When a passenger train loses its main supply - a catenary gap, a tripped auxiliary converter, a dead section in a tunnel - the carriage does not simply go dark. A defined set of loads must keep working: emergency lighting along the aisle and at the doors, the public address and passenger intercom, the door control that lets the crew release and open the right doors, the marker and side lamps that keep the train visible, and on modern high-speed trains the emergency ventilation and control electronics that manage the evacuation. This is Paper A of a three-part series on the battery behind those loads. It maps the operating conditions and the working principle of the carriage emergency battery, with the electrical, thermal, mechanical and fire envelope the pack actually sees. Paper B converts that profile into a pack sizing and design; Paper C covers the qualification and certification. The values cited below come from the published European and Chinese standards and from representative rolling-stock equipment; the binding numbers on a real vehicle always follow the contract specification and the edition cited on the vehicle's certificate.
Key fact 1. Two backup levels coexist on a modern coach. The train-level battery is the large DC battery that carries the whole vehicle's essential loads - classically a vented nickel-cadmium or lead-acid battery qualified to the IEC 62973 series (IEC 62973-2 for nickel-cadmium, IEC 62973-3:2024 for lead-acid). Distributed inside the saloon, each emergency luminaire, door controller, amplifier or emergency module carries its own small sealed rechargeable battery - very often a sealed NiMH pack - which guarantees the local emergency function even if the train battery or the wiring to it is lost. The OEM NiMH packs discussed in this series are the distributed ones.
Key fact 2. The electrical envelope for electronic equipment is EN 50155:2017 (consolidated through the 2021 edition). Nominal DC supplies are 24, 28, 36, 48, 72, 96 and 110 V, with 24, 72 and 110 V recommended; equipment must operate continuously at the nominal voltage plus 25% and minus 30%, ride a 40% supply dip for up to 100 ms during supply change-over, and tolerate a transient of plus/minus 40% for 0.1 s and an extended overvoltage of +40% for 1.0 s. Complete interruptions are classified S1, S2 (default, hold through 10 ms) and S3 (hold through 20 ms).
Key fact 3. The emergency lighting duty is a fixed autonomy, not a few minutes of courtesy light. UIC 555 requires emergency lighting at no less than 50% of normal illuminance in the aisle and 30% in the seated areas for a minimum of 90 minutes, and the TSI LOC & PAS (Regulation (EU) No 1302/2014) and EN 13272 family carry the same 90-minute evacuation philosophy. EN 13272-1:2019 (heavy rail) further requires that for high-speed and long-distance units the general lighting is sustained for at least 10 minutes after loss of the main power supply before the system settles into the emergency mode; EN 13272-2:2019 covers urban rail. China specifies EMU emergency lighting under TB/T 3414 and emergency ventilation under TB/T 3582-2023.
Key fact 4. The environment is far harsher than a building emergency luminaire. EN 50155:2017 defines six operating temperature classes OT1 to OT6; the default is OT3 at -25 C to +70 C in technical cabinets, with OT1/OT2 covering passenger and driver compartments at -25 C or -40 C cold limits and OT5/OT6 reaching +85 C for special applications. Mechanical shock and vibration follow EN 61373 (body, bogie or axle mounted, with category 2 class B as the default reference), EMC follows EN 50121-3-2, and fire behaviour follows EN 45545-2 and the fire-control measures of EN 45545-6.
Key fact 5. The sealed NiMH cells that back the distributed emergency modules are qualified under IEC 61951-2:2017+A1:2022 (performance), IEC 62133-1:2017 (safety of sealed nickel-containing cells; note the -1 part), IEC 63115-2:2021 (industrial NiMH) and IEC 61959 (mechanical operation), and ship under UN 3496 with IATA Special Provision A199. NiMH does not need the UN 38.3 lithium test summary and never travels under UN 3480/3481; it contains no cadmium, which is the recurring environmental objection to the old NiCd train-battery chemistry.
