Comparison of the effect of internal thermal gradients on the performance of Lithium Ion batteries caused by external air convection versus surface conduction and the consequences on electrochemical model parameters fitting

Abstract

Electrochemical models are essential to solving many problems involving lithium ion batteries. However, they require many parameters, some of which cannot be directly measured and must be inferred by fitting to cell testing data. Battery performance is strongly affected by temperature which is well documented in the literature and can be mathematically represented by the Arrhenius equation. When fitting data to a cell model, the temperature must be constant, known, and uniform, to be mathematically consistent. However, it is impossible to test a cell without causing its surface temperature to vary. This will eventually induce thermal gradients which could affect the cell performance. Despite this, there is a significant amount of literature on fitting test data to isothermal models. It is difficult to quantify the non-uniformity inside the cell, therefore a battery model which accounts for internal and surface region has been adopted. By using this model, the deviation of current between surface and internal region can be up to 4% at the constant surface temperature of 5 degree Celsius.

To explore the thermal variation issues, the effect of applying different thermal boundary conditions, using forced air convection and surface cooling plates, was investigated experimentally. This was not to evaluate cooling effectiveness, but rather to show the effect of using data generated with different thermal boundary conditions on parameter estimation when fitting to an isothermal model. When fitting the model to the data, the estimated diffusion coefficient of the positive electrode was four times larger using the data gathered using forced air convection compared to surface cooling at low operating temperature. This was achieved by using a four-point surface cooling rig, which was designed to maintain tabs/surfaces of a pouch cell at the constant temperature, to be as close as experimentally possible to isothermal conditions without interfering with the cell.