Combine beans and 8 cups of water in a large saucepan.
Bring to a boil, and reduce heat.
Turn down heat and let simmer for 2 minutes.
Remove from heat and let stand for 1 hour (covered).
Drain water and rinse beans in a colander.
Add beans to slow cooker and add 8 cups of water.
Add chopped onion, ham, carrots, salt, and pepper. (Add celery if desired).
Cook on low for 5 hours, or medium for 3 hours.