Abstract Stack configuration of multiple bioelectrochemical system (BES) modules is considered nowadays as the best option for a successful scale-up of this technology, either in case of electricity-producing microbial fuel cells (MFC) or in case of electricity-consuming microbial electrolysis or electrosynthesis cells (MEC or MES, respectively). While the parallel electrical connection allows to independently operate each BES in a stack without major issues, serially stacked BES are more appealing from the point of view of energy conversion, as they suffer lower energy losses and it is possible to operate them at higher voltages. However, in the case of MEC/MES cells connected in series, high performing bioanodes can push the less-performing ones in the stack outside their “working zone”, resulting in unfavorable potentials, uncontrolled voltage drops, and the temporal or permanent damage of the electroactive biofilm. A few cell balance systems (CBS) were proposed in the past but requiring expertise in power electronics. In this study an easy, passive and low-cost CBS based on commercial diodes is proposed. Three double-chamber MECs were adopted. A first set of experiments were performed to characterize the cells and understand reasons for voltage unbalance in a series-connected stack. Then, the CBS was adopted and validated.