Use a 6-quart slow cooker. Place the beans in a large pot, cover with water by 2 inches and soak overnight. In the morning, place the drained beans into the slow cooker insert. Add the onion and water. Stir in the barbecue sauce, maple syrup, mustard powder, salt, and pepper. Cover and cook on low for 10 hours, or until the beans have fully softened and begun to split.