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Electrical, thermal and lifetime modeling

Lupe

To enable proper and accurate designing of energy storage devices for equipment like electric and hybrid powered vehicles, or for stationary equipment (e.g. temporary storage for photo-voltaic or for wind powered devices) we require reliable simulation models, with the electrical and thermal characteristics provided by these models it is possible to predict the lifetime. Furthermore, battery management systems use model-based methods for state estimation of the system and the cells. There are various modeling concepts available:

 

 

Impedance based modeling

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The aim of impedance based modeling is to develop an equivalent circuit of the electrical characteristics of the energy storage device. Equivalent circuit models contain simple models consisting of a variable voltage source, an internal resistance, an inductance and multiple RC elements, fractional models, which are capable of modeling special impedance characteristics by using Warburg elements or constant-phase elements. Other models are designed especially for the high frequency behaviour of the cell. The equivalent circuits and the values for the various elements can be determined through impedance spectroscopy or the evaluation of the voltage response to a current pulse. By fitting the impedance spectra and the voltage responses the model parameters can be extracted.

For all battery types the impedance spectrum / voltage response is altered by a variety of external influencing factors, foremost of these are DC ripple current, temperature and state of charge. Therefore the broad range of measurements required to cover the varied tasks performed by a battery must be implemented. Another problem is the size of batteries and hence the associated irregularity in temperature, power distribution etc.

Physical-chemical modeling

In contrast to an impedance based model, a physical-chemical model is not based on measurements from an equivalent circuit but rather on emulating the expiring processes.

The model parameters will be established by means of geometrical data and material characteristics. The benefit of this approach is that you can also simulate new battery concepts without actually having a battery. On the other hand all the relevant processes must be known and modeled for the copy to exhibit accurate behaviour. Likewise the interior structure must be known, which often requires a battery to be opened. The ageing process will also have to be incorporated into the model so that any influences from e.g. operating strategy or the effects of internal structures on the different ageing mechanisms can be examined. Models like the electrode average model, the single particle model and the pseudo two-dimensional model, which are e.g. programmed in Matlab, and finite element models, which are modelled in special multiphysics software solutions.

Thermal models

Thermal models are essential for determining the temperature in a battery system model and on the other hand to simulate the battery’s thermal characteristics. These will be required for the designing of cool or hot system applications, but also to determine the ageing behaviour. The models could be based on thermal equivalent circuits or finite element models and they are parameterized by conducting special thermal measurements or by using the material data. In virtually all battery technologies increasing the temperature accelerates the ageing process, where extremely low temperatures can cause harm to the device.

Ageing models

Also ageing models can be splitted into two different model types, the physical-chemical and the semi-empirical models. The latter is based on the determination of capacity and internal resistance over the course of the ageing process. These two factors will be determined by the ageing tests where the different influencing factors are varied, so that the influence can be quantified. These are the most common models to design battery systems and to estimate the battery cell ageing in battery management systems.

In the area of physical-chemical modeling the aim is to identify and reproduce the ageing processes, whereas in the semi-empirical modeling the aim is to reveal the different influencing factors on the ageing processes and their part in them. In doing this we try, above all, to discover the largely unknown mutual influences from different factors.

System models

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The knowledge acquired from single cell modeling could be applied to model battery systems. That concerns the cell connections and the electrical, thermal and ageing behaviour. The system model is used for research regarding current distributions, resulting thermal behaviour and the possible inhomogeneous ageing. Even contact and conductor resistances could be taken in to account.

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Technische Universität Berlin
Electrical Energy Storage Technology
Institute of Energy and Automation Technology
Faculty IV
sec. EMH 2
Einsteinufer 11
D-10587 Berlin

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+49 (0)30 314-21633
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