Grease your large slow cooker (6-8 quart) with nonstick spray. Stir together chili powder, cayenne pepper, garlic powder, salt, and pepper. Rub mixture all over the top side of the ribs.
Brush about ⅔ of the bbq sauce over the ribs (reserve remaining bbq sauce), top and bottom. Place the ribs in the slow cooker so that they are standing up on one side and curl to fit into around the inner edge of the slow cooker. Cover and cook on low for 8 hours.
Preheat your oven to high broil. Use tongs to transfer ribs to a greased baking sheet so that the ribs are laying flat. Transfer to the oven and cook for 5-8 minutes. (Set a timer for 5 minutes and check them to make sure they aren’t burning). Allow to cool just long enough so that the ribs can be handled and serve immediately