1 Cylindrical lithium-ion battery
Lithium ion batteries come in various specifications and external dimensions, so measuring the thermal performance parameters of lithium batteries involves multiple testing methods and equipment. We first chose to conduct research on the thermal performance testing of cylindrical lithium-ion batteries, especially on the radial thermal conductivity testing technology for cylindrical lithium-ion batteries, mainly for the following considerations:
(1) Cylindrical lithium-ion batteries are currently one of the most common types of batteries and have a wide range of applications. However, the radial thermal conductivity testing technology for cylindrical lithium-ion batteries is not yet mature and is still in its infancy both domestically and internationally. The various testing methods reported have significant errors and cannot meet the needs of battery thermal modeling and management.
(2) The cylindrical structure of lithium batteries is very special, especially with only one circumferential surface in the radial direction. When conducting thermal performance tests without damaging the battery, only one circumferential outer surface can be used to generate corresponding test boundary conditions. This is often the most difficult test in thermal performance parameter testing technology. If thermal performance parameter testing can be achieved in the radial direction of cylindrical batteries with satisfactory measurement accuracy, the testing technology can be easily promoted and applied to prismatic and pouch batteries.
(3) The self heating heat in cylindrical lithium-ion batteries is usually the lowest, lower than that in prismatic and pouch batteries. Similarly, if the testing method studied can achieve satisfactory measurement accuracy on cylindrical lithium batteries with low heat, it can achieve higher measurement accuracy in high heat measurements of prismatic and pouch batteries.
(4) In addition, through the research of radial thermal conductivity testing technology for cylindrical lithium-ion batteries, efforts can be made to achieve multifunctionality, modularity, rapidity, and low cost of lithium-ion battery thermal performance testing instruments.
This article will specifically focus on the radial thermal conductivity of cylindrical lithium-ion batteries and conduct research on testing methods. Establish corresponding constant temperature testing models under the boundary conditions of non-destructive batteries and only the outer surface of the battery circumference, and verify the accuracy of the testing model and explanatory expressions through finite element simulation. It is expected to provide effective guidance for the design of testing instruments.
2 Analytical model for thermal testing
According to the internal structure and heat transfer direction of cylindrical lithium batteries, the radial heat transfer mode of cylindrical lithium batteries is a typical radial circumferential scattering mode. Therefore, cylindrical coordinates are used to describe the testing model of cylindrical batteries, while other forms of testing models cannot accurately describe the heat transfer mode of cylindrical batteries. For a cylindrical lithium battery with a radius of R and a height of H, the boundary conditions for radial thermal conductivity testing can only occur on the outer surface of the circumference at r=R.
If we assume that the upper and lower end faces of a cylindrical battery are adiabatic surfaces, then the boundary conditions on the outer surface of the battery are no different from the three types of boundary conditions in heat transfer, namely constant temperature, linear heating, and alternating temperature. Due to the relatively large size of the tested battery and the difficulty in implementing and analyzing the third type boundary condition of alternating temperature, which is very complex, we only conducted corresponding testing method research for the first and second type boundary conditions of constant temperature and linear heating.
Heat only flows radially, and the temperature distribution is one-dimensional in space. The heat flux is also one-dimensional, and it is assumed that the radial heat transfer coefficient is uniform and independent of temperature within a small temperature range.
2.1. Constant temperature testing
The first type of boundary condition is a constant surface temperature, which means that during the testing process, a battery with an initial temperature of T0 is suddenly placed in an environment with a temperature of Ts, and this environment temperature must be higher than the initial temperature T0 and remain constant. As a result, heat is transferred radially through the battery, and the two ends of the battery are in an adiabatic state.
2.2 Linear temperature rise test
The second type of boundary condition is a linear increase in surface temperature, which means that during the testing process, a constant amount of heat is applied to the outer surface of the battery to heat it up, and it is assumed that the constant heat will not be lost over time throughout the entire heating process. In addition, due to the axisymmetric structure of cylindrical batteries, the heating of the sides around the battery will result in an adiabatic state along the axis of the battery.
3 Conclusion
Special research has been conducted on the radial thermal conductivity testing technology for cylindrical lithium-ion batteries, and a simple and easy to operate testing method has been established. The testing method has been validated using finite element simulation, and the following conclusions have been drawn from the entire research work:
(1) A constant temperature testing model and corresponding testing method were established for the radial thermal conductivity of cylindrical lithium-ion batteries. Finite element simulation has proven that this testing method has high measurement accuracy and can be fully applied in practical testing, which is of great significance for the thermal performance testing of lithium-ion batteries.
(2) The established testing method can obtain radial thermal conductivity, radial thermal diffusivity, and specific heat capacity values through a single heating test. It is also possible to measure the thermal performance parameters over a wide temperature range as a function of temperature, and even to measure the thermal performance throughout the entire phase transition process.
(3) The established isothermal testing method has basically acquired the function of the commonly used accelerated adiabatic calorimeter, which can replace and supplement the accelerated adiabatic calorimeter for detecting thermal runaway of batteries.
(4) The established testing method is simple and easy to implement, with convenient experimental operations, making it very suitable for loading other variables in battery performance assessment, such as thermal performance testing during battery charging and discharging processes.
(5) The breakthrough in the radial thermal conductivity testing method for cylindrical lithium batteries can be extended to the thermal performance testing of other specifications of lithium-ion batteries by using both constant temperature and constant current testing methods. Various loading conditions and directions can be used to test the thermal performance of lithium batteries.
(6) The principle of the constant temperature testing method studied is simple, and the boundary conditions are easy to implement, which is very conducive to instrumentation and modularization, as well as integration with other testing instruments.