Investigation of the relationships between the parameters of electrochemical and equivalent circuit model of lithium-ion battery for improving battery state estimation

This thesis through the analysis of the results of the simulations of the Pseudo 2 Dimensional (P2D) chemical model of a lithium ion cell aims to find a relationship between the chemical components of the cell and the electrical parameters of the equivalent circuit model (ECM). Rechargeable lithium-ion batteries are widely used as energy storage systems for portable devices due to their multiple advantages over other chemicals. Li-ion batteries must be used within a Safe Operating Area (SOA) bounded by specific temperatures, voltages and currents. More the cell is used outside the SOA more its health will be affected, and its performance will be worse. The Battery Management System or BMS is the device that monitor the cell and uses protection mechanisms to prevent irreversible damages. One way to monitor the health of the cell is to estimate online, therefore during the operating phase of the cell, its most significant parameters such as the state of charge, the state of health and the parameters of the ECM. ECMs are a mathematical modelling of the cell and replicate its electrical behaviour. These models need few computational resources to be calculated, so they are very simple to calculate online during the cell operational phase. The problem is that the reliability in predicting the results is poor. This is because mathematical modelling of lithium ion cells with good predictions need more computational requirements. A widely used modelling is P2D because it is quite accurate and its results are very similar to the experimental ones, but it needs greater computational requirements. The purpose of this thesis is to link to each electrical parameter of the ECM one or more parameter of the P2D model of the cell. Then merge the simplicity of ECM simulation with the best predictive reliability of the P2D model. By identifying online the parameters of the ECM, during the operating phase of the cell, the parameters of the P2D model and therefore the phenomena within the cell will also be clear. A P2D chemical model of a lithium-ion cell developed on COMSOL was initially validated with a large data set. Subsequently various charges and discharges were simulated by varying some parameters of the model, in order to modify the chemists within the cell. After each II simulation, all the results that kept track of the electrical behaviour of the model (voltage, current, temperature, etc.) are saved. The results are then processed by a MATLAB script that identified the parameters of an ECM with two-time constants that replicated the electrical behaviour of the P2D chemical model. The parameters of the ECM have been optimized to reduce the error between the electrical results and those of the chemical model as much as possible. The data saved after the script was run was analysed and correlations were found between the electrical parameters of the ECM and the changed parameters in the P2D model simulations. Finally, correlation indices were used to find the chemical components within the cell corresponding to each parameter of the ECM. The results obtained show that each chemical phenomenon of the cell can identify in one or more electrical parameters of the ECM. This thesis wants to highlight that one way to keep track of the chemical components inside the cell and therefore of its health is to simply analyse the variations of the ECM parameters. However, further research and refinements of the model and of the conversion algorithms are necessary to obtain more reliable results and to extend them to any operating condition. In addition, in the P2D chemical model with subsequent research and more targeted variations, cell aging can be implemented with the insertion of degradations. With this, by analysing the electrical parameters of the ECM one could make a prediction about life span of a cell