Paper A of this series mapped the load profile of a passenger coach and high-speed train emergency system: the aisle and doorway lighting, the door control and release, the public address and intercom, the marker lamps and, on modern EMUs, the emergency ventilation and control electronics that must survive a loss of the main supply. This Paper B converts that profile into a pack. It walks through the sizing sequence an OEM engineer actually follows - list the emergency loads, turn them into duty blocks, compute the energy and current for the mandated autonomy, apply the cold, end-of-life and depth-of-discharge margins, choose the cell count and protection, and design the charge and transfer arrangement against the EN 50155 electrical envelope. Every worked number here is labelled illustrative: the binding figures on a real vehicle are the ones in its contract specification and the standards edition cited on the vehicle's certificate. Paper C then covers the type tests and certification that prove the design.
Key fact 1. Sizing starts from a duty-block energy sum, not from a nominal wattage multiplied by a time. The steady loads (emergency lighting, control electronics) run for the full autonomy; the door controllers and PA amplifier are standing loads with short high-current pulses; emergency ventilation may be duty-cycled. Each block is integrated separately, the pulse energy is added, and the result is an ampere-hour demand at the module's nominal voltage, before any margin is applied.
Key fact 2. The autonomy target is fixed by the lighting standards. UIC 555 (with Amendment 7:2013) 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 EN 13272-1:2019 (heavy rail) 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. EN 13272-2:2019 (urban rail) sets a minimum average emergency illuminance of 5 lx at floor level along the escape-route centre line and 40 lx at the vehicle access step when the door is open.
Key fact 3. The usable capacity is always lower than the nameplate. A sealed NiMH cell delivers less at the cold end of the operating range, less at the end of its service life, and less if the design limits the depth of discharge to protect the string. Emergency-luminaire practice conventionally assumes a multi-year design life and sizes against the capacity still available at end of life rather than at day one; the derating chain below turns that into a single sizing factor, and the example uses conservative illustrative factors of about 0.85 for cold performance and 0.85 for end-of-life.
Key fact 4. A sealed NiMH cell is 1.2 V nominal, roughly 1.45 to 1.50 V at the end of a full charge, and should not be discharged below about 0.9 to 1.0 V per cell; pack protection commonly disconnects at 0.85 to 0.9 V per cell to prevent cell reversal. Strings of 3, 4, 5, 6, 8 and 10 cells give 3.6, 4.8, 6.0, 7.2, 9.6 and 12.0 V nominal modules that match the low-voltage rails inside emergency luminaires, door controllers and amplifiers. Charging is terminated on -delta V, temperature or delta T/delta t, followed by a small trickle current of about 0.05 to 0.1 C.
Key fact 5. The distributed NiMH pack is not the train battery. The large train-level battery across the 110 V bus is classically a vented nickel-cadmium or lead-acid battery covered by the IEC 62973 series (IEC 62973-2 for nickel-cadmium, IEC 62973-3:2024 for lead-acid); the sealed NiMH modules in this article are the local batteries inside each emergency device, qualified under IEC 61951-2:2017+A1:2022, IEC 62133-1:2017 (the nickel-system part, note -1) and IEC 63115-2:2021, and shipped under UN 3496 with IATA Special Provision A199.
The first sizing artifact is a load schedule taken from the vehicle's emergency function list, not from a single number on a luminaire label. For each function the engineer records the normal power, the emergency (reduced) power, the supply voltage, whether the load is continuous or pulsed, and the pulse duration and repetition. Emergency LED luminaires are driven at a reduced current in emergency mode so that the mandated fraction of normal illuminance is met with the least energy; the control and monitoring electronics draw a small steady current; the door controller holds a standing current and draws a short pulse when a door is released or opened; the PA amplifier idles at a low quiescent current and peaks when the driver speaks; marker and side lamps are steady low-current loads; emergency ventilation, where fitted, is a heavy rotating load that is often duty-cycled to preserve the energy budget.
