Place all ingredients, except for one can of beans, in the slow-cooker and set to low for 8 hours.
Around hour 7, use stick blender in soup for about 2 minutes. Alternately, if you don't have a stick blender, take about 2-3 cups out and whirl in blender or food processor for a minute or two. Add back into cooker and add the other can of beans. Cook for another 45min to an hour.