In this paper we consider Resistive switching RAM (ReRAM) devices as memristors and we introduce a memristor model in flux-charge domain rather than the usually prefered voltage-current one. We employ the proposed model during the high resistance state of the device. We relate the parameters of the memristor model with a state variable, the temperature, by using a quasi-static thermal model. The emerging results using this simple model show a very good agreement with the experimental ones, correspondingly.