In that case all you will need would be a way to cut the power from the solar panel when the battery is full. The panel may put out 18 volt open but will pull down to battery voltage.

If you want to get the maximum out of the solar cell, early morning , late afternoon, cloudy day, you can use a boost circuit to bring it up to charging voltage. Then when the cell is producing charging voltage on its own the boost circuit drops out and the solar cell is back to straight into the battery.