Place the chicken, carrots, celery, onion, bay, peppercorns and salt (if using) in a slow cooker and cover with water (fill to one inch above the chicken).

Cover, set to low and simmer for 8 hours. Pick the meat off of the carcass and reserve for another use, and return to the pot and continue to cook on low for an additional 4 hours (really you could cook it up to an additional 16 hours, it's pretty forgiving and just gets richer).

Cool slightly and carefully pour through a strainer into a bowl. Ladle broth into jars or another storage container, leaving a couple of inches of head-room. If desired, chill in the fridge for several hours until the fat congeals at the top (I never do this as I like a little extra fat in my stock). Scoop the fat from the top, cover and use within 5-6 days, or freeze for 6 months.