Lithium ion batteries have become a promising candidate as an energy storage in contemporary devices, especially in the automotive field. However, new challenges arise from the flat state of charge to open circuit voltage curve, which puts the Electrochemical Impedance Spectroscopy (EIS) in the lead of condition monitoring. Usually, in the EIS the Fast Fourier Transformation (FFT) is used. The FFT is known to be less precise if the excitation frequency is not centered in the bin, i.e. not an integer multiple of the sampling frequency. This work proposes an alternative approach to calculate amplitude and phase of a frequency contained in a signal, which can be used for EIS or an in operando impedance calculation. To evaluate the new algorithm, a comparison with the FFT is provided initially with artificial data, then using a battery model. Using the proposed algorithm, the error is slightly higher compared to the FFT in its optimum working point, but features almost two times higher precision in an unfavorable constellation for FFT. Furthermore, it is not depending on the sampling frequency.