There is no risk when the e-bike is riding with the electrical system on, as the battery is self-heating by circulating electricity through the cells. However, if the electrical system is off, the battery mustn’t be exposed to the cold. If the battery is left in temperatures of -10 °C or lower with the electrical system off, it may be damaged and lose efficiency.
It is therefore advisable not to leave the battery outside in the cold during the winter. The ideal temperature for storing the battery is between 0 °C and 20 °C.