High frequency battery impedance measurements for EMI prediction

In embedded applications which include power converters, the battery that provides the energy is a part of the EMI path. As conducted emissions generated by power converters spread into a large frequency band, the battery characterization should be extended to cover this range of frequencies. In this paper, a method to measure the battery impedance using an impedance analyzer is explained and applied to a cell-phone battery from 1MHz to 100 MHz. In this frequency range, measurements show that the battery impedance is unrelated to the battery state of charge and that a model including just inductances and resistances matches very well with the measured behavior. The measured impedance is validated by predicting the frequency spectrum of the battery´s voltage when feeding a switched-mode power converter. The converter switches at 1.2MHz and the related harmonics extend beyond 100 MHz. The measured and predicted voltages match up to 80 MHz.