The ongoing transformation of the mobility sector towards more environmentally friendly transport systems is leading to increasing market penetration of electrified mobility solutions. As part of these solutions, personal micromobility represents a steadily growing market. The need for fast charging options results in increased requirements for thermal management and state estimation algorithms to operate battery storage systems within acceptable operating ranges.
For this purpose, the aim of this study was to reveal the potential of online impedance spectroscopy to determine single cell temperatures on a commercially available e-bike battery. By performing a systematic cell characterization in the frequency domain, the state of charge and temperature dependencies are analyzed and frequency ranges relevant for temperature determination are identified. Figure 1a shows the cell impedance at different temperatures and states of charge, from which, considering the state-of-charge as a disturbance, a suitable frequency range can be determined, as shown in Figure 1b.
Within the scope of this work, different algorithms for impedance determination will be investigated with respect to their suitability for online operation in e-bikes. A final verification of online impedance spectroscopy using realistic loads, as shown in time domain in Figure 2a, will demonstrate the potential of impedance-based temperature determination under real conditions at the cell level. Preliminary results of two of the algorithms are shown in Figure 2b and demonstrate, first, strong fluctuations of a Goertzel filter due to its frequency selectivity and, second, stable results of the RMS algorithm after a necessary settling time.