The emergency battery is sized against a list of functions, not against a single lamp. The first and most visible load is emergency lighting: luminaires along the aisle, at each doorway, in the vestibules and over the escape route, typically at a reduced drive current so the available energy lasts the full autonomy. The second is the door system - the door control electronics, the obstruction detection and, on coaches fitted with electro-pneumatic or electric doors, the release or opening cycle that lets the crew select and open the correct side after a stop. The third is passenger communication: the public address amplifier, the passenger-to-driver intercom units and the emergency-talk facility, which must keep the crew in command during an evacuation. The fourth is the set of external lamps that keep the train visible and declared, including side and marker lamps. On high-speed EMUs and modern metros the list also includes emergency ventilation (TB/T 3582-2023 in China), the control and monitoring electronics, and the network or repeaters that keep the emergency information flowing.
Not every load is continuous. Emergency lighting and the control electronics are steady loads for the entire autonomy; the door controllers are a standing load with a short high-current pulse when a door is released or opened; the PA amplifier is a standing quiescent load with a much higher audio pulse when the driver speaks; emergency ventilation is a heavy rotating load that may be pulsed or duty-cycled to preserve energy. A correct battery sizing treats these as separate duty blocks and adds them, rather than multiplying a single nominal wattage by a single time. The table below is a typical load map; the exact wattage, pulse amplitude and duty always follow the vehicle's emergency load schedule.
| Emergency load | Load character | Why it must survive |
|---|---|---|
| Aisle, doorway and vestibule emergency lighting | Steady, reduced LED drive for the full autonomy (at least 90 min) | Illuminates the escape route and the step boards; defined by UIC 555 / EN 13272 / TB/T 3414 |
| Door control and release | Low standing current plus a short release/opening pulse per door | Lets the crew choose and open the correct side; doors must not trap passengers |
| Public address and passenger intercom | Amplifier quiescent current with short audio-current peaks | Maintains command and two-way communication during the evacuation |
| Side, marker and indicator lamps | Steady low-current lamps for the required duration | Keeps a stopped train visible and declared to trackside and rescue staff |
| Emergency ventilation and control electronics | Heavy rotating load, often duty-cycled; control bus is steady | Avoids overheating and loss of command in a tunnel; EMU ventilation per TB/T 3582-2023 |
It helps to trace the energy path. In normal running the train draws traction energy from the overhead line or third rail; the auxiliary converters produce the on-board AC and the low-voltage DC that feeds the vehicle battery bus, which on most mainline and metro vehicles is 110 V DC, with 24 V, 72 V and other nominals on regional and urban fleets. The train battery sits across this DC bus as a float-charged reserve, charged by the auxiliary battery charger, and it supplies the bus when the main supply is lost. The big train battery has historically been a vented nickel-cadmium battery or, increasingly, a valve-regulated lead-acid or lithium battery; the IEC 62973 series is the rolling-stock auxiliary battery standard, with IEC 62973-2 covering nickel-cadmium and IEC 62973-3:2024 covering lead-acid. The same battery appears in Chinese national practice under GB/T 21413.1 for the general equipment conditions and the dedicated EMU specifications.
The distributed emergency packs are a second, independent line of defence. A sealed NiMH pack built into the luminaire, the door controller or the emergency amplifier normally sits on float or on a maintenance charge from the local DC/DC converter; when the local supply falls away or the luminaire loses its feed, the module switches to the pack and keeps the emergency function alive. This distributed design matters because a single failure in the train battery, the battery charger or a carriage feeder cannot extinguish the whole train: each luminaire or module guarantees its own function, and the failure domain is reduced to a single device. It also matches the way retrofit and modernization programs are actually sold, where an emergency LED luminaire or a control unit is delivered with its own certified battery rather than relying on a vehicle battery the integrator cannot change.
EN 50155 is the horizontal standard every on-board electronic device is designed against, and it defines an electrical world that is much rougher than a stationary UPS. A nominal 110 V system does not mean a steady 110 V: equipment must run continuously between 0.7 and 1.25 times the nominal voltage (the standard's continuous range is nominal minus 30% to plus 25%, with the rated voltage defined at nominal plus 15%), ride a transient of plus/minus 40% for 0.1 s, survive an extended overvoltage of +40% for one second, and accept a 5% DC ripple on the supply. For the most common nominal voltages that gives continuous working ranges of roughly 14.4 to 33.6 V at 24 V, 50.4 to 108 V at 72 V, and 66 to 154 V at 110 V. These wide limits are why the charger and the emergency-module DC/DC must be specified against the same envelope, not against a clean bench supply.
