Fighting the Heat: Thermal Management for EV Batteries

The EU has set the objective to have at least 30 million zero-emission vehicles on the roads by 2030, with the automotive industry making huge leaps in battery technologies for electric vehicles to reach this target.

However, consumer uptake of electric vehicles lags behind expectations, as a majority of drivers have concerns related to prolonged charging times and the practical driving range between charges.

Additionally, the car’s temperature during charging and driving can have a considerable influence on the lifetime and performance of a lithium-ion battery, so the focus has shifted towards developing new methods of battery thermal management.

Why are we concerned with the thermal management of batteries?

Temperature plays a significant role in the battery operating performance and capacity: Both the discharge capacity and charge – i.e., the rate at which the car uses energy and the rate at which it charges – are significantly affected by the temperature.

The faster the rate of charging or the discharge capacity of a battery, the more sharp the increase in temperature is, so there is a delicate balance struck between charge/discharge rate and capacity.

Temperature is also a stress factor that diminishes the capacity of a lithium-ion battery in due course; this phenomenon is known as ‘capacity fade,’ whereby the battery lifespan is reduced and impacting an EV’s long-term performance.

With considerable consumer pressures and key environmental targets, manufacturers are inclined to examine thermal management capabilities for optimizing battery longevity. 

What is a battery thermal management system?

Battery thermal management systems (BTMS) are methods that are used to maintain a particular temperature range within a battery pack – preferably between 20 and 40°C – preventing excessive variations and sustaining an even temperature from cell to cell. Methods to manage a battery’s temperature typically follow two routes: active or passive management.

Active BTMS alludes to technologies that use a source of energy to force a change of battery temperature, normally incorporating the use of an air- or liquid-based cooling medium.

Comparatively, passive thermal management solely relies on the thermo-dynamics of conduction, convection, and radiation. There has been extensive debate as to which is the most suitable. For example, Tesla Motors incorporates the active circulation of a coolant fluid in its cars, whereas the Nissan Leaf utilizes a passive air-cooled battery. 

Active Thermal Management: Keeping cool or maintaining control?

There are numerous types of active thermal management systems, and the main difference between them is their primary objective; some have been developed to cool the battery, while others stabilize temperature extremes. But which is the better method?

Air Cooling

Active air-cooling systems circulate air across the battery pack, usually from an AC unit or drawn in from outside; this cooling method utilizes convection to maintain stable temperatures. The main benefits of air-cooling systems are that they are relatively straightforward and inexpensive.

However, they are only designed to cool and avoid potential overheating. This means they lack the capacity to manage a broad range of ambient temperatures. While this isn’t an issue in mild or even warm climates, in colder climates, it can result in battery degradation – EVs are not optimized for going out into a blizzard.

Even at moderate temperatures, the air is not so effective when attempting to transfer heat away from the battery due to the specific restricted heat capacity.

As batteries are becoming much more powerful and hold greater charge, concerns are expressed regarding the safety of depending on an air-cooling system for high-power applications.

Liquid Cooling

Liquid cooling – where pumping a liquid coolant, such as glycol, circulates coolant in a closed loop around the battery – presents a more definite method for thermal condition management, helping to keep them in a suitable range.

Typically, heat is transferred via thermally conductive metal pipes, drawing the liquid away from the source so that it can be distributed effectively. Direct liquid cooling methods – where the battery is immersed in a non-conductive liquid – are in the early stages of development.

Liquid-based cooling is much more efficient, enabling lighter and more compact systems without the addition of needless mass or power drains.

This is most valuable as the automotive industry drives towards achieving the most lightweight systems; approaches to liquid thermal management have been endorsed by Tesla, BMW, and Chevrolet.

Thermoelectric Coolers

Positioning semiconductors between the heat source (the battery) and a heat sink is an additional method of thermal management that has made an impact on rippling across the automotive industry.

A temperature differential between the sink and the source is generated when a voltage is applied, meaning heat transference is achieved through conduction.

This enables exact control of temperatures via a basic change in voltage, and in situations that necessitate the application of heat, the direction of heat transfer can be adjusted by reversing the current.

Passive Management: Relying on the Science

Unfortunately, the largest restriction on all active BTMS is that they depend on energy release from the battery, stripping the vehicle of invaluable power. The goal is, therefore, to self-regulate the temperature of the battery using passive thermal management without dependence on an energy source.

While active management strategies are preferred for their efficiency, there are a number of passive cooling methods in the development phase.

For instance, heat pipes – a closed cycle of liquid evaporation and condensation that utilizes the heat energy from a battery – are particularly effective when it comes to transferring heat in smartphones, but these options only have the capacity to absorb heat from the battery, not draw it away from the source.

With a sustained drive to limit parasitic power consumption in EVs, it is anticipated that more of these passive techniques will be employed in the future.

Materials

Another factor that can have a significant influence on thermal regulation is the materials used in the manufacture of battery cases. Generally, fibrous composite materials – including glass and carbon fiber – have a low thermal conductivity in comparison to traditional metallic materials.

This is due to the fact they are usually an amalgamation of 50-60 % fiber and thermoset or thermoplastic polymers, which serve as binders to form a matrix. Composite structures can be engineered further to enhance insulative performance by integrating structural foam or honeycomb cores.

This technique enables minimal use of fibrous material whilst preserving structural stiffness and strength at a fraction of the weight. The insulative qualities of composites can be advantageous to the design of the battery system as it facilitates the stabilization of temperatures within the enclosure, thus limiting the required energy when cooling or heating the cells determined by the external environmental conditions.

Best of Both Worlds

Currently, the optimal solution is the application of passive cooling in combination with active thermal management systems to improve efficiency.

For instance, positioning thermally conductive cooling fins between two cells expands the surface area to conduct heat away from the battery pack, which then dissipates it to the air via convection.

In the Chevrolet Volt, adding grooves to each fin produced channels for the coolant liquid to flow through, which traverses either a heater or heat exchanger to circulate over the whole face of the cell.

This class of combined approach assists in balancing out the constraints of individual solutions, and it is likely that in the future, an increasing number of hybrid thermal management systems are used, which combine passive and active cooling technologies with sophisticated composite materials to help improve temperature control efficiency while drastically limiting parasitic power consumption.

This information has been sourced, reviewed and adapted from materials provided by TRB Lightweight Structures Ltd.

For more information on this source, please visit TRB Lightweight Structures Ltd.

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