Place the mung beans into a large container and cover with several inches of cool water; let stand 3 hours to overnight. Drain and rinse before using.
Combine the beans with 5 1/4 quart water in a large pot over medium heat; cook until the beans are tender, about 30 minutes. Stir the sugar into the mixture; cook and stir until the sugar is completely dissolved. Remove from heat and serve warm.