As of my last knowledge update in September 2021, flexible graphene-based batteries were still in the experimental and developmental stages, and their exact specifications could vary widely depending on the specific research and advancements in the field.
Graphene-based batteries have been explored for their potential to provide high energy density, fast charging, and flexibility due to the unique properties of graphene. However, I don't have information on a specific voltage rating for a "typical" flexible graphene-based battery, as there isn't a standardized or widely accepted commercial product or specification available at that time.
It's important to note that battery technologies and their characteristics can evolve rapidly, so there may have been developments since my last update. 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 journals, and news articles related to graphene battery advancements or directly contacting experts in the field.