Alternative Battery Models
While reviewing research on battery modelling techniques, the team found many different modelling approaches. Each of these models provides unique outputs which can be used by the eRallyCross team for a range of purposes.
Multi-Cell Battery Model
This prebuilt Simulink/SimScape model provides an approach to simulate an entire battery pack and provides the basis for an in-depth analysis of fundamental battery cell characteristics and battery pack performance. The model utilises the cell modelling approach outlined in the Equivalent Circuit Battery Model. This model is capable of accounting for the convective heat transfer between cells/ modules and the small differences between each cell’s characteristics. The predominant advantage of multi-cell modelling is the ability to conduct investigations regarding fundamental cell characteristics, thermal/battery management systems and general battery pack design. Moreover, this model can be utilised for determining a suitable battery type or the number of required cells through analysing parameters such as state of charge and temperature. Although the team did not create this model, research was conducted into how it can be used by the eRallyCross to meet their goals.
Empirical Modelling
This approach presents terminal voltage as a function of current and state of charge. Essentially a simplified electrochemical model, only the mandatory nonlinear characteristics of the chosen battery cells are represented within the less complex mathematical functions. There are three main classical models, Shepherd, Unnewehr Universal and Nernst. Each of these models can be used independently and provide various pros and cons, depending on what analysis and results the user requires. However, these classic models can be enhanced by either combining multiple models or altering an individual model. Further parameters and influencing inputs can be added to provide additional accuracy. This gives the client the opportunity to create a computationally efficient simulation by only including parameters that are completely necessary for the specific investigation, such as powertrain analysis during a race scenario.
Electrochemical Modelling
Modelling the battery using an electrochemical approach takes the chemical reactions occurring within the cells into account. Several physical laws are utilised in order to model the inner workings of the cells. Two main methods are used for electrochemical modelling; Single Particle Method (SPM) and Pseudo-2D Model (P2D). The P2D method attempts to model each individual particle within the electrodes and the Li-Ion transition within the electrolyte. This is primarily used for battery cell design and investigating processes within the cells. SPM can be used for SOC (State of Charge) estimation and is a simplified version of the P2D model. This can allow the electrodes to be assumed as single spherical particles, making the calculations much less complex. Although the SPM method is a simplified electrochemical method, the procedure for estimating SOC is still complex and computationally heavy. These methods can accurately describe lithium ion batteries, but are unnecessary unless extremely precise SOC estimation is required.