An access-controlled doorway hides a deceptively hard power problem. An intruder alarm spends almost all its life asleep; a door held by an electromagnetic lock spends its entire locked life drawing current, continuously, just to stay shut. When the mains fails, that continuous load does not pause - and the life-safety codes simultaneously demand that the door release for egress on a fire signal. Paper A, the first of three on access-control and door-entry backup, dissects this continuous-load duty, frames it against EN 50133-1 for access systems and UL 294 for access-control units, and explains why the fail-safe/fail-secure tension makes the battery a genuinely safety-relevant component.
The animated current trace shows the shape of the duty. A magnetic lock - a maglock - holds the door by energising an electromagnet, and it draws its holding current for as long as the door is locked, twenty-four hours a day. Superimposed on that plateau are short release dips each time the door is opened by a credential, a request-to-exit (REX) device or a push bar, plus brief inrush spikes from the reader, controller and any electric strike. A building door-entry intercom adds its own standing load for the camera, amplifier and backlight, with talk bursts when a visitor calls.
Contrast this with an alarm panel, whose battery exists to feed a sleeping controller. The access battery must feed an active, continuous load for the entire outage, which is why its ampere-hour calculation is dominated by holding current multiplied by the number of doors and the required backup time - a fundamentally different, and larger, energy problem than a pulsed or sleeping duty.
Life-safety codes decide what a lock is allowed to do when power is removed. A maglock is inherently fail-safe: remove power and the electromagnet releases, so the door opens for free egress, which is why NFPA 101 and the International Building Code accept maglocks on egress paths only with reliable release on loss of locking power and with independent REX hardware. Hardware that preserves free mechanical egress - certain strikes and mortise locks - may instead be fail-secure, remaining locked on power loss. The choice is a safety decision before it is a security decision.
This creates the central design tension. On a plain mains failure a security operator often wants the doors to remain controlled, which requires backup power to keep fail-safe maglocks energised; on a fire-alarm signal, however, those same maglocks must release regardless of backup, so that occupants can evacuate. The backup battery therefore keeps the door secure through an outage while the fire-alarm tie-in retains an unconditional override. Request-to-exit and panic hardware always, without exception, take priority over lock power.
In Europe, EN 50133-1 classifies access-control systems into security levels and sets functional expectations for the system and its power provision; where access is integrated with intrusion it also falls under the EN 50131 family. In North America, UL 294 governs access-control system units - panels, readers, locks and their power supplies - and its standby-power provisions call for a defined minimum period of operation on backup; the commonly cited Level II requirement is at least four hours of standby, with higher levels demanding more.
The animated chart converts a single NiMH pack into backup hours as the number of held maglocks grows. It is an illustrative model using a representative holding current, but the relationship is exact and unforgiving: because every maglock adds a continuous load, backup hours fall in direct proportion to door count. A pack sized for one door that 'happens to work' will not carry four or eight doors through the same outage. Door count, lock holding current and the required standby period are three halves of one decision.
Typical surface maglocks are rated for several hundred kilograms of holding force and draw on the order of 0.3-0.5 A at 12 V (or roughly half that current at 24 V for the same power); shear locks and gate locks can draw more. Electric strikes draw a similar holding current but often a larger momentary inrush, and readers, controllers and intercoms add tens to hundreds of milliamps each. Real power-supply datasheets reflect this directly: a UL 294-listed door power supply may be designed to accommodate a 1-4 Ah battery with its output fused by a 2.5 A resettable PTC, sized to hold one or two locks through the required standby window.
Multiplying these currents across a multi-door site is what turns a small battery into a substantial pack. A four-door lobby at 0.5 A per lock already presents a 2 A continuous load before a single reader is counted - and that load runs for the entire outage, not just during a transaction.
The access power supply lives in the same world as an alarm PSU - a warm cabinet, years of connected readiness - but its battery experiences a distinct regime. While mains is healthy the battery is held at readiness under a maintenance charge, exactly as in an alarm panel; the instant mains fails it must deliver a sustained, relatively high current rather than a sleeping trickle. The cell therefore needs both good float endurance and a low enough internal resistance to hold multiple locks and strike inrushes on a stable rail without sagging below the lock's dropout or the controller's brown-out voltage.
A matched NiMH string meets both halves: the flat 1.2 V plateau and low impedance hold a steady rail under the continuous lock load and through strike spikes, while the aqueous chemistry tolerates the warm-cabinet float when the charge regime is temperature-aware. Paper B builds the exact energy budget and compares chemistries.
Sealed lead-acid has historically dominated access power supplies because of first cost, but it is heavy, sulphates under perpetual float and delivers poorly in cold vestibules; lithium adds protection electronics and transport complexity to a duty that is volume-tolerant and cost-sensitive. NiMH offers a cadmium-free, memory-free aqueous chemistry with a stable plateau, good high-rate delivery for strikes and maglocks, a simple air-freight classification and a cycle life well matched to a product that is briefly discharged on every outage and test. The discipline is the same as across the standby family - temperature-aware maintenance charging and a real end-of-life margin - but the payoff is a door that stays controlled through an outage yet always releases for life safety.
Paper B turns this duty into a sizing and selection method; Paper C walks the UL 294, EN 50133-1, IEC 61951-2 and IEC 62133-1 evidence trail.
Weijiang Power manufactures matched NiMH packs for access-control panels, magnetic-lock power supplies and building door-entry intercoms: 12/24 V welded strings sized to the UL 294 four-hour (or higher) standby duty across multiple held locks, low-impedance cells for strike inrush and stable lock rails, temperature-aware float designs, and IEC 61951-2, IEC 62133-1 and UN 38.3 documentation. Send your door count, per-lock holding current, intercom load and required standby period and we will size a pack that keeps doors controlled while preserving fail-safe egress.
