Place the chicken carcass in the crock pot.
Add the carrots, celery, onion, and garlic to the crock pot.
Add the bay leaves, black peppercorns, kosher salt, and fresh thyme sprigs.
Pour 12 cups of cold water over all the ingredients, ensuring they are fully submerged but not overfilled.
Cover the crock pot with the lid and cook on low for 8 hours.
After 8 hours, turn off the crock pot and allow the broth to cool slightly for about 15 minutes.
Using a slotted spoon, remove and discard the large solids (bones, vegetables, herbs).
Carefully strain the broth through a fine mesh sieve or cheesecloth into a large bowl or pot to remove any remaining solids.
Let the broth cool to room temperature, then refrigerate. Once chilled, skim off any solidified fat from the surface if desired.
Use the broth immediately or store it in airtight containers in the refrigerator for up to 5 days or freeze for up to 3 months.