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IEC 62928: Railway Applications — Rolling Stock — Onboard Lithium-Ion Traction Batteries
Comprehensive requirements for design, testing, and safety of Li-ion battery systems in railway rolling stock
IEC 62928, published in 2017, defines the design, performance, and safety requirements for onboard lithium-ion traction battery systems installed in railway rolling stock. As the railway industry accelerates its transition toward cleaner, more flexible energy solutions, Li-ion battery technology has emerged as a pivotal enabler for catenary-free urban tramways, hybrid multiple-unit trains, and emergency traction power applications. The standard establishes a unified qualification framework that addresses the unique challenges of the railway operating environment, including extreme vibration, wide temperature ranges, high transient currents, and stringent fire safety requirements.
The scope of IEC 62928 covers battery systems with a nominal voltage exceeding 60 V DC, intended for traction power and onboard auxiliary services. It applies to cells, battery modules, battery trays, and the complete battery system including the battery management system (BMS), thermal management system, and enclosure. The standard classifies railway applications into three categories based on power demand and operational profile: urban rail (frequent starts/stops, short distances), commuter rail (moderate speed, medium distances), and mainline rail (high speed, long distances). Each category imposes distinct requirements on the battery system design in terms of power density, energy capacity, and cycle life.
IEC 62928 represents a significant milestone as the first international standard specifically addressing Li-ion batteries in railway traction applications. It bridges the gap between general battery safety standards (IEC 62660 series for automotive) and railway-specific rolling stock standards (IEC 60077 series for electrical equipment), providing a dedicated qualification path for railway battery systems.
Battery System Design and Safety Requirements
The standard mandates a multi-level safety approach spanning cell, module, and system levels. At the cell level, lithium-ion cells must pass a comprehensive set of abuse tests including overcharge, external short circuit, thermal abuse, mechanical crush, and nail penetration. Cells intended for railway applications must achieve a minimum energy density target of 120 Wh/kg at the module level while maintaining thermal stability up to 150 deg C without thermal runaway propagation. For battery modules, the standard requires that a thermal runaway initiated in any single cell must not propagate to adjacent cells — a requirement known as “no propagation” or “cell-to-cell propagation prevention.” This is validated through a forced thermal runaway test where one cell is deliberately heated until it enters thermal runaway while monitoring the temperature of all neighboring cells.
The battery management system (BMS) is considered a safety-critical component and must provide at least the following protection functions: overvoltage protection (per-cell voltage monitoring with trip thresholds defined by cell chemistry), undervoltage protection, overcurrent protection (both charge and discharge), short-circuit detection with hardware-based fast interruption (response time below 100 microseconds), overtemperature protection at multiple locations within the battery pack, insulation monitoring (per IEC 61557-8 for IT systems), and state-of-charge (SOC) and state-of-health (SOH) estimation with defined accuracy requirements. The BMS must achieve a Safety Integrity Level (SIL) of at least SIL 2 per IEC 61508 for protection functions that could lead to a hazardous event if they fail. For systems installed in passenger-carrying vehicles, SIL 3 is required for critical functions such as overvoltage and overtemperature protection.
| Protection Function | Response Time | Required SIL | Test Verification |
|---|---|---|---|
| Overvoltage protection | < 100 ms | SIL 2 (SIL 3 passenger) | Injection test at 110% of max voltage |
| Undervoltage protection | < 100 ms | SIL 2 | Discharge to minimum voltage + margin |
| Short-circuit protection | < 100 μs (hardware) | SIL 3 | Direct short with pre-charged battery |
| Overtemperature protection | < 1 s | SIL 2 (SIL 3 passenger) | Heater element test at Tmax + 5 deg C |
| Insulation monitoring | < 10 s | SIL 2 | Calibrated leakage resistor injection |
Thermal runaway prevention is the single most critical safety concern for railway Li-ion battery systems. IEC 62928 requires demonstration that the battery system can withstand a “worst-case” single point failure — including internal cell short circuit, BMS failure, or cooling system failure — without leading to fire or explosion. The forced propagation test is conducted at system level with all adjacent modules and the enclosure in place, as thermal propagation paths can involve heat transfer through busbars, enclosure walls, and cooling system ducts.
Performance Testing and Environmental Qualification
The standard specifies an extensive test program covering electrical, mechanical, and climatic qualification. Electrical performance tests include capacity determination at multiple discharge rates (C/3, 1C, and maximum rated current), power capability testing at various states of charge, cycle life testing (minimum 2,000 cycles to 80% residual capacity for urban rail applications, 5,000 cycles for light rail with frequent regenerative braking), and energy efficiency measurement over a full charge-discharge cycle at 25 deg C with a minimum requirement of 90% round-trip efficiency. For mainline applications where batteries serve primarily as backup or peak-shaving devices, the cycle life requirement may be reduced to 1,000 cycles but calendar life must extend to at least 10 years.
