Julia Kowal

A review of current automotive battery technology and future prospects

In this article, today’s battery technologies and future options are discussed. Batteries have been one of the main focuses of automotive development in the last years. Technologies that have been in use for a very long time, such as the lead–acid battery, are indispensable but need improvement. New technologies such as the lithium-ion battery are entering the market. Supercapacitors (also known as electrochemical double-layer capacitors) can be used for high-power requirements such as regenerative braking. The variety of vehicles has increased with the introduction of hybrid vehicles, plug-in hybrid vehicles and electric vehicles and, for each type, suitable battery types are being used or under development. Appropriate battery system designs and charging strategies are needed. Battery technologies can be classified according to their energy density, their charge and discharge characteristics, system integration and the costs. Further relevant performance parameters are the calendar lifetime, the cycle lifetime, the low- and high-temperature performances and the safety.

Efficient battery models for the design of EV drive trains

Energy storage devices have become an important part of many mobile applications, such as conventional, electric or hybrid electric vehicles. For an optimal and efficient system design, simulation models have become an inevitable tool. Accuracy over a wide range of operating conditions is very important. However, electrochemical storage devices are strongly nonlinear and often show inhomogeneous current distribution leading to performance and lifetime limitations.

Model Parameterization of Nonlinear Devices Using Impedance Spectroscopy

In this paper, a new approach is used to identify the parameters of a large-signal lumped-element model for nonlinear devices based on small-signal impedance measurements at several operating points. Starting from the large-signal impedance of an equivalent circuit with nonlinear devices, the relationship between current and voltage is derived by the determining nonlinear system equation. This differential equation for a first-order model is then linearized via nonlinear series expansion using the perturbation approach. The linearized ac part (which is the fundamental frequency part) is then transformed into the frequency domain to find a representation of the complex small-signal impedance for a given operating point. Large-signal modeling based on small-signal measurements is successfully demonstrated for organic light-emitting diodes.

Elektrische Batteriespeichermodelle : Modellbildung, Parameteridentifikation und Modellreduktion

This work deals with the electrical behavior of lithium-ion batteries. The aim is to gain as much information as possible by only monitoring the current and voltage behavior of a battery without being able to look inside. For this, a model representation of the processes inside a lithium-ion battery is developed. For all important phenomena, the influence on the electrical voltage response is derived and merged into a mathematical framework. For parameter identification based solely on the voltage curve, these model equations must be simplified in order to obtain the necessary computation speed for a parameter estimation algorithm. The parameters whose sole influences on the electric behaviour can not be separated without changing the system are summarized so that a model with purely electrical quantities is derived. Using different methods, the model order is significantly reduced while minimizing the approximation error. The model thus transformed, based on the model equations of porous electrodes according to Newman, can even be used for voltage prognosis or for parameter tracking in onboard diagnostics. Furthermore, a method is presented with which the individual processes can be quantitatively measured and separated without destroying the battery cell. In total, more than sixteen different types of batteries were extensively measured within the scope of the work. On the basis of the findings derived from this, the development of the parameterization procedure was carried out. Thus, it is now possible to separate the individual processes occurring in the battery and to support a hypothesis of the assignment to positive and negative electrodes on the basis of several indicia. For this purpose, the alternating current electrochemical impedance spectroscopy (EIS) and a correlation analysis are used to calculate the distribution function of the time constants (DRT) occurring in the impedance. Together with a series of tests with direct current excitation, a measuring method is obtained which, with the correct evaluation methodology, allows a broad characterization of the battery behavior. Finally, the described method is used in two studies. In the first, the possibilities of a current-voltage model derived from the measurements are demonstrated by fully parameterizing the battery cell of the first generation Mitsubishi iMiEV. The overvoltages of both electrodes can be viewed separately and the effects of so-called blend electrodes are also visible. The appendix of the work additionally includes the parameters of the cell of the first generation BMW i3. The second study examines the changes in the model parameters during the cyclic aging of batteries. Further information on the assignment of the processes to the two electrodes is derived from this data. This precise knowledge of the composition of the voltage and the assignment of the overvoltages to the individual electrodes can in the future allow a completely new control of the operation of a battery. The prognosis quality of the condition and the remaining lifetime of the system will be improved significantly by applying the presented methods.

Speicherung der elektrischen Energie

Die Speicherung der elektrischen Energie im Fahrzeug und ihre bedarfsabhangige Abgabe gehoren neben den elektrischen Maschinen (siehe auch Kapitel 2.2 und 2.3) und der Leistungselektronik (siehe Kapitel 4.1) zu den Schlusseltechnologien fur die Elektrifizierung des Antriebsstrangs. Die Batterie (genau gesprochen der Akkumulator wegen der Wiederaufladbarkeit) ist der heute ublicherweise eingesetzte Energiespeicher. Schon vor etwa 100 Jahren sind die damals dominierenden Elektrofahrzeuge an der Batterietechnik gescheitert. Die Batterien waren zu schwer, zu gros und hatten eine zu geringe Speicherkapazitat. Die jahrzehntelange Entwicklung fuhrte inzwischen zu Alternativen zur konventionellen Bleibatterie, wie der Nickel-Cadmium-, der Nickel-Metall- Hydrid- oder der Lithium-Ionen- Batterie. Nachfolgend werden die Grundlagen der Batterietechnik beschrieben. Die Lithium-Ionen-Batterie als besonders erfolgversprechende Entwicklung fur den Fahrzeugeinsatz wird in Kapitel 3.2 vorgestellt.