Trim excess fat and any small tail pieces from the chicken carcasses.
Place the chicken bones in a large stock pot with the carrots, celery, onion, parsley, bay leaf, and peppercorns.
Pour in the cold water until everything is covered.
Slowly bring to a gentle boil over medium heat.
Skim off any foam that rises to the surface.
Reduce heat and simmer uncovered on very low heat for 3–4 hours.
Strain through a fine mesh sieve.
Let cool and store in containers or freezer bags.