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The impedance of power-net systems in cars is described and modeled. The effects of power sources, electric devices, and cabling on the systems' impedance are investigated to predict frequency-dependent voltage fluctuations and resultant possible interferences. Then, a rapid and accurate method for measuring the impedance of electrical power-net systems in cars is presented. This method is based on a time-domain measurement. At an access point, where an electrical load is to be connected, the power net is loaded with a current pulse with peak amplitudes up to 100 A. The current pulse and the resulting voltage response are digitally recorded versus time. By means of a Fourier transform, the impedance at the access point is computed in a frequency range between 0 Hz [direct current (dc)] and 100 kHz.