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IEC 62973-1: Railway Applications — Rolling Stock — Batteries for Auxiliary Power Supply Systems
Requirements and test methods for secondary batteries used in railway rolling stock auxiliary power systems
IEC 62973-1, published in 2018 by IEC Technical Committee 9 (Electrical Equipment and Systems for Railways), specifies requirements and test methods for secondary batteries installed in railway rolling stock for auxiliary power supply systems. As modern trains increasingly rely on electrical systems for essential functions including lighting, HVAC, door control, train control and monitoring systems (TCMS), emergency lighting, and communication systems, the reliability and performance of onboard batteries have become critical for both operational availability and passenger safety.
The standard addresses battery systems with rated voltages up to 150 V DC, covering the three primary battery technologies used in railway applications: vented nickel-cadmium (Ni-Cd), valve-regulated lead-acid (VRLA), and lithium-ion (Li-ion) batteries. Each technology presents distinct characteristics in terms of energy density, power capability, cycle life, maintenance requirements, safety behavior, and operational temperature range. IEC 62973-1 provides a unified framework for qualifying any of these technologies for railway service while acknowledging their specific characteristics through technology-specific test requirements and acceptance criteria.
Railway auxiliary batteries serve critical safety functions including emergency lighting for at least 45 minutes (per TSI PRM requirements), emergency door release, train radio communication backup, and control voltage stabilization during main power interruptions. Unlike automotive starter batteries that experience brief high-current discharges, railway batteries must support sustained loads for extended periods during both normal operation and emergency scenarios, often with deep discharge cycles exceeding 60% depth of discharge.
Battery Technology Requirements and Selection
The standard classifies railway auxiliary batteries according to their application type: Type A for emergency-only backup (typically sized for 30 minutes to 2 hours of emergency operation), Type B for combined emergency and intermittent loads (the most common configuration in modern rolling stock), and Type C for continuous auxiliary power including peak load shaving. This classification directly impacts the required battery capacity, discharge rate capability, and cycle life expectations. For Type B systems, the battery must support both the emergency load during main power loss and provide power smoothing for auxiliary loads such as HVAC compressors, air conditioning fans, and door operating mechanisms during normal operation.
Ni-Cd batteries have historically been the dominant technology in railway rolling stock due to their excellent performance at low temperatures, high mechanical robustness, tolerance to overcharge and overdischarge, and very long service life (15-20 years in many applications). However, the memory effect and the need for periodic electrolyte maintenance have driven interest in alternative technologies. VRLA batteries offer lower initial cost and maintenance-free operation but have significantly shorter service life (3-5 years in railway service) and reduced performance at low temperatures. Li-ion batteries, particularly lithium iron phosphate (LFP) and lithium nickel manganese cobalt oxide (NMC) chemistries, have gained significant adoption in recent years, offering 3-5x higher energy density than Ni-Cd (120-180 Wh/kg vs 30-50 Wh/kg), higher efficiency (95%+ vs 70-80%), and substantially longer cycle life in partial state-of-charge operation.
| Parameter | Ni-Cd (Vented) | VRLA (Lead-Acid) | Li-ion (LFP/NMC) |
|---|---|---|---|
| Energy density | 30 – 50 Wh/kg | 30 – 40 Wh/kg | 120 – 180 Wh/kg |
| Cycle life (80% DoD) | 1,500 – 2,500 | 300 – 600 | 3,000 – 8,000 |
| Operating temp. range | -20 deg C to +50 deg C | -10 deg C to +45 deg C | -20 deg C to +55 deg C |
| Charge efficiency | 70 – 80% | 75 – 85% | 95 – 99% |
| Self-discharge per month | 5 – 15% | 3 – 8% | 1 – 3% |
| Maintenance requirement | Periodic electrolyte refill | None (sealed) | None (BMS monitored) |
| Typical service life | 15 – 20 years | 3 – 5 years | 8 – 15 years |
| Overcharge tolerance | Excellent | Moderate | Low (requires BMS protection) |
| Relative cost (per kWh) | Moderate | Low | High to Moderate |
Li-ion batteries for railway applications require a robust Battery Management System (BMS) that is not needed for Ni-Cd or VRLA technologies. The BMS must provide overvoltage, undervoltage, overcurrent, overtemperature, and short-circuit protection. IEC 62973-1 requires that the BMS maintain cell voltage balance within +/- 1% and provide continuous monitoring of state of charge (SOC), state of health (SOH), and insulation resistance. Failure of the BMS is a single-point failure risk that must be addressed through safety analysis per IEC 62278 (EN 50126).
