Heat management systems for battery packs.

The article presents the advantages and disadvantages of active and passive heat management systems in battery sets as well as the latest, perspective materials and solutions.

Battery Thermal Management System (BTMS) is a device responsible for the management and / or dissipation of heat generated during electrochemical processes taking place in the cells, allowing the safe and efficient operation of the battery. The goal of BTMS is to prevent the accelerated deterioration of the quality of the battery by managing the heat generated by its components so that it constantly operates under optimal temperature conditions.

Although existing commercially available cells can safely operate at temperatures ranging from -40 to + 60 ° C, the operating range recommended by manufacturers to maximize their efficiency is indeed 15 to 35 ° C. In addition, it is recommended that there is no more than 5 ° C difference between the cells in the battery pack.

It should be noted that exposing the battery to extreme conditions can have fatal consequences. For example, its operation at very high temperatures (> 80 ºC) can cause the already known temperature instability, resulting in fire and, in the worst case scenario, even explosion of the battery.

Heat management technologies

There are many alternatives to choose from when choosing a BTMS for your battery pack. The figure below provides an overview of the leading thermal management technologies that are commercially available or researched by scientists:

Source: © cicenergigune.com

The BTMS classification is based on the distinction between systems in which the fluid is in motion and those in which it is not. The former are known as active BTMS, the latter as passive BTMS.

Active BTMS

Currently, in electric vehicles, active BTMS based on forced air circulation or coolant are most often used. For example, both Toyota and Lexus use fans that circulate cold air through the battery cells. On the other hand, Tesla and Audi use channels in direct contact with the cells through which the coolant (usually a mixture of water and ethylene glycol) circulates.

When liquid coolants are used, they may come into direct contact with the cells (immersed in the liquid) or circulate inside the channels and act indirectly (all the examples of liquid cooling mentioned above are indirect systems).

One of the main disadvantages of indirect systems compared to direct systems is the loss of heat transfer efficiency, mainly due to the heat transfer resistance at the interface between the pipe / channel containing the refrigerant and the cell itself.

However, since there is no direct contact between the fluid and the battery's electrical components, indirect systems allow the use of conventional coolants already used in internal combustion vehicles. For this reason, and due to its low cost, it is currently the solution favored by car manufacturers.

In recent years, immersing cells in cooling fluids has been very popular both at the scientific and industrial level. The main advantage of this configuration is the direct contact of the coolant with the cells, which allows for more efficient heat transfer. Research shows that transfer can be increased up to four times compared to indirect systems.

However, there are serious challenges that make it difficult to implement this solution in electric vehicles today. The main one is the need for further research on dielectric fluids that guarantee the correct operation of the cells, do not react with any of the elements of the battery pack (cells, electronics), have a reasonable cost and guarantee the safety of the vehicle in the event of a collision.

A more extreme case of direct systems is the use of liquids with a boiling point in the cell's operating temperature range in order to take advantage of the liquid-vapor phase change. There are scientific studies on this type of fluids, where it is estimated that heat transfer can be increased up to 10 times compared to the use of fluids without phase change. However, these fluids have a very low TRL (Technology Readiness Level) and are not expected to be used in vehicles in the next few years.

Overall, the advantages and disadvantages of active BTMS can be summarized as follows:

Advantages:

• Relatively simple design of systems based on forced air circulation.
• Highly efficient in keeping the battery pack within the desired temperature range for batteries in fluid based systems.

Disadvatages:

• High operating costs for forced air installation due to the need to create an intensive air flow.
• Low efficiency in achieving uniform temperature between cells.
• Leakage problems can arise in liquid-based systems.
• Occupied volume and complexity of fluid systems.

Source: © cicenergigune.com

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