Flexible lithium-polymer (LiPo) batteries can come in various voltage ratings, but one of the most common voltage ratings for a single cell (1S) flexible LiPo battery is 3.7 volts. This is the nominal voltage, and the fully charged voltage can be around 4.2 volts, while the discharged voltage can drop to around 3.0 volts.
It's important to note that flexible LiPo batteries can also be manufactured in multi-cell configurations, such as 2S (7.4V), 3S (11.1V), and higher, by connecting multiple individual cells in series. The voltage of the multi-cell configuration will be a multiple of the nominal voltage of a single cell. For example, a 2S battery will have a nominal voltage of around 7.4 volts, a 3S battery will have a nominal voltage of around 11.1 volts, and so on.
Always refer to the manufacturer's specifications and guidelines for accurate information about the voltage ratings and usage of specific flexible LiPo batteries, as there can be variations and developments in battery technology over time.