As of my last knowledge update in September 2021, flexible graphene-based batteries were still in the research and development phase, and there wasn't a standardized or typical voltage rating for them. The voltage rating of a battery is often determined by the specific materials, design, and configuration used in its construction. Graphene-based batteries were being explored for their potential to offer high energy density, flexibility, and other advantageous properties, but their commercialization and standardization were not yet well-established.
It's possible that there have been advancements or developments in the field since then, so I recommend checking recent research papers, news articles, or battery manufacturer websites for the latest information on the voltage rating of flexible graphene-based batteries.