Mechanical testing is particularly demanding for railway applications. The standard mandates vibration testing per IEC 61373 (railway equipment shock and vibration tests) Category 1 Class B for body-mounted battery systems, which includes random vibration in all three axes at RMS acceleration levels up to 1.34 m/s² in the vertical axis over a frequency range of 5-150 Hz. For underframe-mounted battery systems, the more stringent Category 1 Class A applies with RMS acceleration up to 5.4 m/s² and includes 5,000-cycle endurance testing. Shock testing requires 10 g half-sine pulses in each axis, simulating coupler forces and track irregularities. The battery enclosure must also pass a 15-minute fire resistance test per EN 45545-2 (railway fire protection) demonstrating that the enclosure prevents the spread of fire from the battery compartment to the passenger cabin for at least 15 minutes.
| Test Type | Standard Reference | Conditions | Acceptance Criterion |
|---|---|---|---|
| Vibration (body-mounted) | IEC 61373 Cat 1 Cl. B | 5-150 Hz, 1.34 m/s² RMS, 5 h/axis | No performance degradation, no loose parts |
| Vibration (underframe) | IEC 61373 Cat 1 Cl. A | 5-150 Hz, 5.4 m/s² RMS, 15 h/axis | No structural failure, capacity > 90% initial |
| Thermal cycling | IEC 60068-2-14 | -40 deg C to +85 deg C, 5 cycles | No leakage, no cracking, full function after test |
| Salt fog (corrosion) | IEC 60068-2-52 | Severity 3 (7 days exposure) | No corrosion affecting safety or function |
| Fire resistance | EN 45545-2 | 15 min flame exposure at 800 deg C | No flame penetration, limited heat release |
| Altitude/decompression | IEC 60068-2-13 | Equivalent to 4,000 m altitude | No leakage, no venting, correct function |
A well-designed railway Li-ion battery system per IEC 62928 delivers 8-12 years of service life in urban rail applications, withstanding over 10,000 charge-discharge cycles from regenerative braking daily. The combination of robust mechanical design, active thermal management, and multi-layer safety architecture ensures reliable operation under the most demanding conditions, from arctic winter temperatures to tropical heat and humidity.
Engineering Design Insights for Railway Battery Integration
From a system integration perspective, several design considerations deserve special attention. First, thermal management system design must account for the wide ambient temperature range encountered in railway operation (-40 deg C to +55 deg C for most applications). Active liquid cooling with a water-glycol mixture is typically required for high-power urban rail applications where charge and discharge rates can exceed 3C during acceleration and regenerative braking. The cooling system must maintain cell temperatures within the optimal operating window of 15-35 deg C and ensure that the maximum temperature difference between any two cells in the battery system does not exceed 5 deg C to prevent uneven aging and capacity imbalance. Air cooling may be acceptable for lower-power applications, but the standard requires demonstrated performance at ambient temperatures up to 55 deg C with solar load on the roof-mounted enclosure.
Second, the mechanical integration of the battery system into the vehicle structure requires careful analysis of the vehicle crashworthiness. The battery enclosure must be designed as a load-bearing structural element in some installations or must be demonstrated not to compromise the vehicle’s crash energy management. The standard requires that battery mounting points withstand acceleration forces of up to 5 g in the longitudinal direction (emergency braking), 3 g laterally, and 2 g vertically simultaneously without permanent deformation of the mounting structure. The battery enclosure must also meet IP 65 ingress protection per IEC 60529 for underframe-mounted systems to withstand water spray from track washing and rain, and IP 67 for roof-mounted systems that may be fully submerged during extreme weather events.
Third, the electrical integration must consider the interaction between the battery system and the existing traction system. The battery DC-DC converter or direct traction coupling must maintain stable bus voltage during all operating modes, including transition between powering and regenerative braking. The standard recommends that the battery system provide grid-forming capability for catenary-free operation, where the battery system serves as the primary energy source for traction through a DC link voltage of typically 600-750 V for trams and 1,500-3,000 V for mainline applications. For hybrid applications where the battery operates in parallel with a diesel engine or fuel cell, the power management strategy must be validated through hardware-in-the-loop (HIL) simulation before vehicle integration, covering at least 20 different failure scenarios including sensor faults, actuator faults, and communication loss between the battery management system and the vehicle control unit.