Electrical Performance Testing
IEC 62973-1 specifies a comprehensive electrical test regime to verify battery performance under railway operating conditions. Capacity testing is conducted at the C-rate (the current that discharges the battery in 1 hour), at 25 deg C +/- 2 deg C, with acceptance criteria depending on the battery technology: minimum 100% of rated capacity for new Ni-Cd and Li-ion batteries, and 95% for VRLA batteries. High-rate discharge testing verifies the battery’s ability to deliver peak currents required for emergency operation, typically at 3-5 C-rate for Ni-Cd and Li-ion, and 2-3 C-rate for VRLA batteries, with voltage remaining above the minimum system operating voltage.
The standard specifies charge acceptance testing to verify that the battery can be recharged within the available charging time typical of railway operation, which may be as short as 2-4 hours for inter-city trains or longer for main line locomotives. For Ni-Cd batteries, the charge acceptance test typically requires the battery to reach 80% state of charge within 4 hours at the standard charge voltage. Li-ion batteries must accept a full recharge from 20% to 100% SOC within 3 hours using the recommended CC-CV charging profile, with the constant current phase typically at 0.3-0.5 C-rate and the constant voltage phase at the manufacturer-specified maximum voltage per cell (typically 3.65 V for LFP and 4.2 V for NMC).
| Test | Condition | Acceptance Criteria | Applicable Technology |
|---|---|---|---|
| Capacity test | C-rate discharge at 25 deg C | >= 100% rated (Ni-Cd, Li-ion), >= 95% (VRLA) | All |
| High-rate discharge | 3-5 C (Ni-Cd, Li-ion), 2-3 C (VRLA) | Voltage > Vmin throughout discharge | All |
| Charge acceptance | Standard charge regime, 25 deg C | >= 80% SOC in 4 h | Ni-Cd |
| Rapid recharge | CC-CV, recommended profile | >= 95% SOC in 3 h | Li-ion |
| Cycle life | 80% DoD, at operating temperature | >= specified cycles to 80% capacity | All |
| Self-discharge | 28 days storage at 25 deg C | Capacity retention >= 85% (Li-ion), >= 70% (Ni-Cd) | All |
| Insulation resistance | 500 V DC megger | >= 1 MOhm (dry), >= 0.1 MOhm (wet) | All |
The transition from Ni-Cd to Li-ion batteries in new rolling stock designs has resulted in weight savings of 60-70% for the same energy capacity. For a typical metro train with a 100 Ah / 110 V battery system, this represents a weight reduction from approximately 350 kg (Ni-Cd) to 120 kg (Li-ion), contributing to reduced energy consumption and increased passenger capacity. The higher efficiency of Li-ion charging (95%+ vs 75%) also reduces the load on the train’s auxiliary power converter.
Mechanical and Environmental Qualification
Railway batteries are subjected to extreme mechanical and environmental conditions that differ significantly from stationary or automotive applications. IEC 62973-1 mandates vibration testing per IEC 61373 (Railway applications – Equipment for rolling stock – Shock and vibration tests), with the battery mounted in its operational orientation on a vibration table. Test categories are specified according to the battery installation location: Category 1 for body-mounted equipment (3-5 Hz, 10-20 m/s² acceleration), and Category 2 for bogie-mounted equipment (more severe, 5-150 Hz, up to 30 m/s²). The battery must pass the test without mechanical damage, electrolyte leakage (for Ni-Cd), or capacity loss exceeding 5%.
Environmental testing includes temperature cycling from -40 deg C to +70 deg C, damp heat cycling (95% RH at 55 deg C), salt fog exposure for coastal and tunnel applications, and low air pressure testing for high-altitude operations. For Li-ion batteries, the standard also requires thermal runaway propagation testing to verify that a failure in one cell does not propagate to adjacent cells – a critical safety requirement given the high energy density of Li-ion chemistry. The test is performed by initiating thermal runaway in one cell (typically through nail penetration or localized heating) and observing whether adjacent cells enter thermal runaway within a 1-hour observation period. A successful result is defined as no propagation to adjacent cells, no explosion, and no fire beyond the initiating cell compartment.
Thermal runaway propagation is the most critical safety concern for Li-ion batteries in railway applications. A single cell failure in a 100-cell battery pack can release sufficient thermal energy to heat adjacent cells to their thermal runaway threshold. IEC 62973-1 requires cell-to-cell propagation resistance for a minimum of 15 minutes (allowing the train to reach a safe stopping point) with a growing industry trend toward &quto;no propagation&quto; (0-minute propagation) as the preferred safety target. Thermal barrier materials between cells and advanced BMS thermal monitoring are essential design elements for meeting these requirements.
