Almost all kinetic energy harvester (KEHs) are inherently nonlinear. In addition, most practical systems introduce supplementary nonlinearity to improve their performance. Qualitatively and quantitatively understanding the nonlinear behaviour in these devices can highlight important parameters with regard to optimisation and design. Capacitive energy harvesters require an initial electrical bias, resulting in electromechanical coupling. Thus, the electromechanical transducer force is present in all capacitive harvesters. It is the cause of numerous well studied phenomena, such as the modification of the resonant frequency, for alternative bias voltages. The effects of the coupling in two of the most common conditioning circuit configurations has been described and compared with experimental results. Furthermore, semi-analytical analysis of the coupled systems has been completed, giving further insight into the behaviour and potential optimisations.