The design of power aware wireless sensor systems has gained increasing importance in recent research. Challenges in designing such devices are a throughout energy scalable design of hardware, signal processing and communication and an energyeffective energy management. The power supply should be capable to 'harvest' ambient energy and to accumulate superfluous energy in an energy storage unit in order to be able to bridge low energy availability. After an overview of the state-of-the-art in key technologies for power-aware portable sensor systems, we focus in this paper on energy management. An effective energy management plays a key role for energy-aware adaptation of system operation. An accurate up-to-date measurement of stored energy and remaining usable storage capacity allows to optimize operation and to extend maintenance-free periods. As challenging example we present recent results for the diagnosis of electrochemical batteries by impedance spectroscopy [24].

Soil moisture sensor systems need extensive calibration processes because of the strong dependence on soil properties. Therefore a suitable method for in-situ soil characterization is required. Using impedance spectroscopy, more information can be extracted from real and imaginary part of the complex admittance for several frequencies at the same moisture value. In this paper, an automatic measurement set-up was developed. It allows keeping similar conditions of the soil during measurements. Data consistency was investigated by means of the Kramers-Kronig relation after compensation of cable effects. For soil type characterization suitable signal features were extracted and subjected to Principal Component Analysis (PCA) via SingularValue Decomposition (SVD). The results are correlated with soil composition. The first two principal components were sufficient for classification of unknown soil type by the k-nearest neighbor method.

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