Pour off and reserve 3 cups water from soaking liquid. Discard remaining soaking liquid.
Add beans and reserved soaking liquid, bacon, celery, and onion to slow cooker (see notes for how to cook in a bean pot).
In small bowl, whisk together ketchup, brown sugar, and molasses. Pour into the slow cooker and stir to combine.
Cover and cook on HIGH for 4 to 6 hours or LOW for 8 to 10 hours, or until beans are tender. Season to taste with salt and pepper if desired.
To soak beans: Add 4 quarts water to a large bowl and add beans. Soak at least 8 hours or overnight.To quick soak beans: Place 1 pound of rinsed beans in a large saucepan and cover with 3 inches of water. Bring to a boil. Boil rapidly for 5 minutes, then remove from heat, cover, and let stand for 1 hour.To cook the beans in a bean pot: you do not need to pre-soak the beans. Preheat the oven to 250°F. In a medium sauce pan, combine beans and 1/4 tsp. baking soda. Cover with water, bring to a boil, and simmer for 3 minutes. Drain the beans and place in a bean pot. Add remaining ingredients and bake for 5 hours.Adapted from Shirley's Hill of Beans recipe.