Temperature affects lithium-ion battery performance, health and safety. Therefore, obtaining temperature is quite necessary for battery management system. Impedance is widely adopted and proved to be feasible to estimate temperature. In the paper, wavelet transform is applied to process the step current and the response voltage of the battery cell to obtain the impedance. And to reduce the computational complexity and process the signals automatically for online application, the basic method is improved. Considering that the step current used is a kind of non-steady perturbation and it may change the operating points of the electrochemical system insider the battery, a series of tests are performed to study the effect of the changing step current conditions on the impedance calculated. The results show that the impedance calculated at low frequencies with the method is always different from electrochemical impedance spectroscopy. The impedance in the mid-high/high frequency range is nearly not affected and provide guidance to use the impedance to estimate the temperature. With the battery impedance phase at 10 Hz calculated online, the temperature is finally estimated. The average error is 0.33 °C during step current test and max error 0.5 °C during NEDC test, showing a feasibility of the fast impedance calculation method in the battery temperature online estimation.