According to the disclosure there is provided a method for characterizing an electrical connection between an energy storage device of an electrical or hybrid vehicle and an external power supply. The method may include, in the vehicle, receiving an alternating waveform from the power supply, determining a fundamental frequency of the waveform, determining if the waveform is distorted, and, if it is determined that the waveform is distorted, determining if the distortion is an indication of a loose connection between the vehicle and the power supply. There is further provided a charging system for characterizing an electrical connection between an energy storage device of an electrical or hybrid vehicle and an external power supply.