Fourth, the battery system must include provisions for diagnostic monitoring over the life of the vehicle. The BMS must record and store key operational data including charge/discharge cycles, depth of discharge, maximum and minimum cell voltages, maximum temperature, and cumulative energy throughput. This data is essential for predictive maintenance and for determining the optimal time for battery replacement. The standard requires a minimum data storage capacity of 10 years of operational data and that the data be retrievable via standard diagnostic interfaces (typically MVB or Ethernet-based train communication networks per IEC 61375). Additionally, the battery system should support remote health monitoring and over-the-air BMS firmware updates to extend service life and adapt to changing operational patterns without requiring the vehicle to return to the depot.
| Application | Energy Capacity | Peak Power | Voltage Range | Cycle Life | Cooling Method |
|---|---|---|---|---|---|
| Urban tram (catenary-free) | 50-200 kWh | 300-600 kW | 600-750 V DC | > 5,000 cycles | Liquid cooling (water-glycol) |
| Light rail / metro | 30-100 kWh | 200-500 kW | 600-750 V DC | > 8,000 cycles | Liquid or forced air cooling |
| Hybrid multiple unit (DMU) | 100-500 kWh | 500-1,500 kW | 1,500-3,000 V DC | > 2,000 cycles | Liquid cooling (dielectric fluid) |
| Mainline locomotive (assist) | 200-1,000 kWh | 1-4 MW | 3,000 V DC | > 1,000 cycles | Liquid cooling + immersion cooling |
Q1: What is the difference between IEC 62928 and IEC 62660 for automotive Li-ion batteries?
A: IEC 62660 focuses on automotive traction batteries for road vehicles and addresses different mechanical stress profiles (road vibration, crash impact), while IEC 62928 addresses railway-specific requirements including rail vibration per IEC 61373, train fire protection per EN 45545-2, and railway EMC per IEC 62236. Additionally, IEC 62928 requires higher SIL levels for BMS safety functions and addresses catenary-free operation which is unique to rail applications.
A: IEC 62660 focuses on automotive traction batteries for road vehicles and addresses different mechanical stress profiles (road vibration, crash impact), while IEC 62928 addresses railway-specific requirements including rail vibration per IEC 61373, train fire protection per EN 45545-2, and railway EMC per IEC 62236. Additionally, IEC 62928 requires higher SIL levels for BMS safety functions and addresses catenary-free operation which is unique to rail applications.
Q2: Can NMC (Nickel Manganese Cobalt) batteries be used in railway applications per IEC 62928?
A: Yes, NMC cells are commonly used for railway traction batteries due to their high energy density (200-260 Wh/kg at cell level). However, the standard also permits LFP (Lithium Iron Phosphate) cells which offer lower energy density (140-180 Wh/kg) but superior thermal stability and longer cycle life. The choice depends on the application requirements: NMC for weight-sensitive mainline trains where energy density is critical, LFP for urban rail where cycle life and safety are prioritized. The standard is chemistry-neutral — any lithium-ion chemistry can be qualified provided it passes all required abuse and performance tests.
A: Yes, NMC cells are commonly used for railway traction batteries due to their high energy density (200-260 Wh/kg at cell level). However, the standard also permits LFP (Lithium Iron Phosphate) cells which offer lower energy density (140-180 Wh/kg) but superior thermal stability and longer cycle life. The choice depends on the application requirements: NMC for weight-sensitive mainline trains where energy density is critical, LFP for urban rail where cycle life and safety are prioritized. The standard is chemistry-neutral — any lithium-ion chemistry can be qualified provided it passes all required abuse and performance tests.
Q3: What is the minimum cycle life required by IEC 62928?
A: The standard requires a minimum of 2,000 cycles to 80% residual capacity for most railway applications. For urban rail systems with frequent regenerative braking that undergo multiple partial cycles daily, the standard recommends 5,000-8,000 cycles. The end-of-life criterion is defined as the point when the battery can no longer deliver 80% of its rated capacity at the C/3 discharge rate at 25 deg C.
A: The standard requires a minimum of 2,000 cycles to 80% residual capacity for most railway applications. For urban rail systems with frequent regenerative braking that undergo multiple partial cycles daily, the standard recommends 5,000-8,000 cycles. The end-of-life criterion is defined as the point when the battery can no longer deliver 80% of its rated capacity at the C/3 discharge rate at 25 deg C.
Q4: How does the standard address second-life or repurposed batteries?
A: IEC 62928 is primarily intended for new battery systems. For second-life batteries (e.g., repurposed automotive batteries), additional testing is required to validate residual capacity, impedance, self-discharge rate, and safety characteristics. The standard notes that second-life batteries must meet all the same performance and safety requirements as new batteries, including the full type test program. The BMS parameters must be recalibrated for the second-life application, and the battery system must be clearly labeled as containing repurposed cells.
A: IEC 62928 is primarily intended for new battery systems. For second-life batteries (e.g., repurposed automotive batteries), additional testing is required to validate residual capacity, impedance, self-discharge rate, and safety characteristics. The standard notes that second-life batteries must meet all the same performance and safety requirements as new batteries, including the full type test program. The BMS parameters must be recalibrated for the second-life application, and the battery system must be clearly labeled as containing repurposed cells.