Two events in this standard decide the battery's first moments. Supply change-over is the moment the vehicle switches between the main source and the battery; the standard requires equipment to ride a 40% dip of the supply voltage for up to 100 ms during change-over. Complete interruptions are graded S1, S2 and S3: S1 only requires the equipment to resume afterwards, S2 (the default) requires it to maintain normal operation through an interruption of up to 10 ms, and S3 raises the ride-through to 20 ms. In practice the hold-up capacitor or the battery contactor must bridge these short events so the controller never resets, while the sealed NiMH pack takes over the longer outages. The automatic transfer to emergency lighting is generally specified to act when the normal supply voltage drops below roughly 0.6 times nominal for more than about half a second, with no visible dark interval.
Figure 1. An illustrative carriage-bus voltage trace: normal running, a change-over dip and short interruption, then the distributed NiMH pack holding the emergency load over the 90-minute autonomy until recovery. The curve is qualitative and not to scale; the real limits follow EN 50155 and the vehicle's contract specification.
| Supply condition (EN 50155) | Limit | What it demands from the emergency pack |
|---|---|---|
| Continuous range | Vn +25% / -30% (e.g. 66 to 154 V at 110 V) | Charger and module stay within their own charge window across the bus range |
| Transient / extended overvoltage | +/-40% for 0.1 s; +40% for 1.0 s | No overcharge stress or protection trip; float control must be robust |
| Change-over | 40% dip for up to 100 ms | Hold-up or battery bridge with no reset and no visible dark gap |
| Interruption S2 (default) / S3 | Hold through 10 ms / 20 ms complete interruption | Seamless transfer; the controller never loses memory or state |
| DC ripple | 5% ripple factor, continuous | Float charge stays stable; no ripple-induced micro-cycling of the cells |
The emergency lighting requirement is the heart of the carriage battery specification. UIC 555, still widely used as the global baseline for passenger rolling stock and updated by Amendment 7 (2013) for LED lighting, requires that on loss of the main lighting supply the emergency lighting provides at least 50% of normal illuminance in the aisle and 30% in the seated areas for a minimum of 90 minutes, switches on automatically when the supply drops below about 0.6 times nominal for more than half a second, and provides a manual test switch for routine checks. EN 13272, which is the harmonised standard for new rolling stock under the Interoperability Directive, sets the same evacuation philosophy and is the mandatory reference for new EU builds: EN 13272-1:2019 for heavy rail and EN 13272-2:2019 for urban rail, with the heavy-rail text requiring that high-speed and long-distance units sustain general lighting for at least 10 minutes after loss of the main supply before the emergency mode. The TSI LOC & PAS, Commission Regulation (EU) No 1302/2014, carries the rolling-stock subsystem requirements that these standards support.
For the Chinese high-speed and intercity market the same duties are written into the railway industry standards: TB/T 3414 specifies EMU emergency lighting and is cited in the current intercity EMU technical specification, while TB/T 3582-2023 covers the EMU emergency ventilation system that has become a specific safety function on trains that may stop in long tunnels. The practical point for battery sizing is that the autonomy is not the time the cells last at a comfortable temperature in a lab: it is the time the emergency loads are delivered at the cold end of the operating range, at the end of the battery's service life, after years of float charging, and with the LED drive current reduced but the light output still above the mandated fraction of normal.
The emergency pack sees a wide temperature range because it is installed where the luminaire or controller is. EN 50155:2017 splits operating temperature into six classes: OT1 (-25 C to +55 C) and OT2 (-40 C to +55 C) for passenger and driver compartments, OT3 (-25 C to +70 C, the default) and OT4 (-40 C to +70 C) for equipment in technical cabinets, and OT5 (-25 C to +85 C) and OT6 (-40 C to +85 C) for special applications; equipment must also start across a -40 C to +85 C envelope for short periods. The standard also defines useful-life classes L1 (5 years) to L4 (20 years, the default) and LX for special cases. A luminaire in a roof cavity can see far higher daytime temperatures than the saloon air, and an unheated vestibule can sit at -30 C overnight - both extremes land on the same battery.
