FEATURE POWER ELECTRONICS
eMOBILITY: IMPOSSIBLE WITHOUT IMPEDANCE SPECTROSCOPY
The battery is a key element in the step towards electromobility. Within this context, central aspects such as range, charging time, and guaranteed battery life are decisive. Precise information about the battery’s state of health is crucial. The foundations for this have been laid by a new procedure developed by the Professorship for measurement and sensor technology at Chemnitz University of Technology in cooperation with its research partner Rutronik, enabling the use of impedance spectroscopy in embedded systems
T
he move towards developing a 48V electrical system for gradual
adoption into electromobility has been taken by OEMs. They are, therefore, placing their trust in a 12V and 48V hybrid electrical system. With regard to integration into the 48V subsidiary electrical system, the most suitable items to begin with are assemblies with high power consumption, such as the windshield, rear window, and seat heating systems, and the PTC auxiliary heaters. Dynamic loads are also assumed by the 48V electrical system. Besides various compressors, it is primarily the e-boost that provides a hybrid vehicle featuring a drive that is virtually free of loss with the typical low-speed torque curve of a diesel combustion engine and the corresponding driving experience. In addition to this, there is the start-stop generator, which, as a central component of hybrid vehicles, enables the recuperation of braking energy and thus emission-free sailing/coasting. The SoH describes a battery’s overall capacity and the usable capacity remaining for the intended application. For example, a 100Ah battery with a SoH of 80% has a maximum capacity of 80Ah. It is hard to know or even to predict how quickly a battery or each cell of a battery pack will age. Before a battery is installed, current laboratory testing, similar to that used for engines, involves carrying out numerous measurements for various parameters, e.g. different temperature points, charge states, and load profiles. The data obtained in this manner is stored as characteristics in the battery system and provide the basis for estimating the state and safeguarding selected operational areas. In this case, it is assumed that the battery’s performance is the same in the real world as during testing. The battery is a complex, multi-dimensional system
20 SEPTEMBER 2017 | ELECTRONICS
with individual patterns of behaviour, resulting in numerous reciprocal effects. To gain more reliable information about the state of the battery, it needs to be diagnosed during ongoing operation. In other words, a method is required that can directly investigate the current state of the battery and thus provide an alternative to the data collected in the test facility. The ideal method for this is impedance spectroscopy. The battery is additionally tested with varying levels of current and calculated as the impedance along with the resulting voltage. This enables the measurement and assessment of non-invasive battery internal processes such as charge transfer, electrode degradation, and diffusion. In this manner, conclusions about the state of the analysed battery are possible based on measurements. To enhance utilisation of the battery
Figure 1: The demonstrator of the Professorship for
measurement and sensor technology at Chemnitz University of Technology with modular current excitation and analogue signal processing based on the STM32F4 evaluation board. (Photo: Rutronik)
significantly, the Professorship for measurement and sensor technology at Chemnitz University of Technology has developed a procedure based on impedance spectroscopy that allows constant and precise diagnosis of a fully operational battery in just a few minutes. The measurement and analysis occur in real-time within the application. Statements about the state of the battery are, therefore, derived directly from data captured in the application. These provide a reliable basis for statements about the actual state of the battery (state-of-X and remaining useful life). Rutronik supports research through partnerships for undergraduate and postgraduate dissertations and provides the electronic components and development tools. Furthermore, as an official distribution partner and supplier of Li-ion batteries for Samsung SDI, Rutronik is an ideal partner for exchanging know-how on all aspects of battery cells and battery management systems during research projects. To reduce the measurement time
Figure 2:
Factors for installation and optimisation of an impedance measuring system for battery management (Thomas Günther, Chemnitz University of Technology)
from roughly one hour for current methods to just a few minutes, new algorithms have been developed for the excitation signals and enable all calculations to be carried out at the same time as the measurement. The measurement time is thus between three seconds and five minutes depending on how comprehensively the battery is to be analysed. These characteristics allow the
solution to meet all the development needs for controllers in the automotive sector as well as other demands of the target application: It is small, robust, and and can be implemented with an embedded microcontroller.
Rutronik
www.rutronik.com T: 01204 363311
/ ELECTRONICS
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