So I'm looking into buying a portable fan, most I've looked at run on D batteries (8) aka 12V. Now I don't feel like buying 8 D batteries every time I want to run this thing and I can't find rechargeable Ds (im sure they would be expensive). I was thinking however, I do have number of 18v drill batteries, is it feasible to fun the fan on off of those?
Yes. You just need to lower the voltage to 12V. A simple PWM circuit is all that you need. Something like this: