Battery life, ability to charge and discharge rapidly, and safety are three critical issues for different rechargeable batteries to become the workhorse of electrical vehicles (EV) and many other applications. How to manage the temperature inside the battery cells so that it stays at near room temperature (25 C) is a key to solving problems related to all three issues. This is because 1) excessive heat during operation or shelf time above this temperature will cause excessive irreversible chemical reactions, which in turn will cause ion channels to be blocked and batteries to lose their ability to be re-charged; 2) charge or discharge more rapidly through battery internal resistance will cause the temperature inside the cell to increase more with a given heat dissipation rate for a given packaging technologies, and cell will lose their capacity and life; 3) if the temperature exceeds a catastrophic failure temperature (thermal runaway) causing strong chemical reaction, the cells will explode.
Conventionally, there are four different basic methods of heat management for batteries. Common to all, batteries are first manufactured in small cells with different geometries and then packaged into a large volume; this approach will decrease the temperature difference between cell surfaces and hottest point inside the cell. Different heat management methods are different in their ways to dissipate the heat from the surface of the cells to outside of the large battery package. The first method is forced air convection method, i.e. using electrical fans to cause air flow in the wind tunnel and cool off the cell surfaces in the wind tunnel. This method is simple, but not energy efficient, since a decrease in the temperature difference between cell surfaces and air in the tunnel entrance requires a linear increase in the speed of air flow, while the required electrical power increases as the third power of the air flow speed. This is evident in the cooling methods of power plants.
The second method is forced liquid convection. Liquid, instead of air is used in the forced convection. Since the liquid has a larger heat capacity than gas, the cooling is more effective, but the power required to speed up liquid flow and increase cooling is still the third power of the speed of liquid flow. For liquid to directly flow over cell surfaces in the space between cells, the flow resistance is too large to keep required electrical power low enough for a dense cell packaging.
The third method (US2009004556A1) is to package solid to liquid phase change materials (PCMs) with the cells. As the cell surfaces heat up, the PCMs absorb the heat and melt into liquid, storing the heat energy while keeping the temperature constant. This method is effective to increase the charging and discharging speed for a given heat dissipation rate due to temporary heat storage by the PCMs. However, the PCM capacity is limited since for the PCM volume far away from the cell surface to reach the melting temperature, the cell surface temperature still is required to be much higher than the melting temperature of the PCMs due to the poor thermal conductivity of PCMs.
The last method (US2011206965A1) is a 2-D heat pipe method. Two-dimensional heat pipes in the form of thin sheets are fabricated and the hot ends are mounted on the battery cell surfaces and the cool ends are mounted with fins for more effective forced air-cooling. The liquid to vapor PCM with a desired boiling temperature is sealed inside the 2-D heat pipes. As the cell surface temperature rises, which can exceed the PCM boiling point, the PCM on the heat pipe inside wall surface vaporizes and bring the heat energy to the cool ends to condense back into liquid again. The heat energy transferred to the cool ends is dissipated by the airflow. Fabrication of 2-D heat pipes is too expensive to implement in industrial applications, and cooling is still limited by the forced air cool method.