Mechanical robustness follows EN 61373, which sorts equipment by mounting location - body mounted (category 1), bogie mounted (category 2) and axle mounted (category 3) - with category 2 class B as the default reference for general equipment, using random vibration spectra and shock tests that the welded tabs, inter-cell connectors and protection devices inside the pack must survive. Electromagnetic compatibility follows EN 50121-3-2, the rolling-stock apparatus standard, with fast transients on power and signal lines (plus or minus 2 kV, 5/50 ns, 5 kHz), surges (line to ground plus or minus 2 kV, line to line plus or minus 1 kV), and radio-frequency common-mode immunity up to 80 MHz. Fire protection follows EN 45545, with Part 2 defining the fire behaviour of materials and components - including the plastics, labels and insulation around the battery - and Part 6 covering fire control and management. The pack cannot be qualified on electrical merit alone; it has to pass all four envelopes together.
The distributed emergency pack is a demanding but narrow duty: long periods on float or standby, a small number of genuine discharge events, wide temperature, mandatory readiness after years of service, and a strong preference for a chemistry that is safe to ship and acceptable in the cabin. Sealed NiMH matches each point. Its nominal 1.2 V per cell stacks cleanly into the small pack voltages used in emergency modules (3.6, 4.8, 6.0, 7.2, 9.6 V and higher strings), it keeps useful capacity at sub-zero temperatures where lead-acid weakens, and it tolerates the long float and occasional deep discharge of emergency duty better than a lead-acid block that sulphates. Unlike nickel-cadmium it contains no cadmium, the heavy metal that drives the environmental objection to the old vented train battery; and unlike lithium, sealed NiMH is not classified as a lithium battery for transport, so it ships under UN 3496 with IATA Special Provision A199, without the UN 38.3 test summary and without UN 3480/3481 handling.
Figure 2. The working sequence from main-supply loss to emergency autonomy (illustrative): the detection threshold, the EN 50155 change-over, the transfer to the distributed NiMH pack, the emergency loads, the 90-minute autonomy and the recovery when the main supply returns. Timing values are illustrative; thresholds follow EN 50155, UIC 555, EN 13272 and the vehicle specification.
The qualification basis for the cells is explicit. Sealed portable nickel-metal hydride cells are covered for performance by IEC 61951-2:2017 consolidated with Amendment 1:2022, and for safety by IEC 62133-1:2017 - the -1 part is the nickel-system part, while IEC 62133-2 is lithium and must not be confused. Industrial NiMH cells and batteries are covered by IEC 63115-2:2021, and the mechanical operation of sealed portable secondary cells by IEC 61959. These standards, together with the vehicle-level EN 50155 / EN 61373 / EN 50121-3-2 / EN 45545 tests and the transport classification, form the complete evidence chain that Paper B uses to size the pack and Paper C follows through certification. The chemistry is not chosen for its energy density; it is chosen for reliable readiness, cold performance, long float life, no cadmium and the simplest compliant logistics path.
The defined emergency loads include the aisle, doorway and vestibule emergency lighting, the door control and release, the public address and passenger intercom, the side and marker lamps, and - on modern high-speed and metro trains - the emergency ventilation and the control electronics. Lighting and control run for the full autonomy; doors and PA are standing loads with short high-current pulses; the train-level battery and the distributed local batteries together cover these functions.
UIC 555 and the TSI LOC & PAS / EN 13272 family use a minimum of 90 minutes at reduced illuminance - UIC 555 requiring at least 50% of normal illuminance in the aisle and 30% in seated areas. EN 13272-1:2019 additionally requires high-speed and long-distance units to sustain general lighting for at least 10 minutes after loss of the main supply before the emergency mode. Chinese EMU emergency lighting is specified under TB/T 3414 and emergency ventilation under TB/T 3582-2023.
