As of my last knowledge update in September 2021, flexible graphene-based batteries were still a topic of ongoing research and development, and there wasn't a standardized "typical" voltage rating for such batteries. The voltage of a battery is determined by the materials and design used in its construction, and graphene-based batteries can vary widely in their specifications based on the specific technology and advancements being pursued by researchers.
Graphene has unique properties that can potentially be utilized to create batteries with high energy density and improved performance, but commercial applications and standardized voltage ratings might not have been established yet.
To get the most accurate and up-to-date information on the voltage rating of flexible graphene-based batteries, I recommend checking recent research papers, scientific articles, and news from reputable sources in the field of battery technology and materials science.