A metro station is a machine for moving people safely at very high density, and two of its most safety-critical powered elements sit at the platform edge and the paid-area boundary: platform screen doors (PSD), including full-height and half-height platform gates, and the automatic fare-collection (AFC) flap, retractable and speed gates that control entry and exit. This paper explains the working principle and power/battery duty of both, starting from the life-safety principle that dominates every design decision: in an emergency these barriers must not trap people. Platform barrier systems are addressed by ISO 18298 (Railway applications - Platform barrier systems, developed from the European prEN 17168), which covers the fixed structure, the passenger sliding doors, emergency doors, driver access doors and platform extremity doors, and defines an emergency egress device for manual opening from the trackside; the train's own bodyside doors are covered by EN 14752, which requires an emergency egress device on each passenger door. AFC gates follow contactless ticketing standards such as ISO/IEC 14443 and station life-safety codes that require fail-safe release. The paper shows how normal operation - repeated, high-cycle motorised opening and closing driven by station controllers and obstacle detection - is powered from the station supply, while a station blackout triggers a deterministic response: mechanical fail-safe mechanisms spring or release the barriers open, and small sealed battery modules power a defined number of controlled door cycles, the emergency egress logic, indicators and, on gates, the completion or safe release of the current transaction and the audit trail. It develops the characteristic battery duty - long idle standby, rare but mandatory short high-current door or flap actuation, and a guaranteed safe-state transition - and explains why sealed nickel-metal hydride is well matched to these small, distributed, maintenance-poor, station-environment modules.
A platform screen door system is a row of fixed panels and powered sliding door units aligned with the train doors, controlled by a station-level PSD controller that receives train stopping position and door commands, manages obstacle detection, and reports status to signalling and SCADA. Each door unit has a motor drive, a controller, locking and detection, and local indicators; full-height systems seal the platform from the track environment while half-height platform gates provide edge safety on older or ventilated platforms.
Normal duty is a high-cycle, moderate-power motor load: every station stop opens and closes dozens of doors in synchrony with the train, with obstacle-detection re-open events adding partial cycles. ISO 18298 manages the safety risks particular to barrier systems - the gap between train and platform, entrapment, and emergency evacuation - and specifies emergency doors and the trackside emergency egress device. The power system must support both the repetitive service load and, separately, the emergency behaviour.
The governing requirement is that loss of power must never lock passengers onto the track side or block evacuation. ISO 18298 provides emergency doors and manual emergency egress devices operable from the trackside, and EN 14752 requires an emergency egress device at each train door. At station level, PSD and gate designs combine mechanical fail-safe elements - spring-applied or gravity-released mechanisms, breakout emergency panels - with electrical control so that, depending on the station's safety concept, doors open automatically or can be pushed open with defined force when power is lost.
Industry PSD designs integrate an emergency backup battery bank that, on total station power loss, supplies the door controls and drives for several full open/close cycles or holds a defined evacuation configuration, alongside spring-loaded breakout panels. AFC gates are commonly designed so the flap or barrier retracts on power-off (mechanically or via a small battery or capacitor), opening the egress path; life-safety codes in many jurisdictions require egress gates on designated routes to open within seconds of a fire alarm or mains loss and remain open. The battery's role is therefore not to run the station indefinitely but to make the safe transition reliable and controlled.
An AFC gate is a self-contained access unit: a contactless reader (ISO/IEC 14443 for cards and phone wallets, often within Calypso- or MIFARE-based schemes), a controller and passenger-facing indicators, infrared anti-tailgating and anti-pinch sensors, and one or two motorised flaps, retractable barriers or speed-gate wings, typically on a 24 V safety-voltage bus with residual-current protection. Throughput can reach several tens of passengers per minute per lane in peak service.
Its emergency duty is short and decisive. On mains loss the gate must release the barrier for evacuation (mechanically fail-open or via a local battery/capacitor that retracts the wing), finish or safely discard the transaction in progress, and preserve the audit log and fare data so revenue and entry/exit records are not corrupted. A small sealed backup module provides the last-gasp energy for these controlled actions and for a period of reduced operation or indication; it is a high-reliability, low-energy, long-standby application rather than a traction load.
Across PSD and AFC equipment the battery sees the same fundamental profile: months or years of idle float at full readiness, interrupted rarely by an emergency that demands a short, high-current, guaranteed action - driving a door or flap, holding a logic rail, writing a log - followed by a return to charge. The first animated figure traces this duty, contrasting the repetitive station-service load with the rare emergency pulse and the safe-state hold that follows.
This profile penalises chemistries that self-discharge, need routine maintenance, or fail abruptly after long idle, and it favours sealed cells with very good charge retention, a flat voltage for predictable motor and logic performance, adequate short-pulse current, and no liquid servicing in cabinets distributed along a platform or concourse. The station environment - under-platform heat, concourse temperature swings, dust and vibration from passing trains - adds derating and enclosure requirements that the second paper addresses in the sizing and selection work.
Sealed nickel-metal hydride is well suited to the small distributed backup modules in PSD local control cabinets and AFC gate pillars: it is sealed with no electrolyte top-up, cadmium-free, tolerant of the shallow discharge and long float that emergency backup entails, capable of useful pulse current at the modest low temperatures found in unheated stations, and simpler to transport and dispose of than some alternatives. Low-self-discharge variants keep their readiness over long intervals between tests.
It is not the energy source for the large station UPS or for the normal high-cycle door drives - those remain grid-fed with their own uninterruptible supply - but it is an excellent fit for the local, safety-related 'last metre' of power that guarantees a controlled fail-safe action and protects data. The next paper sizes these modules, sets the charger and supervision architecture, and compares NiMH with lead-acid, lithium and capacitor/mechanical alternatives.
The consolidated requirement is for sealed, maintenance-minimum battery modules sized to deliver a defined number of emergency door or flap cycles and a defined safe-state hold at end of life and at station temperature extremes, with pulse current sufficient to avoid motor stall or logic brown-out, integrated into fail-safe mechanical designs that remain safe even if the battery itself is exhausted. Supervision must log every emergency action and prove readiness through periodic self-test.
The following papers convert this into hardware and evidence: the second sizes the packs and selects the chemistry for PSD controls and AFC gates, and the third maps the qualification campaign - ISO 18298 and EN 14752 functional safety, station EMC, environmental and cycle testing, and the IEC 61951-2 / IEC 62133-2 / UN 38.3 battery dossier that a transit authority or system integrator requires.
Weijiang Power designs and manufactures sealed nickel-metal hydride cells and matched industrial packs for remote, off-grid and safety-related equipment, and supports OEM partners with IEC 61951-2 performance files, IEC 62133-2 safety evidence, pulse-load characterisation, wide-temperature testing and charger/pack co-validation. Tell us your duty cycle, peak current, temperature envelope, autonomy target and the standards your product must meet, and our engineers will specify a cell-and-pack combination that protects runtime, reliability and service life. Review the range on the products page.
