Battery Pack Safety, Battery Standards, and Tests

Lithium-ion battery (LIB) safety is a major concern for designers. LIBs are generally safe to use if they have been properly manufactured and are integrated correctly into systems. However, the use of sub-standard materials and the possibility of manufacturing or design defects can result in hazardous conditions. As a result, there are many global safety standards related to LIBs. This FAQ first looks at the potential for, and consequences of, thermal runaway in LIBs, it looks at the most-commonly-cited LIB safety standards, it reviews the non-standard nature of so-called “nail penetration” testing, looks at the hazard levels associated with LIBs in electric vehicles and closes with a brief look at coin cell safety standards.

Figure 1: Three stages of battery thermal runaway.

Thermal runaway (TR) in LIBs can be a catastrophic event. It can have several causes, such as dendrite formation, manufacturing defects, physical damage during use, and so on. It doesn’t happen all at once but in a series of stages. The first stage is the beginning of overheating (Figure 1). If it is not extinguished, the solid electrolyte interface (SEI) decomposes, and heat continues to build up, resulting in more side reactions, possibly melting the separator. As the heating continues and the pace of temperature rise increases, the TR phase is entered. TR is defined as a self-heating rate of at least 10°C /min. As it accelerates, TR leads to uncontrollable temperatures, and the cell may burst or even explode. Temperatures of 200 °C or even higher will be experienced, and the cell will catch on fire. 

Considering safety, larger cell sizes are more challenging. The physical construction also has a major impact on safety, with cylindrical cells the safest, prismatic cells the least safe, and pouch cells somewhere in the middle. In part, that is related to keeping the cells cool; the heat dissipation of prismatic cells is the worst, while the gaps between cylindrical calls in battery packs tend to simplify thermal management. Cylindrical and prismatic cells have metallic casings that can withstand high pressures. That can be a negative if there is an internal short circuit since pressures can build up to potentially explosive levels. Pouch cells tend to burst under lower pressures and can catch fire but rarely experience explosions. Selection of the ‘best’ cell format is a complex process, and each of the three packaging formats is used by different EV makers. Adding to the complexity, there are various regional and global EV battery safety standards with differing requirements (Table 1).

Hazard levels

In EVs, hundreds to thousands of cells are combined in the battery pack, increasing the challenges for battery safety. Under normal operating conditions, EV battery thermal management is difficult. When abnormal conditions occur, such as an accident, the challenges can become unmanageable. That’s where the concept of “hazard levels” is applied. The EUCAR and SAE-J Hazard Levels and the associated criteria are widely applied to determine the safety of EV battery packs (Table 2).

Battery Pack Safety

Battery pack safety is critical to ensure the reliable and safe operation of batteries, particularly in electric vehicles (EVs) and portable devices. It involves several aspects including design, monitoring, and protective measures to prevent hazards such as fires, explosions, or leakage.

Key Aspects of Battery Pack Safety:

1. Thermal Management

   • Heat Generation: Batteries generate heat during charging and discharging. Efficient thermal management systems (e.g., liquid cooling, air cooling) are essential to maintain safe operating temperatures.
   • Thermal Runaway: Preventing thermal runaway, a condition where excessive heat leads to uncontrollable temperature increases, is crucial.

2. Electrical Safety

   • Overcharge and Over-discharge Protection: Prevents damage from exceeding the voltage limits.
   • Short Circuit Protection: Detects and stops short circuits that can cause fires or explosions.

3. Mechanical Safety

   • Structural Integrity: Ensures the battery pack can withstand physical impacts and vibrations.
   • Encapsulation: Protects cells from external environmental factors such as moisture and dust.

4. Chemical Safety

   • Stable Materials: Uses stable and less reactive materials to minimize the risk of chemical reactions.
   • Vent Mechanisms: Includes vents to release pressure buildup safely.

5. Integrated Safety Systems

   • Battery Management System (BMS): Continuously monitors voltage, current, temperature, and state of charge (SOC) to manage the battery's health and safety.
   • Safety Circuits: Includes various electronic safety measures such as fuses, relays, and circuit breakers.

Battery Standards

Battery standards are established by international organizations to ensure the safety, reliability, and performance of batteries. These standards provide guidelines for design, testing, and certification of batteries.

Key Battery Standards:

1. ISO 26262

   • Scope: Functional safety standard for automotive electrical and electronic systems.
   • Relevance: Ensures the safe operation of batteries in vehicles, particularly focusing on the avoidance of hazards.

2. IEC 62660 Series

   • Scope: Standards for secondary lithium-ion cells for the propulsion of electric road vehicles.
   • Relevance: Covers performance, reliability, and abuse testing.

3. UN 38.3

   • Scope: Testing requirements for the transport of lithium batteries.
   • Relevance: Ensures batteries can withstand conditions encountered during transportation.

4. UL 1642

   • Scope: Standard for lithium batteries.
   • Relevance: Addresses safety requirements to prevent fire or explosion hazards.

5. SAE J2464

   • Scope: Electric vehicle battery abuse testing.
   • Relevance: Ensures batteries can endure extreme abuse conditions without catastrophic failure.

6. IEC 62133

   • Scope: Safety requirements for portable sealed secondary cells.
   • Relevance: Applies to batteries used in portable applications.

Battery Tests

Battery tests are conducted to verify compliance with safety standards and to evaluate performance under various conditions. These tests can be categorized into performance tests, safety tests, and environmental tests.

Key Battery Tests:

1. Performance Tests

   • Capacity Test: Measures the battery's ability to store and deliver energy.
   • Cycle Life Test: Assesses the number of charge-discharge cycles a battery can undergo before its capacity significantly degrades.
   • Rate Capability Test: Determines how the battery performs at different charge and discharge rates.

2. Safety Tests

   • Overcharge Test: Evaluates the battery's response to being charged beyond its maximum voltage.
   • Short Circuit Test: Assesses the battery's ability to handle an internal or external short circuit.
   • Thermal Stability Test: Tests the battery's behavior under high-temperature conditions.
   • Crush Test: Simulates the effects of mechanical deformation on the battery.
   • Penetration Test: Assesses the battery's resistance to being punctured by sharp objects.

3. Environmental Tests

   • Temperature Cycling Test: Exposes the battery to repeated temperature changes to evaluate its durability.
   • Humidity Test: Tests the battery's performance in high-humidity environments.
   • Vibration Test: Simulates the effects of vibrations experienced during operation or transportation.

Conclusion

Ensuring battery pack safety involves a comprehensive approach that includes proper thermal management, electrical and mechanical protections, chemical stability, and integrated safety systems like a BMS. Adhering to established battery standards and conducting rigorous tests are essential to verify that batteries meet safety, reliability, and performance criteria. These measures help prevent potential hazards and ensure the safe use of batteries in various applications.