Usually I simmer vegetables in water on the stovetop and this results in a lot of evaporation. It makes a good rich broth (I cook it for 2 1/2 - 3 hours) but as well as liquid evaporating I find a lot of the broth is absorbed by the vegetables and 3-4 cups of water can become just one cup of broth plus some tasty looking vegetables that I can't eat because I'm juice fasting!
I had thought because of the lack of evaporation that maybe a slow cooker wouldn't make a full flavoured broth but I was wrong - it was the best broth I've ever made.
Here I used half a large swede diced, one Spanish onion and one brown onion plus about a cup and a half of chopped celery. I covered the vegetables with 4 cups of water and a pinch of sea salt.
...Five hours in the slow cooker later, the result was four and a half cups (!) of rich, golden broth as full-flavoured as my stovetop broth! A bonus with the slow cooker is that I could leave it cooking while I was out. I expect I'll never make broth any other way again :)
Annie.