This study focuses on the coupling characterisation of a structure displaying a geometric elastic nonlinearity and paired with piezoelectric elements. In the case of linear transducers, several equivalent definitions based on electrical and mechanical parameters are used to estimate the electromechanical coupling. The validity of these formulations are questioned in the non-linear case, which requires to reconsider the definition of the coupling factor. The definition based on energy transfer inside the resonator is extended to monitor its evolution with the dynamic amplitude of the device. It is supposed that the coupling would not only be function of the equivalent parameters but also of the solicitation amplitude. The definition based on the natural frequencies is also extended to exploit the backbones curves of the transducer under different electrical configurations. An experimental study is performed an on a clamped-clamped beam covered with piezoelectric patches to compare the results obtained with these two methods.