Add chicken drumsticks to slow cooker.
Halve the onion, head of garlic, and lemon. Add to slow cooker.
Chop the carrots and celery into big pieces (you want them to easily be removed when the broth is done). Add to slow cooker.
Add salt, pepper and bay leaves to crockpot. Fill the slow cooker with water (about 16 cups)
Turn on high and let cook for 4 hours.
After 4 hours, remove the drumsticks. Separate meat and remove bones (make sure you remove the thin, needle-like bone as well. Separate skin and cartilage and throw away. Add the 8 thick bones back to the slow cooker.
Turn temperature to low and let cook overnight or for 10-12 hours.
Cut slow cooker off and let the broth cool. Poor through a fine mesh strainer and voila! You have chicken bone broth.