Eckhard Karden

Modeling the dynamic behavior of supercapacitors using impedance spectroscopy

This paper presents a new approach of modeling the dynamic behavior of supercapacitors using impedance spectroscopy. The new Matlab/Simulink model can be used in simulating the voltage response and energy efficiency, e.g. for automotive power systems. The model in the frequency domain, the parameter calculation as well as the inverse transformation into the time domain are discussed in detail. Furthermore, simulation results are compared with measured data obtained on a test bench.

Impedance-based non-linear dynamic battery modeling for automotive applications

This paper presents a systematic approach to employ electrochemical impedance spectroscopy for determining model structure and parameters of a simulation model for a VRLA battery. It focuses on the interpretation of the impedance data in terms of equivalent circuit models and describes the Matlab/Simulink implementation of the model as well as its time-domain verification. Furthermore, the advantages and limits of the impedance-based model as well as the possible simplifications are discussed.

A method for measurement and interpretation of impedance spectra for industrial batteries

Abstract Impedance spectroscopy is a promising tool for the modeling and diagnosis of industrial batteries. This paper discusses methodological questions connected with the measurement and interpretation of the impedance of such batteries, especially nonlinearity, voltage drift, stability, reproducibility, half-cell measurements, model structure and parameter extraction with respect to quantities like state of charge (SOC). On the basis of this discussion, a specialized impedance spectroscope for industrial batteries has been developed, as well as modifications of the standard electrochemical impedance spectroscopy (EIS) algorithm. A mini-cycle technique is suggested that gains additional information compared to classical measurements with continuous dc current offset. Impedance spectra from lead/acid batteries for different dc currents, SOCs, and temperatures are presented and analyzed. Reference-electrode measurements allow for separation of the half-cell impedances. Emphasis is placed on the limits of experimental reproducibility due to “history” of the battery.

A frequency-domain approach to dynamical modeling of electrochemical power sources

Electrochemical impedance spectroscopy can be used to obtain simulation models for the non-linear, non-stationary dynamic behavior of electrochemical power sources. Some extensions of porous-electrode theory are necessary for this purpose, which are not provided by standard EIS data evaluation software. This paper presents experimental data and consistent equivalent-circuit models for different dc conditions, regarding three electrochemical systems: supercapacitors as almost ideally blocking porous electrodes, lead/acid batteries under overcharge (water electrolysis) operation, and the same battery type under discharge/charge operation. The series inductance which masks the electrochemical behavior for frequencies as low as 100 Hz for a 100 Ah cell can be attributed mainly to the macroscopic cell geometry. The dependence of impedance parameters on direct current (non-linearity), temperature, state-of-charge, and previous discharge/charge regime is investigated. It is shown that model parameters extracted from the impedance spectra are closely linked with the charge-transfer kinetics, double-layer capacitance, transport limitation, and porous structure of the electrodes. Consequences for impedance-based determination of the batterys state-of-charge or state-of-health are discussed.

Charge equalizing and lifetime increasing with a new charging method for VRLA batteries

A new charging method for valve-regulated lead-acid (VRLA) batteries is presented which limits the maximum of all block voltages in a battery string. A commercially available VRLA battery was equipped with cell-pressure and cell-voltage sensors. The new method was applied to the battery in comparison to conventional IU- and IUIa-charging methods. Voltage and pressure measurements during these experiments show clearly that the conventional charging methods lead to excessive overcharge of single cells correlated with irreversible water losses. These problems are efficiently avoided by the new charging method. Capacity data from cycling operation prove as well that the new charging method guarantees charge-equalizing between the inhomogeneous cells while avoiding additional hardware.

Electrochemical modelling of lead/acid batteries under operating conditions of electric vehicles

In order to overcome deficiencies of phenomenological models of batteries for electric vehicles, this paper analyses models based on electrochemical laws. Considering one space coordinate, there are essentially two approaches: (i) macrohomogeneous and (ii) single-pore models. It is shown that they are equivalent under general assumptions. A macrohomogeneous model of a valve-regulated lead/acid battery is calculated for a highly dynamic discharge current pattern. Simulated voltage as a function of time matches experimental results, but further work is necessary, especially in a careful determination of several empirical parameters. Finally, battery models restricted to ordinary differential equations are discussed as an approximation of spatial models.

On the impedance of the gassing reactions in lead-acid batteries

Oxygen and hydrogen evolution reactions in flooded lead-acid batteries during float charging were studied by galvanostatic steady-state polarization and impedance spectroscopy techniques. Given the very low relaxation frequencies of such processes (between 2 and 0.05 mHz), impedance measurements needed to be extended to the ultra-low frequency domain. Investigation of their dynamic behavior provided a complete description of the porous character of the active zone. The steps involved in the reaction mechanisms are also discussed.

Supercapacitors and lithium-ion batteries for power electronic applications

This article employs the method of electrochemical impedance spectroscopy (EIS) to extend the physics-based, nonlinear equivalent circuit models of supercapacitors (SCs) to describe lithium-ion batteries (Li-ion). The following section briefly introduces the method of electrochemical impedance spectroscopy and presents measured impedance spectra. From these spectra, appropriate equivalent-circuit models are deduced. After this, the MATLAB/Simulink implementation of the new simulation models is discussed, and simulation results, as well as verification measurements, are provided. Finally, conclusions are drawn, and future perspectives of the new impedance-based modeling approach are outlined.

Stop-Start Micro Hybrid: An Estimation of Automatic Engine Stop Duration in Real World Usage

Legislative emissions requirements, customer expectation and environmental concerns are driving the introduction of Hybrid Electric Vehicle (HEV) technologies. In the European market, where diesel powertrain technology has high penetration, Micro Hybrid technology, featuring engine stop/start plus regenerative charging, is attractive due to system cost versus CO2 emission benefits. The availability of the engine stop/start feature in real world usage depends on the control logic taking account of, for example, safety, comfort or other factors. The research reported here involved developing tools to analyze the duration of automatic engine stop events in real world usage taking account of the situations where automatic engine stop would be unavailable. These tools help determine the durability requirements for key system components, in particular the battery, and estimation of the likely fuel savings as a function of the system calibration.

Analysis and evaluation of charge balancing circuits on performance, reliability and lifetime of supercapacitor systems

Supercapacitors, also known as ultracapacitors or electric double layer capacitors (ELDC), are electrical energy storage devices, which offer high power density, extremely high cycling capability and mechanical robustness. Due to their electrical performance, supercapacitors have a high potential to be used in industrial applications. To improve the performance, reliability and lifetime of these capacitors, charge balancing circuits are employed. In this paper, different equalization concepts are analyzed and evaluated. In addition, a simulation approach for the design of supercapacitor systems is proposed. As an example, results from an automotive application are presented.Supercapacitors, also known as ultracapacitors or electric double-layer capacitors (ELDCs), are electrical energy storage devices, which offer high power density, extremely high cycling capability, and mechanical robustness. Due to their electrical performance, supercapacitors have a high potential to be used in industrial applications. To improve the performance, reliability, and lifetime of these capacitors, charge-balancing circuits are employed. In this paper, different equalization concepts are analyzed and evaluated. In addition, a simulation approach for the design of supercapacitor systems is proposed. As an example, results from an automotive application are presented.