Craig Ormerod, senior manager at TÜV SÜD, on the major considerations of battery safety testing, and why electric vehicle battery specifications and compliance are ever more critical.
Driving the electric vehicle revolution with safe and secure batteries
Comparatively lightweight and long lasting with good performance, lithium-ion (Li-ion) batteries have proven invaluable in EV development, but they carry with them potential safety hazards which must be managed. Also, while consumers are familiar with the traditional combustion engine, and therefore accept the well-known risks associated with fossil fuel-powered cars, there is still an element of distrust relating to relatively new and unfamiliar EV technologies.
Improvements in design, materials, construction and manufacturing processes mean that the safety of Li-ion has dramatically improved. However, ensuring their safety and reliability requires thorough and accurate testing, which includes:
- Lifecycle testing – verifies how long a battery lasts and demonstrates the quality of the battery. These tests include environmental cycle testing and calendar life testing.
- Performance testing – demonstrates the efficiency of batteries, such as performance testing under various climatic conditions.
- Environmental and durability testing – demonstrates the quality and reliability of a battery through tests including vibration, shock, EMC, thermal cycling, corrosion, dust, salt and humidity.
- Abuse testing – simulates extreme environmental conditions and scenarios to test batteries beyond limits.
- Dynamic impact tests – simulates a real vehicle accident to determine the true safety performance of the battery when the car body is deformed.
- Transportation tests – UN 38.3 is a series of tests to verify the robustness of batteries against conditions encountered in shipment.
Single battery cells typically come in three package styles – cylindrical, prismatic and pouch – and can be particularly sensitive to mishandling, inappropriate packaging, deformation and contamination. They can also fail due to overcharging and extreme temperatures. Repeated overcharging of a battery cell can create unwanted electrical paths, as well as short circuits that grow and create instability. High temperatures can drive excessive ionic flow which damages the crystalline structure of the cathode and can ignite electrolyte. Meanwhile, charging at low temperatures can lead to metallic plating, creating instability through short circuits.
When these individual cells are connected in series/parallel combinations (depending on end-use requirements) the resulting modules deliver increased voltage and capacity. Although the individual cells are now mechanically ‘protected’ with a mechanical support/enclosure, care must be taken due to the potentially high voltages and high currents presented.
For EVs, large battery packs connect to the vehicle’s electric powertrain. These packs are constructed by connecting modules together, adding sensors and a battery management system (BMS). They deliver an extremely high voltage and can be molded to fit the host vehicle and may also form part of its structure.
Safety tips for module and pack designs include:
- Use physical partitions and fire breaks to minimize fire propagation.
- Employ good thermal management.
- Use pressure vents/relief mechanisms to safely deal with excessive pressures.
- Utilize sensors and BMS to identify abnormal behaviors.
- Use materials appropriate for foreseeable temperatures.
- Use constructions with adequate mechanical strength appropriate for the real world.
Hardware and software
The BMS consists of both hardware and software elements, which contribute to vehicle safety and performance. The hardware generally includes current sensing capabilities for state-of-charge (SoC) estimation and for safety. It must also detect leakage current faults, which could render the vehicle chassis ‘live’ and therefore highly dangerous, if not lethal. Effective fusing will also provide overcurrent protection. A pre-charge element should be incorporated to energize circuits via current limiting components to minimize inrush currents. Relays and contactors will also provide safe and reliable connection/disconnection to and from the vehicle.
The software element of the BMS provides the interface and communications to the vehicle (CANbus). The incorporation of diagnostics and health software monitors SoC (under/over charge), which is important for control, safety and vehicle range estimation. State-of-health functions will also determine battery degradation over time and predict end of usable life. The software delivers control over the battery’s function, including electrical isolation, thermal management, charge/discharge and cell balancing.
5G will also be a driver of smart battery maintenance, using data over-the-air and software-over-the-air. This means that real-time data can be used to optimize battery charging and discharging, and support predictive maintenance and failures, as well as remote troubleshooting. On-the-fly software updates will deal with battery aging and extreme operating conditions, such as hot or cold environments.
Because of the energy requirements to power EVs, high-voltage/high-capacity battery packs are needed. Depending on the configuration, battery modules can be high voltage (>50V DC), therefore presenting an electric shock and energy hazard, and vehicle battery packs will certainly present both. It is therefore essential that people working with high-voltage systems are aware of the potential dangers and protective measures. Battery safety and testing must also be a key consideration and EV battery specifications and compliance is becoming ever more critical.