In a 5- to 6-quart slow cooker combine chicken carcasses and skin, carrots, onion, celery, parsley, thyme, garlic, salt, and peppercorns. Add enough cold water to cover mixture in cooker. Cover and cook on low-heat setting for 12 hours.
Strain broth through a large colander lined with damp cheesecloth; discard solids. Cover broth and chill overnight. Before using, lift off and discard fat layer.