A duty block is the combination of a power and a time. The energy of a continuous load is its emergency power multiplied by the autonomy; the energy of a pulse is its pulse power multiplied by the number of pulses and the pulse duration. Summing the blocks gives the gross energy the module must deliver. The table below shows an illustrative single-zone emergency module - one luminaire group plus a share of control and door logic - and deliberately keeps the numbers round so the method is clear; a real carriage schedule has dozens of rows and the contract values replace these.
| Duty block (illustrative one-zone module) | Emergency power | Time over 90 min | Energy |
|---|---|---|---|
| Reduced emergency LED lighting (zone) | 6.0 W | 90 min continuous | 9.0 Wh |
| Control and monitoring electronics | 1.5 W | 90 min continuous | 2.25 Wh |
| Door control standing + opening pulses | 0.5 W stand + 18 W x 2 s x 4 doors | Standing + four short pulses | 0.79 Wh |
| PA / intercom (idle + short announcements) | 0.8 W idle + 12 W x 10 s x 6 | Idle + six announcements | 0.32 Wh |
| Marker / side lamps (shared) | 0.7 W | 90 min continuous | 1.05 Wh |
Summing the illustrative blocks gives roughly 13.4 Wh delivered over the 90-minute autonomy. The pulses are a small fraction of the energy but they set the peak-current rating of the cells, the tabs and the protection device, so they are never dropped just because their energy is small: a pack that has enough total energy but cannot deliver the door-opening pulse will still fail the evacuation sequence.
With the energy in hand, the nominal module voltage is chosen from the rail inside the device. Emergency luminaires and small control modules commonly use 3.6, 4.8, 6.0 or 7.2 V strings (3, 4, 5 or 6 NiMH cells), while door controllers and emergency amplifiers that need more power use 9.6 or 12 V (8 or 10 cells) or higher. The capacity required at that nominal voltage is the energy divided by the voltage. In the illustrative one-zone example, a 7.2 V nominal module needs 13.4 Wh / 7.2 V = 1.86 Ah of delivered capacity before margins; the same energy at 4.8 V would be 2.79 Ah and at 9.6 V would be 1.40 Ah. The choice of voltage is therefore a decision about cell size and current, not just a wiring detail: a higher module voltage lowers the current for the same power, reducing I-squared-R losses and the peak-current demand on each cell.
The average emergency current is the continuous power divided by the average discharge voltage, but the pack must also be checked against the end-voltage. A NiMH string does not hold 1.2 V per cell all the way down; the terminal voltage sags under load and falls through the discharge, and the LED driver or DC/DC must keep working down to the cut-off of the lowest permitted string voltage. The engineer verifies that the load electronics operate from the fully-charged string voltage (about 1.45 to 1.5 V per cell, so roughly 8.7 to 9.0 V for a 6-cell pack) down to the cut-off (about 0.9 to 1.0 V per cell, so roughly 5.4 to 6.0 V), and sizes the downstream converter to that window rather than to the nominal 7.2 V.
Figure 1. The sizing and design sequence for a distributed carriage emergency module (illustrative). The factors and order follow common emergency-battery practice; the contract specification and the standards edition on the vehicle certificate set the binding values.
The nameplate capacity of a cell is measured at 20 C on a fresh cell at a defined discharge rate, which is exactly the condition the battery will not see during a real winter evacuation. Three reductions are applied before the pack is declared adequate. The first is temperature: at the cold end of the EN 50155 operating class - -25 C for OT3 equipment, -40 C for OT2/OT4/OT6 - the available capacity and the deliverable voltage under load are both lower, so the design uses a cold-temperature factor based on the cell manufacturer's discharge curves at the actual emergency current. The second is end-of-life: emergency-luminaire practice assumes a multi-year design life and sizes against the capacity still available at the end of it, because a 90-minute test must still pass in the final year of service; the illustration here uses a conservative 0.85 factor. The third is depth of discharge: limiting how deeply the string is discharged reduces stress and guards against a weak cell being driven into reversal, and the design carries a residual margin rather than planning to drain the pack to the protection threshold.
