As of my last knowledge update in September 2021, the voltage rating of a typical paper-based microbial fuel cell (MFC) battery can vary depending on the specific design, materials, and configuration used in its construction. However, in general, microbial fuel cells tend to produce relatively low voltages compared to traditional batteries.
At that time, a paper-based microbial fuel cell might generate a voltage in the range of a few hundred millivolts to a volt or so, depending on factors such as the type of microorganisms used, electrode materials, and the specific design of the MFC.
It's important to note that MFC technology is still a relatively emerging field, and researchers are continually working on improving the efficiency and voltage output of microbial fuel cells. For the most up-to-date and accurate information about the voltage rating of paper-based microbial fuel cell batteries, I recommend checking recent scientific literature, research papers, or articles published after September 2021.