Soak beans in water for 6 hours. Rinse and drain off excess water. Remove the outer skin.
Blend 3 cups of water with the beans in a blender.
Pour mixture into a muslim bag, and squeeze out all the liquid.
Heat the remaining 12 cups of water in a saucepan, and pour in the juice from the beans as well. Bring to the boil, then add sugar and cook until melted. Serve hot or cold.