These factors compound, so they are multiplied rather than added. In the illustrative example the 7.2 V module must deliver 1.86 Ah; applying a cold factor of 0.85, an end-of-life factor of 0.85 and a depth-of-discharge allowance of 0.90 gives a required nameplate capacity of 1.86 / (0.85 x 0.85 x 0.90) = 1.86 / 0.650 = 2.86 Ah, which rounds up to a standard 3.0 Ah 6-cell pack. The same chain at 4.8 V lands near 4.3 Ah and at 9.6 V near 2.1 Ah. These numbers are illustrative and the factors must be replaced by the chosen cell's real cold and ageing data, but the method - gross delivered energy, divide by voltage, then divide by the compounded margin - is what keeps the module meeting the full autonomy at the end of a cold night in the last year of service.
| Sizing stage (illustrative 7.2 V module) | Value | Basis |
|---|---|---|
| Gross energy over 90 min | 13.4 Wh | Sum of duty blocks above |
| Delivered capacity at 7.2 V nominal | 1.86 Ah | 13.4 Wh / 7.2 V |
| Cold factor (illustrative) | 0.85 | Cell cold discharge curves at the OT class cold end |
| End-of-life factor (illustrative) | 0.85 | Multi-year design-life, EOL capacity still passes the test |
| Depth-of-discharge allowance (illustrative) | 0.90 | Residual margin to protect the weakest cell |
| Required nameplate capacity | 2.86 Ah -> select 3.0 Ah | 1.86 / (0.85 x 0.85 x 0.90); round up to a standard cell |
Once the capacity and voltage are set, the cell format is chosen to deliver both the average current and the pulse current with margin. A sealed NiMH high-rate or industrial cell in an A, sub-C, C or welded-button format can be specified to the discharge rate the door and PA pulses require, with cells in parallel added where the capacity or pulse current exceeds one cell and cells in series set the voltage. The inter-cell connectors are welded tabs sized for the pulse current and the EN 61373 vibration profile; spot-welded tabs and strain-relieved interconnects survive the body-mounted (category 1, with category 2 class B as the default reference) random-vibration and shock tests where soldered or hand-crimped joints fail.
The protection is sized to the chemistry rather than borrowed from a lithium design. Sealed NiMH does not need a lithium-style balancing BMS, but a distributed emergency pack still carries a thermal cut-off or PTC element against external short and abnormal charge, a series protection FET or relay that disconnects at the pack cut-off voltage (about 0.85 to 0.9 V per cell) to prevent cell reversal in a deep discharge, reverse-polarity protection at the module terminals, and, on multi-cell strings, attention to cell matching so one weak cell is not reverse-charged by the others at the end of a long discharge. Because a failed protection device must never silently extinguish an emergency light, the protective elements are selected as fail-safe components and the module is designed to be testable on the vehicle's routine test switch.
A distributed emergency module spends almost all of its life on standby, so the charge strategy is a design driver. Sealed NiMH chargers terminate the main charge on -delta V, on the rate of temperature rise (delta T/delta t) or on an absolute temperature cut-off, and then switch to a small trickle current of about 0.05 to 0.1 C, or to a timed pulse/maintenance charge, which keeps the pack full without the overcharge stress that shortens float life. The charger is fed from the carriage bus through an EN 50155-qualified DC/DC converter, so it must keep the charge window across the full continuous range of nominal minus 30% to plus 25% and ride the transient and extended overvoltage without overcharging; the 5% DC ripple on the bus must not induce micro-cycling of the cells.
The transfer arrangement has two distinct jobs. The very short events - the EN 50155 change-over dip of 40% for up to 100 ms and the complete interruptions of 10 ms (S2, default) or 20 ms (S3) - are bridged by hold-up capacitance in the converter so that the controller never resets, exactly as railway DC/DC practice specifies with added hold-up capacitors. The long event - a genuine loss of the main supply lasting the full autonomy - is handled by the NiMH pack, which is switched in with no visible dark interval, typically when the supply falls below about 0.6 times nominal for more than roughly half a second, matching the UIC 555 change-over threshold. Designing the hold-up and the battery transfer separately avoids two common failures: resetting electronics on every short dip, and cycling the battery on millisecond glitches that the capacitor should absorb.
