomy.
The animated chart converts one fixed NiMH pack into available standby hours across a range of panel quiescent currents. It is an illustrative model, but the lesson is exact: because hours are inversely proportional to standing current, the same pack that comfortably clears the 12-hour Grade 2 line at 60-90 mA falls short once a radio expander and cellular communicator push the load toward 180-240 mA. Grade is therefore not a label you choose after the battery; the grade you quote sets the standby hours the battery must demonstrably deliver.
EN 50131-6 contains a clause that is easy to underestimate: the transfer between the mains-derived supply and the standby battery, and back again, must not generate an alarm, a tamper condition or a fault condition. A panel that logs a spurious 'power fault' every time the mains blips, or that resets its clock and loses its armed state on transfer, fails the requirement even if its battery is enormous. In electrical terms the cell string must hold the panel's DC rail through the transfer without a dip deep enough to brown-out the microcontroller or the radio module.
This is where cell internal impedance and the flatness of the discharge curve matter. A freshly charged NiMH cell sits near 1.35 V and delivers a long, flat plateau around 1.2 V nominal before a defined end knee; a matched, welded NiMH string presents a low, stable rail that rides through the transfer and through the communicator's transmit surge without a brown-out. Designers still specify a bulk capacitor on the rail and a clean OR-ing path, but the battery's dynamic voltage under load is what makes the transfer genuinely seamless - a point Paper B develops when comparing chemistries.
Control panels live in hall cupboards, ceiling voids and metal enclosures above a boiler or beside network equipment. Environmental Class II assumes an indoor general location, but real enclosure ambients of 30-40 degrees C are routine, and a panel sealed above a warm riser can see more. A battery kept at, or near, full charge in that environment ages on an accelerated clock: for aqueous nickel chemistries, sustained charging above roughly 45 degrees C promotes internal gas evolution and gradual electrolyte dry-out, which is precisely why the maintenance-charge regime and the cell design matter more than the headline capacity.
A correctly engineered NiMH standby pack uses a temperature-appropriate maintenance current - long-term trickle is conventionally kept at or below C/20 - together with cells whose alloy and separator are chosen for endurance under float-like readiness. The contrast with legacy nickel-cadmium is deliberate: NiCd tolerates abuse but contains cadmium, a restricted substance under RoHS with only narrow exemptions, and exhibits memory effect that erodes apparent capacity after years of shallow self-tests. NiMH removes the cadmium and the memory behaviour while keeping the aqueous, intrinsically safe family character.
A graded panel is exercised far more often than it is attacked. Installers perform walk tests, maintenance engineers trigger bell tests and communicator tests, and many panels run periodic automatic battery or load checks. Each event is a small discharge-recharge cycle: the battery carries the system while the mains-derived supply is tested, then is recharged. EN 50131-6 requires a depleted standby battery to be rechargeable to 80 percent within 72 hours, a deliberately generous window that recognises the low charge currents available from a small panel PSU.
Over a five-to-ten-year service life those micro-cycles accumulate into hundreds of shallow events. IEC 61951-2 sets the reference endurance expectation for sealed nickel-metal hydride cells - a minimum of 500 charge-discharge cycles under its reference test - and a pack that survives that regime must combine matched cells, a charger that returns the string to readiness without overcharge, and a capacity margin that still clears the graded standby hours at the declared end of service life, not only on day one.
Pull the requirements together and the profile is specific: years of near-full readiness in a warm enclosure, a low but non-zero standing load, occasional high-current bursts, a seamless transfer that must never raise a fault, and a graded autonomy that has to survive hundreds of micro-cycles. Sealed lead-acid is cheap and familiar but is bulky for a compact panel, sulphates under partial-state readiness and loses usable capacity sharply in the cold; lithium adds protection circuitry, transport classification weight and cost to a duty that rarely needs its energy density. NiMH offers a flat 1.2 V-per-cell plateau, low internal impedance for the transfer and transmit surge, cadmium-free aqueous chemistry with no memory effect, and the simplest air-transport profile of the rechargeable family.
Paper B turns this duty into a sizing and chemistry-selection method, including the quiescent-plus-alarm energy budget and the choice of cell size and string voltage. Paper C walks the EN 50131-6, IEC 61951-2 and IEC 62133-1 evidence trail that lets a panel manufacturer declare a grade with confidence.
Weijiang Power manufactures matched, welded NiMH cells and packs for intruder and hold-up alarm control panels: AA, C and sub-C strings sized to the EN 50131-6 12-hour or 24-hour graded standby duty, low-impedance cells for seamless changeover and communicator bursts, temperature-aware charge profiles for warm enclosures, and IEC 61951-2, IEC 62133-1 and UN 38.3 documentation. Send your panel quiescent current, alarm and siren load, target grade and enclosure ambient and we will size a pack that still clears its graded standby hours at end of life.
