As of my last knowledge update in September 2021, flexible graphene-based batteries were still in the realm of research and development, and there wasn't a standardized or widely accepted "typical" voltage rating for such batteries. Graphene-based batteries have been explored for their potential to offer high energy density, faster charging times, and flexibility, among other advantages. However, the exact specifications and performance characteristics can vary based on the specific design, materials used, and manufacturing processes.
Since technology and research developments can change rapidly, I recommend checking the latest scientific literature, research articles, or news releases from reliable sources for the most up-to-date information on the voltage rating of flexible graphene-based batteries.