When it comes to electric vehicle batteries lithium-ion is king, but as up to the job as Li-ion may be, it doesn’t come without its risks. Richard Poate, senior manager at TÜV SÜD, a global product testing and certification organisation, gives us some potentially life saving tips on battery safety, maintenance and management that we should all be aware of.
According to the International Energy Agency (IEA), in 2019 electric cars registered a 40% year-on-year increase in sales. It puts part of this growth in demand down to significant improvements in technology. For example, research from the IEA reveals that the 2018-19 versions of some common electric car models display a battery energy density that is 20-100% higher than their counterparts in 2012, with battery costs decreasing by more than 85% since 2010.
The UK government is bringing forward a ban on the sale of new combustion engine cars from 2035 to 2030, in an effort to speed up widespread electric vehicle (EV) adoption. Vehicle manufacturers are therefore investing heavily in R&D to radically transform the way we drive, and battery development is at the heart of this process.
Comparatively lightweight and long lasting with good performance, lithium-ion (Li-ion) batteries have proven invaluable in electric vehicle 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 electric vehicle technologies.
Improvements in design, materials, construction, and manufacturing processes means that the safety of Li-ion has dramatically improved. However, ensuring their safety and reliability requires thorough and accurate testing, which includes:
- Life cycle testing: Verifies how long a battery lasts and demonstrates the quality of the battery. These tests include environmental cycle testing and calendar life testing.
- Abuse testing: Simulates extreme environmental conditions and scenarios to test batteries beyond limits.
- 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.
- 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.
Battery designs
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 the potentially high voltages and high currents presented.
For electric vehicles, 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 moulded 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 minimise fire propagation.
- Employ good thermal management.
- Use pressure vents/relief mechanisms to safely deal with excessive pressures.
- Utilise sensors and BMS to identify abnormal behaviours.
- Use materials appropriate for foreseeable temperatures.
- Use constructions with adequate mechanical strength appropriate for the real world.
Battery management
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 energise circuits via current limiting components to minimise 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 (CAN bus). 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 optimise battery charging and discharging, and support predictive maintenance and failures, as well as remote troubleshooting. On the fly software updates will deal with battery ageing and extreme operating conditions, such as hot or cold environments.
Risk management
Batteries used in electric vehicles present many electrical hazards, such as electric shock, arc flash burn, heatwave/fire burns and explosion, which could include shrapnel and hot molten metal. Of course, because of the energy requirements to power electric vehicles, high voltage/high capacity battery packs are needed. Depending on the configuration, battery modules can be high voltage (>50Vdc), 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. This applies to all employees – mechanics and technicians, cleaning staff, office workers, and vehicle owners – anyone who might come into contact with the vehicles. So, this is a real game changer for the electric vehicle market. As the global demand for innovation in electric vehicles increases, so too does the need for qualified testing of lithium-ion batteries to power electric vehicles, and education about their use and care, will also continue to grow.
This article originally appeared in Electrical Review January/February 2021.
