Comparison of calorimetric studies on 18650 lithium-ion cells and equivalent circuit model-based simulation applied to driving cycles

The performance of a lithium ion cell is strongly related to the temperature. Therefore it is important to understand the process of heat generation and dissipation inside a single cell but also in battery packs since this is also closely coupled to safety issues. In this study, commercial 18650 lithium ion cells (1.6Ah) with LiMn2O4 (LMO) cathodes, were tested under isoperibolic and adiabatic conditions in an accelerating rate calorimeter (THT Company) to investigate the heat effects during cycling. Isoperibolic investigations in the range from 25 to 60°C show that the applied environmental temperature does not largely influence the battery thermal behavior. Additionally, the heat capacity and heat transfer coefficient were measured after calibration. By integrating over the heat dissipation rate and the enthalpy accumulation rate the total generated heat was determined in dependence of discharge C‐rate. Under adiabatic conditions at a C/2 discharge rate the temperature on the surface of the cells was increasing over three cycles by 20°C and a 1C rate by more than 40°C before reaching the safety limit temperature of 75°C.As part of the IKEBA project, the measured cell and temperature data have then been compared to the data coming from simulations that are based on the combination of electric and thermal equivalent circuit models. The identification problem of the structure and the parameters of the ECM are discussed in terms of the Current Interruption Technique (CIT). The CIT measurements were carried out at different constant temperatures from ‐10 to 60 °C. During the CIT, the cell is gradually charged and discharged in the calorimeter by interrupting the current after defined percentage of state of charge change, followed by a defined relaxation period. From the voltage drop and the relaxation curve look‐up tables (LUT) have been created for the system parameters of the ECM model. To increase the resolution of the LUTs, the SOC was only decreased in 1% steps by the current pulses. The unknown nonlinear functions for the parameters could then be approximated by interpolation and extrapolation from theLUT's and could be implemented into the simulation platform which is using the electronic system level (ESL) behavioural languages SystemC and SystemC AMS [1]. A good general agreement was found between the experimental data from the electrochemical‐calorimetric measurements and the simulation results. This will be demonstrated for the New European Driving Cycle (NEDC).Reference:[1] Matthias Gulbins, et al., Parametrization and Validation of Li‐Ion‐Battery Cell Models (this Conference)