The rated voltage of the 6V6w solar panel is 6v, the rated power is 6w, when the 6v6w solar panel charges the battery, the rated voltage of the battery should be the same as the rated voltage of the 6v6 watt solar panel, which is also 6 volts, the rated power of the battery is less than the rated power of the solar panel, between 4 and 5 watts. So a 6V6w solar panel can be charged with a 6v4 to 5w battery.