Figure 2. Illustrative string terminal voltage over the 90-minute emergency discharge at the cold end of life, with the cut-off and the sized capacity margin shown. The curve is qualitative and not to scale; the real profile follows the chosen cell's published discharge curves at the emergency current and the load converter's shut-down voltage.
Designers sometimes ask why a distributed emergency module uses sealed NiMH when the train-level battery is something else. The four chemistries occupy different niches. The traditional train battery is a vented nickel-cadmium battery under IEC 62973-2, valued for very high mechanical and thermal robustness and a long record in rail, but it contains cadmium and needs maintenance and ventilation. Valve-regulated lead-acid (IEC 62973-3:2024) is low cost but is heavier, weakens in the cold, sulphates after long partial states of charge, and suits larger blocks rather than small sealed modules. Lithium chemistries offer high energy density but bring cell-level monitoring, the UN 38.3 test summary and UN 3480/3481 shipping controls. Sealed NiMH sits in the middle: cadmium-free, tolerant of long float and occasional deep discharge, capable of useful cold capacity, simple to ship under UN 3496 with IATA A199, and well matched to the small distributed packs in luminaires and controllers. It is not chosen for energy density; it is chosen for readiness, cold performance, float life and the simplest compliant logistics path.
| Attribute | Sealed NiMH (distributed module) | Vented NiCd (train battery) | VRLA lead-acid | Lithium |
|---|---|---|---|---|
| Nominal cell voltage | 1.2 V | 1.2 V | 2.0 V | 3.2 to 3.7 V by type |
| Cadmium / heavy metal | Cadmium-free | Contains cadmium | Lead | No cadmium |
| Cold and float readiness | Good cold; tolerates long float | Very robust; needs maintenance | Weakens cold; sulphates at partial SOC | Good, but needs monitoring |
| Transport classification | UN 3496 / IATA A199 | NiCd battery rules | Lead-acid rules | UN 38.3; UN 3480/3481 |
| Typical role in this series | Distributed local emergency modules | Train-level 110 V battery | Larger train or auxiliary blocks | High-density packs with BMS |
The electrical sizing is necessary but not sufficient. The pack and its enclosure are designed for the mounting location under EN 61373 - body mounted (category 1), bogie mounted (category 2) or axle mounted (category 3) - with the welded tabs, connectors and internal components surviving the matching random-vibration and shock spectra. EMC is designed against EN 50121-3-2, which means the charger and protection electronics withstand fast transients of plus or minus 2 kV (5/50 ns, 5 kHz), surges of plus or minus 2 kV line to ground and plus or minus 1 kV line to line (1.2/50 microsecond), and radio-frequency common-mode immunity up to 80 MHz. Fire behaviour follows EN 45545-2 for the plastics, labels, insulation and enclosure around the pack, and EN 45545-6 for the fire-control measures; ingress protection follows EN 60529 for the luminaire or cabinet environment. The routine maintenance design - the manual test switch, the status indicator and a replaceable module that does not require dismantling the luminaire - is what allows the operator to demonstrate readiness over the service life, and the life classes L1 to L4 in EN 50155 (5 to 20 years, with L4 the default) frame the expected service duration.
List each emergency load and its reduced emergency power, turn each into a duty block (power multiplied by time, with pulses counted separately), sum the energy over the required autonomy, divide by the module nominal voltage to get the delivered ampere-hour demand, then divide by the compounded cold, end-of-life and depth-of-discharge factors. The illustrative 7.2 V module in this article needs 1.86 Ah delivered and about 2.86 Ah nameplate, rounded to 3.0 Ah; the factors must be replaced by the chosen cell's real cold and ageing data.
UIC 555 requires at least 50% of normal illuminance in the aisle and 30% in seated areas for a minimum of 90 minutes, with automatic change-over at about 0.6 times nominal for more than half a second. EN 13272-1:2019 adds at least 10 minutes of general lighting for high-speed and long-distance units before emergency mode; EN 13272-2:2019 sets a minimum average of 5 lx along the escape-route centre line and 40 lx at the access step for urban rail.
