As of my last knowledge update in September 2021, flexible graphene-based batteries were still an area of active research and development, and there wasn't a well-established "typical" voltage rating for such batteries. The voltage rating of a battery depends on various factors, including the specific design, materials, and fabrication methods used in its construction.
Graphene-based batteries are being explored for their potential to offer high energy density, fast charging, and flexibility. However, the actual voltage rating could vary widely depending on the specific configuration and technology used in the battery.
Since research and development in this field are ongoing, I recommend checking the latest scientific literature, research papers, and news from reputable sources for the most up-to-date information on flexible graphene-based batteries and their voltage ratings.