Vibration energy harvesting is a technique for the generation of electricity from ambient vibrations. Electrostatic vibration energy harvesters (e-VEHs) are particularly compatible with microtechnology and can serve as a source of energy for autonomous microsystems. E-VEHs require conditioning electronics to effectively sequence different phases of energy conversion. One of the most promising circuits for e-VEHs is the charge pump, presented here with a resistive return circuit. E-VEH as a system is composed of a mechanical resonator coupled with a conditioning circuit and is strongly nonlinear. We discuss different models of this system and show how steady-state oscillations of the system can be analysed. In order to do so, the system is represented as a nonlinear oscillator and a perturbation technique, such as the multiple scales method, can be applied.