Kidney beans contain a toxin that can cause an upset stomach, so they have to be boiled for at least 10 minutes to kill it off. Hotter-running slow cookers will have them boiling by the end of the cooking time and that’ll do the trick, but to be sure you should boil them first: Put the rinsed beans in a large saucepan, add cold water to cover them by at least 2 inches (5 cm), and bring to a boil on the stovetop over high heat. Boil for 10 minutes, then drain, put them in the slow cooker, and proceed with the recipe.