A Battery Thermal Management System (BTMS) is a system that manages the heat dissipated or generated during the electrochemical process in cells of a battery pack of an electric vehicle (EV). The objective of a BTMS is to avoid accelerated deterioration of batteries and maintain battery operation at optimum temperature. It improves the battery performance, lifetime, and safety of EVs.

When an EV battery is in operation, the electrochemical processes inside it produce heat which increases the battery temperature. Hence it is required to maintain the EV battery's optimum temperature to operate the battery safely and efficiently. The maximum battery performance is obtained when the temperature is between 20^-40⁰C. If the temperature is less than 10⁰C, the charging capacity and efficiency of the battery are reduced. If the temperature rises beyond 80⁰C, thermal runaway may occur resulting in the explosion of the battery.

Functions of BTMS

A BTMS carries out the following functions.

• Cooling: When the battery is in operation it produces heat, and this heat increases the battery temperature. Hence cooling is required to maintain the optimum battery temperature. 
• Heating: When the battery is placed in cold climate conditions, the battery temperature falls. Hence, a heating function is required to increase the battery temperature to an optimum range in a shorter period.
• Insulation: There is an influence of external temperature on battery temperature which causes a difference between inside and outside temperature and hence the battery temperature rises and falls very quickly. This rapid rise or fall in temperature must be prevented using good insulation.
• Ventilation: When the battery is in operation, it produces hazardous gases. These gases generated from the battery must be evacuated for safe battery operation. 

Active BTMS: These systems dissipate the heat generated from the cell through a fluid that is in motion. Active BTMS based on forced air or liquid coolant is commonly used in EVs. For forced air systems, fans may be used to circulate cold air through the battery cells. For liquid coolant systems, the liquid can be in direct contact with the cells (immersed in fluid) or it can be circulated through pipes, acting indirectly with the cells. A commonly used coolant for liquid-based BTMS is a mixture of water and ethylene glycol.

Passive BTMS: These systems dissipate the heat generated from the cell through phase change materials (PCM) and heat pipes. PCMs with solid-liquid phase changes such as paraffin, fatty acids, or hydrated salts are preferred for BTMS. The PCM is filled inside the cavity of the battery case and the cell. When the cell generates heat, the PCM material absorbs the latent heat and starts to melt. The PCM is used as a conductor and buffer in BTMS where the latent heat of molten PCM is conducted to the surrounding environment. However, the above-mentioned PCMs have low thermal conductivity, limiting the heat transfer from the cell to the PCM. To overcome this limitation of PCM, it may be embedded in a porous structure or may be doped with nanoparticles. Heat pipe module uses fluid-filled vacuum tubes (usually water-filled) that undergo vapor-liquid phase change of the fluid. This system as such is not a preferred choice of BTMS in EVs, though it is widely used in the cooling of electronic components.

Hybrid BTMS: These systems take advantage of both active and passive BTMS. The most widely used combinations are PCMs with forced air, PCMs with liquid cooling, and PCMs with heat pipes. The use of PCM with forced air/liquid cooling results in fair temperature distribution within the battery pack by dissipating the generated heat outside. The usage of PCMs with heat pipes enhances the heat transfer from the PCM to the exterior of the cells so that the cells can be cooled by natural convection. Though hybrid BTMS delivers better performance than active or passive BTMS, its cost, and complexity is a regulating factors for use in EVs.

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