The war on heat – the challenges of thermal management for EV batteries

The EU aims to have at least 30 million zero-emission vehicles on the roads by 2030, and the automotive industry has made huge advances in battery technologies for electric vehicles to make this possible. However, the consumer uptake of electric vehicles has not met expectations, with most drivers still concerned about lengthy charging times and the realistic range between charges. The temperature of the car during charging and driving can have a significant impact on the performance and lifetime of a lithium-ion battery, so attention has turned to methods of battery thermal management. Why are we concerned with the thermal management of batteries? Temperature is a significant factor for battery operating performance and capacity. Both the charge and discharge capacity – i.e. the rate at which the car charges and the rate at which it uses that energy – are strongly influenced by the temperature. However, the faster we charge or discharge a battery, the more severe the increase in temperature is, so it is a fine balance between charge/discharge rate and capacity. Temperature is also a stress factor that, over time, lessens the capacity of a lithium-ion battery through what’s known as ‘capacity fade’, reducing battery lifespan and affecting an EV’s long-term performance. With substantial consumer pressures and environmental targets, it’s in manufacturers best interest to consider thermal management for optimising battery longevity. What is a battery thermal management system? Battery thermal management systems (BTMS) are methods employed to keep the temperature in a battery pack within a fixed range – ideally between 20 and 40 °C – avoiding excessive fluctuations and maintaining an even temperature from cell to cell. Methods to control the temperature of a battery tend to follow two routes: passive or active management. Active BTMS refer to technologies that force a change of battery temperature using a source of energy, and typically include the use of an air- or liquid-based cooling medium. In contrast, passive thermal management relies solely on the thermo-dynamics of conduction, convection and radiation. There is huge debate as to which is better. For example, the Nissan Leaf uses a passive air-cooled battery, whereas Tesla Motors incorporates the active circulation of a coolant fluid in its cars. Active thermal management: keeping cool or maintaining control? There are several types of active thermal management systems, and the greatest distinction between them is their purpose; some are designed to cool the battery, while others stabilise the temperature extremes. But which is better? Air cooling Active air-cooling systems blow air, typically from an AC unit or drawn in from outside, across the battery pack, using convection to keep it cool. The biggest advantages of air-cooling systems are that they are simple and inexpensive. However, they are only intended to cool and prevent overheating. This makes them unable to manage a wide range of ambient temperatures. This isn’t an issue in mild or even warm climates, but colder climates can lead to battery degradation – EVs don’t enjoy going out in a blizzard! Even at moderate temperatures, air is not particularly efficient at transferring heat away from the battery, due to its limited specific heat capacity. As batteries become much more powerful and hold greater charge, there are concerns about the safety of relying on an air-cooling system for high-power applications. Liquid cooling Liquid cooling – where a liquid coolant, such as glycol, is pumped and circulated in a closed loop around the battery – offers a more precise method for managing thermal conditions, helping to keep them in the optimal range. Normally, heat is transferred through thermally-conductive metal pipes to the liquid, drawing it away from the source so that it can be dissipated. Direct liquid cooling methods – where the battery is submerged in a non-conductive liquid – are also in the early stages of development. Liquid-based cooling is far more efficient, allowing smaller, lighter and more compact systems without adding unnecessary mass or power drains. This is extremely valuable as the automotive industry strives for the most lightweight systems, and liquid thermal management approaches have been adopted by Tesla, BMW and Chevrolet. Thermoelectric coolers Placing semiconductors between the heat source (the battery) and a heat sink is another method of thermal management that is making waves in the automotive industry. A temperature differential between the source and the sink is produced when a voltage is applied, so that heat is transferred through conduction. This allows precise control of temperatures by a simple change in voltage, and in instances where heat is required, the direction of heat transfer could be changed by reversing the current. Passive management: relying on the science Unfortunately, the greatest limitation of all active BTMS is that they require energy from the battery, robbing the vehicle of valuable power. The aim of passive thermal management is therefore to self-regulate the temperature of the battery, without relying on an energy source. Although active management strategies are currently favoured for their efficiency, there are plenty of passive cooling methods under development. For example, heat pipes – a closed cycle of liquid evaporation and condensation using heat energy from a battery – are highly efficient at transferring heat in smart phones, but these options can only absorb heat from the battery, not draw it away from the source. With a constant push to reduce parasitic power consumption in EVs, expect to see more of these passive techniques used in the future. Materials Another factor that can have a huge impact on thermal regulation is the materials used in manufacturing battery cases. Overall, fibrous composite materials – such as glass and carbon fibre – have a low thermal conductivity compared to conventional metallic materials. This is because they are typically a combination of 50-60 % fibre and thermoset or thermoplastic polymers, which act as binders to form a matrix. Composite structures can be further engineered to improve the insulative performance by incorporating structural foam or honeycomb cores. This approach allows minimal use of fibrous material, whilst maintaining structural stiffness and strength at a fraction of the weight. The insulative properties of composites can be beneficial to the battery system design as it aids the stabilisation of temperatures within the enclosure, thus reducing the energy required in heating or cooling the cells depending on the external environmental conditions. Best of both worlds At present, the best solution is to use passive cooling in conjunction with active thermal management systems to optimise efficiency. For example, placing thermally conductive cooling fins between two cells increases the surface area to conduct heat away from the battery pack, then dissipate it to the air through convection. In the Chevrolet Volt, grooves were added to each fin to provide channels for the coolant liquid, which has passed through either a heater or heat exchanger, to circulate over the entire face of the cell. This type of combined approach helps to balance out the limitations of individual solutions, and we are likely to see a greater incidence of hybrid thermal management systems in the future, combining passive and active cooling technologies with advanced composite materials to help optimise temperature control efficiency while reducing parasitic power consumption.

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