Electrochemical-calorimetric studies on different lithium-ion cells

The lithium-ion batteries used in current electric vehicles evidently have reserves. When the capacity in each cell was optimally utilized, more energy could be put in and delivered again. Thus, the driving range of electric vehicles could be significantly increased. In this work, commercial lithium-ion pouch cells were cycled at varying C-rates under isoperibolic and adiabatic conditions in an accelerating rate calorimeter (ARC) with external battery cycler to investigate their performance and their thermal behavior. A thermal camera was also used to investigate the heat effects and the local temperature distribution during cycling in more details. Different factors that have influence on the performance of energy storage such as temperature, charging/discharging current and state of charge (SOC) have been studied. The isoperibolic investigations were performed at specific temperatures in the range from 25 to 60 °C. The results show that the applied environmental temperature did not largely influence the battery thermal behavior. Both isoperibolic and adiabatic tests were performed at different charging/discharging rates in the range from C/4 to 3C. The results show a considerable temperature rise with increasing rate. Additionally, the heat capacities and the calorimeter constant were determined to calculate the total generated heat during cycling

Experimental Analysis of Thermal Runaway in 18650 Cylindrical Li-Ion Cells Using an Accelerating Rate Calorimeter

In this work commercial 18650 lithium-ion cells with LiMn2O4, LiFePO4 and Li(Ni0.33Mn0.33Co0.33)O2 cathodes were exposed to external heating in an Accelerating Rate Calorimeter (es-ARC, THT Company) to investigate the thermal behavior under abuse conditions. New procedures for measuring external and internal pressure change of cells were developed. The external pressure was measured utilizing a gas-tight cylinder inside the calorimeter chamber in order to detect venting of the cells. For internal pressure measurements, a pressure line connected to a pressure transducer was directly inserted into the cell. During the thermal runaway experiments, three stages (low rate, medium rate and high rate reaction) have been observed. Both pressure and temperature change indicated different stages of exothermic reactions, which produced gases or/and heat. The onset temperature of thermal runaway was estimated according to temperature and pressure changes. Moreover, the different activation energies for the exothermic reactions could be derived from Arrhenius plots.</jats:p

A table-driven Li-ion battery model for a BMS development platform - modeling, measurements, implementation and validation

In this paper a simulation environment for the development of battery management systems (BMS) with respect to Lithium-ion cells is presented. This platform should allow for the verification of the suitability and of the interaction between selected hardware components (Li-Ion batteries and semiconductor circuits for battery monitoring), and of the software of a battery management system (BMS) by simulation. The necessary mathematical models for an equivalent circuit model (ECM) based on the theory of passive electric elements are developed. It contains a general system of nonlinear ordinary differential equations to describe the dynamical behavior of Lithium-ion with respect to internal voltages U i , i = 1,...,n, temperature and the state of charge SOC and the cell temperature T. The parametrization of the cells was carried out on the base of CIT (Current Interruption Technique) measurements in an Accelerating Rate Calorimeter (ARC) at different temperatures T and SOC in the time domain with a Least-Squares approach and subsequent optimization. From the parametrization look-up tables are created which are needed for the model implementation in the software. The models have been implemented in the modeling language SystemC AMS using the lookup-tables. The validation was performed with a current profile for an electric vehicle based on a NEDC (New European Driving Cycle) velocity profile

Parametrization and validation of lithium ion battery cell models: Poster presented at 8. Internationale Fachtagung "Kraftwerk Batterie" 2016, Münster, 26. - 27. April 2016

Li‐Ion battery cells are basic components of the power train of Electric Vehicles. Their behaviour is controlled by battery management systems (BMS). As part of the IKEBA project [1], we create a prototype of a virtual simulation platform for BMS. This platform allows to verify the suitability and the interaction between selected hardware components (Li‐Ion batteries and semiconductor circuits for battery monitoring), and the software of a battery management system (BMS) by means of simulation. The electronic system level (ESL) behavioural languages SystemC and SystemC AMS were used for modelling. In this context, models of battery cells are essential. The cells were modelled by equivalent circuit models (ECM) that consist of a series connection of a voltage source, an internal resistance and two parallel connections of resistances (R) and capacitances (C). The components of the ECM cell model depend on temperature and state of charge (SOC). These dependencies can be described by one‐ and two‐dimensional lookup tables (LUT). Because of effort and uncertainties of material parameters, we resigned to determine ECM model parameter tables based on geometry and low level electrochemical behaviour simulations. Therefore, the data points that establish the tables were determined on the base of current interruption technique (CIT) measurements for a special battery cell type. For more details see [2]. The evaluation of cell voltages delivers the corresponding ECM parameter values. A special challenge is to derive the unique R und C values from observed time constants. For this purpose, only small changes from one CIT period to the next were accepted. We could approve the CIT measurement results by simulation with the parametrized cell model as expected. For better validation, we used a current profile that describes charging and discharging a battery based on the New European Drive Cycle (NEDC). Simulation and measurement results were in very good accordance. Only for a measurement with a SOC of 20% at the beginning, small deviations were detected. That means in our opinion, that the presented approach is a good choice for BMS simulations in the design process.References:[1] IKEBA - Integrierte Komponenten und integrierter Entwurf energie‐effizienter Batteriesysteme. Available: http://www.iam.kit.edu/awp/ikeba/[2] Boxia Lei et al., Comparison of calorimetric studies on 18650 lithium‐ion cells and equivalent circuit model‐based simulation applied to driving cycles (this conference)