Engineering Design Insights for Railway Auxiliary Power Systems
From a system engineering perspective, the integration of batteries into railway auxiliary power systems requires careful consideration of multiple factors. The charging system must be designed to match the specific requirements of the chosen battery technology. Ni-Cd batteries require a constant current-constant voltage (IU) charging profile with temperature-compensated voltage regulation, typically at 1.45-1.55 V per cell at 25 deg C with a temperature coefficient of -4 mV/deg C/cell. Li-ion batteries require a more precisely controlled CC-CV profile with cell voltage monitoring, and the charger must communicate with the BMS to implement charge current limiting, temperature-based charge termination, and equalization charging.
System voltage design must account for the different voltage characteristics of each battery technology. A nominal 110 V DC system (the most common in European rolling stock) requires approximately 89 Ni-Cd cells (1.2 V nominal), 55 VRLA cells (2.0 V nominal), or 30 Li-ion LFP cells in series (3.2 V nominal). The charging voltage range and the minimum system operating voltage must be carefully coordinated to ensure that all auxiliary loads receive their specified supply voltage throughout the battery discharge cycle. Modern trains increasingly use DC-DC converters to provide regulated voltage buses regardless of battery state of charge, decoupling the battery voltage from the auxiliary load requirements and allowing greater flexibility in battery technology selection.
Thermal management system design for Li-ion battery enclosures is particularly critical. Railway battery compartments are often located in enclosed spaces under the train floor or in equipment cabinets with limited ventilation. The thermal management system must maintain cell temperatures within the specified operating range (-20 deg C to +55 deg C for LFP cells) under all ambient conditions and duty cycles. Active cooling using forced air or liquid cooling may be required for high-power applications, while passive cooling with phase-change materials or heat sinks may suffice for lower-power systems. The system must also include provisions for cold-weather operation, as Li-ion batteries have significantly reduced performance below 0 deg C and cannot be charged below this temperature without the risk of lithium plating and internal short circuits.
Q1: What is the typical useful life of a railway auxiliary battery?
A: Service life varies significantly by technology: Ni-Cd batteries typically last 15-20 years with proper maintenance, VRLA batteries 3-5 years, and Li-ion batteries 8-15 years depending on operating conditions and cycle regime. Life is primarily determined by operating temperature, depth of discharge, and charge control accuracy.
A: Service life varies significantly by technology: Ni-Cd batteries typically last 15-20 years with proper maintenance, VRLA batteries 3-5 years, and Li-ion batteries 8-15 years depending on operating conditions and cycle regime. Life is primarily determined by operating temperature, depth of discharge, and charge control accuracy.
Q2: Can different battery technologies be mixed in the same railway vehicle?
A: IEC 62973-1 does not prohibit mixing technologies, but it is not recommended due to different charging requirements, voltage characteristics, and aging behaviors. If multiple battery technologies are used (e.g., Ni-Cd for engine starting and Li-ion for auxiliary power), they must be on separate electrical circuits with dedicated charging systems.
A: IEC 62973-1 does not prohibit mixing technologies, but it is not recommended due to different charging requirements, voltage characteristics, and aging behaviors. If multiple battery technologies are used (e.g., Ni-Cd for engine starting and Li-ion for auxiliary power), they must be on separate electrical circuits with dedicated charging systems.
Q3: What maintenance is required for railway batteries under IEC 62973-1?
A: Maintenance requirements are technology-dependent. Ni-Cd batteries require periodic electrolyte level checking and refill (every 3-6 months), cleaning, and equalization charging. VRLA and Li-ion batteries are maintenance-free but require periodic visual inspection, capacity checks (annually), and BMS data review to detect developing issues before they cause failures.
A: Maintenance requirements are technology-dependent. Ni-Cd batteries require periodic electrolyte level checking and refill (every 3-6 months), cleaning, and equalization charging. VRLA and Li-ion batteries are maintenance-free but require periodic visual inspection, capacity checks (annually), and BMS data review to detect developing issues before they cause failures.
Q4: How is battery state of health monitored during service?
A: SOH is primarily determined through periodic capacity tests (typically annual), supplemented by continuous monitoring of internal resistance, self-discharge rate, and charge acceptance efficiency by the BMS. For Li-ion batteries, incremental capacity analysis (ICA) and differential voltage analysis (DVA) techniques can detect cell degradation modes including loss of active material, lithium inventory loss, and impedance increase before they cause significant capacity fade.
A: SOH is primarily determined through periodic capacity tests (typically annual), supplemented by continuous monitoring of internal resistance, self-discharge rate, and charge acceptance efficiency by the BMS. For Li-ion batteries, incremental capacity analysis (ICA) and differential voltage analysis (DVA) techniques can detect cell degradation modes including loss of active material, lithium inventory loss, and impedance increase before they cause significant capacity fade.
