Voltage reversal is a critical issue for serially stacking microbial fuel cells (MFCs). It occurs when current density in stacked MFCs increases over critical current density (j(critical)). In this study, we clearly show that no voltage reversal occurs in stacked MFCs if current density is maintained below j(critical) where the anode and the cathode potential in an inferior unit become identical, with an external resistance placed between individual MFCs. We define threshold resistance (R-threshold) that enables current density below j(critical) in stacked MFCs, and demonstrate the validity of R-threshold theoretically and experimentally. Voltage reversal is controlled in the stacked MFC equipped with R-threshold by which the current density in the stacked MFC is kept below j(critical). In comparison, a stacked MFC without R-threshold faces voltage reversal over j(critical). Energy loss in R-threshold is comparable to energy loss with other voltage control methods, such as passive or active methods. However, the R-threshold approach is a simple, inexpensive way of controlling voltage reversal, especially for small MFCs (<50 mL). (C) 2015 Elsevier B.V. All rights reserved.