The train battery is the large DC battery across the vehicle battery bus (classically nickel-cadmium or lead-acid, under IEC 62973-2 or IEC 62973-3), carrying the whole vehicle's essential loads. The sealed NiMH packs this series describes are distributed batteries built into each emergency luminaire, door controller or amplifier; they guarantee that local function independently, reducing the failure domain and supporting retrofit, and ship under UN 3496 with IATA A199.
EN 50155 equipment runs continuously from nominal minus 30% to plus 25% (about 66 to 154 V at 110 V nominal), tolerates a transient of plus/minus 40% for 0.1 s and an extended overvoltage of +40% for 1 s, rides a 40% change-over dip for up to 100 ms, and holds through complete interruptions of 10 ms (S2, default) or 20 ms (S3). The battery and its charger must bridge the short events and take over the long ones without reset.
Shock and vibration follow EN 61373 by mounting location (body, bogie or axle, category 2 class B as default); EMC follows EN 50121-3-2, including fast transients at plus or minus 2 kV and surges to line ground at plus or minus 2 kV; fire behaviour follows EN 45545-2 for materials and components and EN 45545-6 for fire control. The cell chemistry is separately qualified under IEC 61951-2, IEC 62133-1 and IEC 63115-2.
No. UN 38.3 and UN 3480/3481 belong to lithium batteries. Sealed NiMH is classified under UN 3496, and IATA Special Provision A199 confirms it is not restricted as lithium battery cargo when the terminals are protected against short circuits. NiMH also contains no cadmium, which is the environmental problem of the old vented nickel-cadmium train battery.
Rolling-stock framework and electrical conditions: EN 50155:2017 (Railway applications - Rolling stock - Electronic equipment, with the 2021 consolidation of supply interruption and battery supply provisions), including the nominal DC voltages (24, 28, 36, 48, 72, 96, 110 V), continuous and transient voltage limits, supply change-over, interruption classes S1/S2/S3 and operating temperature classes OT1 to OT6; EN 50163 for supply voltages of traction systems; EN 61373 for shock and vibration; EN 50121-3-2 for apparatus EMC, including the IEC 61000-4-4 fast transient and IEC 61000-4-5 surge levels. Lighting and autonomy: UIC 555 (Electric lighting in passenger rolling stock, with Amendment 7:2013) and UIC 555-1, EN 13272-1:2019 (heavy rail) and EN 13272-2:2019 (urban rail), Commission Regulation (EU) No 1302/2014 (TSI LOC & PAS), and the Chinese TB/T 3414 (EMU emergency lighting) and TB/T 3582-2023 (EMU emergency ventilation). Fire: EN 45545-2 and EN 45545-6. The voltage traces, duty-cycle blocks and transfer sequence in this article are engineering illustrations and are labelled illustrative; the binding values for any vehicle are the ones in its contract specification and the standards edition cited there.
Battery and transport references: IEC 62973 series for rolling-stock auxiliary supply batteries (including IEC 62973-2 for nickel-cadmium and IEC 62973-3:2024 for lead-acid), GB/T 21413.1 for rolling-stock electrical equipment general conditions, IEC 61951-2:2017+A1:2022 for sealed NiMH cell performance, IEC 62133-1:2017 for sealed nickel-containing cell safety, IEC 63115-2:2021 for industrial NiMH and IEC 61959 for mechanical operation, with UN 3496 and IATA Special Provision A199 for transport. The load wattage and autonomy calculations in the tables follow the published standard duties and representative rolling-stock practice; vehicle-specific load schedules supersede these general values.
Weijiang Power supplies sealed NiMH packs for OEM carriage emergency modules - emergency LED luminaires, door controllers, passenger address and intercom units, and emergency control electronics - built to the 90-minute autonomy and the EN 50155 electrical envelope, with welded tab and protection-device construction for the EN 61373 vibration profile, EN 45545-2 support, and the cell-level IEC 61951-2 / IEC 62133-1 / IEC 63115-2 documentation. The packs are cadmium-free and travel under UN 3496 with the IATA A199 path. Tell us your bus nominal voltage, emergency load list, required autonomy, cold end temperature and charging arrangement, and we will support the cold-autonomy and float-life evidence before the type test rather than during it. See the Weijiang NiMH product range for standard pack formats, or contact our OEM team with your vehicle specification and emergency load schedule.