Six cells in series give 7.2 V nominal. A cell is 1.2 V nominal, about 1.45 to 1.50 V when fully charged, and should not be discharged below about 0.9 to 1.0 V; the load converter must work from roughly 8.7 to 9.0 V (full) down to 5.4 to 6.0 V (cut-off) for a 6-cell string, and protection disconnects at about 0.85 to 0.9 V per cell to prevent cell reversal.
The main charge is terminated on -delta V, temperature or delta T/delta t, followed by a small trickle current of about 0.05 to 0.1 C or a timed maintenance charge. The charger is fed from the carriage bus through an EN 50155-qualified DC/DC that holds the charge window across the full bus range; hold-up capacitors bridge the 10 ms (S2) or 20 ms (S3) interruptions and the change-over dip, while the NiMH pack takes over genuine long outages.
Sealed NiMH is cadmium-free, keeps useful cold capacity, tolerates long float and occasional deep discharge, and ships under UN 3496 with IATA A199, avoiding the UN 38.3 and UN 3480/3481 lithium controls. Lead-acid is heavier and sulphates at partial states of charge, while vented NiCd is the train-level battery under IEC 62973-2 and contains cadmium; NiMH is selected for the small distributed local modules, not to replace the train battery.
The cells are documented under IEC 61951-2:2017+A1:2022 (performance), IEC 62133-1:2017 (nickel-system safety), IEC 63115-2:2021 (industrial NiMH) and IEC 61959; the module meets EN 50155 for the electrical envelope and temperature class, EN 61373 for vibration and shock, EN 50121-3-2 for EMC and EN 45545-2/-6 for fire, with transport under UN 3496 / IATA A199. Paper C follows these through the type tests and certification.
Sizing duty and autonomy: UIC 555 (Electric lighting in passenger rolling stock, with Amendment 7:2013), including the 50% aisle / 30% seated-area emergency illuminance, the 90-minute minimum and the change-over threshold; EN 13272-1:2019 (heavy rail, including the 10-minute general-lighting requirement for high-speed and long-distance units after loss of main supply) and EN 13272-2:2019 (urban rail, including the 5 lx floor-level escape-route centre-line minimum and 40 lx at the vehicle access step); 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). Electrical envelope: EN 50155:2017 (consolidated through the 2021 edition) for the nominal DC voltages, continuous and transient ranges, supply change-over and interruption classes S1/S2/S3 and the temperature classes OT1 to OT6 and life classes L1 to L4; EN 50163; and railway DC/DC converter practice using hold-up capacitors for the S2 10 ms and S3 20 ms ride-through.
Cell, protection and transport references: IEC 61951-2:2017+A1:2022 for sealed NiMH cell performance, IEC 62133-1:2017 for sealed nickel-containing cell safety (the -1 part), IEC 63115-2:2021 for industrial NiMH and IEC 61959 for mechanical operation; the IEC 62973 series for rolling-stock auxiliary batteries (IEC 62973-2 nickel-cadmium, IEC 62973-3:2024 lead-acid) and GB/T 21413.1 for general rolling-stock equipment conditions; EN 61373 for shock and vibration, EN 50121-3-2 for apparatus EMC and EN 45545-2/-6 for fire behaviour, with EN 60529 for ingress protection; UN 3496 and IATA Special Provision A199 for transport. The cell voltage, trickle-current and cut-off values reflect common sealed-NiMH practice and representative cell data sheets; the per-cell cold, end-of-life and depth-of-discharge factors and all duty-block wattages, capacities and timings in this article are illustrative engineering examples, and the binding values are those in the vehicle contract specification and the cited standards editions.
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 - sized to the 90-minute autonomy with the cold, end-of-life and depth-of-discharge margins built in, welded-tab and fail-safe protection construction for the EN 61373 vibration profile, EN 50121-3-2 and EN 45545-2 support, and 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. Send us your bus nominal voltage, emergency load schedule, required autonomy, cold-end temperature class and charging arrangement, and we will work through the cell-count, capacity and charge design and support the cold-autonomy and float-life evidence before the type test. See the Weijiang NiMH product range for standard pack formats, or contact our OEM team with your vehicle specification and emergency load list.
