We investigated the influence of microbial processes on self-potential signals in a Plexiglas tank filled with a water-saturated quartz sand. A small region of the tank was treated with sulfato-reducer bacteria and organic nutrients. Redox potential measurements were performed both in the treated portion containing the bacteria and in the non-treated portion of the tank. Self-potential signals were recorded at the upper (free-air) surface of the tank to monitor biodegradation. We observed a linear correlation between the temporal variation of these self-potential signals and the redox potential difference between the treated and the non-treated portion of the tank. Self-potential signals can therefore be used as a non-intrusive redox sensor. In addition, we propose a geobattery concept in which biofilms and the possible precipitation of metallic particles (biominerals) at the redox front allow electron transfer (therefore a net driving current density) between the reduced and oxidized parts of the system. In turn, this current is responsible for an electrical field in the Maxwell equations.