One might think that the pairing of dry kidney beans -- an economical protein source -- and a slow cooker -- an essential time saver for busy moms -- would be a match made in heaven. The truth is not quite so simple. Red kidney beans contain a toxin, phytohaemagglutinin, that causes nausea, vomiting and diarrhea if the beans are not completely cooked at a high enough temperature. While you can put your slow cooker to good use with kidney bean recipes, you must handle the dry beans properly before putting them in the slow cooker.

Dry beans need to be soaked before cooking. Wash them and cover with 5 cups of water for each cup of beans. Adding salt to the soaking water won't toughen the beans as it would if added during cooking. Soak the beans for at least 5 hours; overnight is fine. Discard the water and rinse the beans. Boil the beans in rapidly boiling water on the stove for at least 10 minutes before placing them in your slow cooker. Boiling the beans for at least 10 minutes will destroy the toxin.

Salt and acid ingredients, such as tomatoes, prevent kidney beans from softening as they cook. Add these and any seasonings, such as ranch-flavored seasoning mix, near the middle of the cooking time once the beans have started to become tender. If your beans never soften when cooking, chances are you've seasoned them too soon.

In a slow cooker, kidney beans need 6 hours on high or 8 to 10 hours on low. Cooking time depends partly on the age of the beans. The longer the beans have been in storage on the pantry shelf, the longer they will take to cook. Taste test a few beans after 4 to 5 hours to check for tenderness. Allow a longer cooking time for older beans.

Kidney beans will swell as they cook, so allow room for expansion in the pot. A good rule of thumb is to fill the pot no more than half full. Add your salt, seasoning mix and acidic ingredients after 4 hours. Another alternative is to prepare a flavorful sauce separately, then stir it into the beans immediately before serving.