Controlling Current with regulator?

I am wanting to power my GPS (TomTom) when I ride my bicycle on super long trips. I have exhausted every other option so I came up with an idea. I was going to either make my own "D" battery pack or use a 12 volt car battery I have in the garage. Either way I want to mount them on my bike. My question is if I make a battery pack how do I control the power coming from it? If I tape 25 "D" batteries together it is more power than the GPS can handle so how do I control how much comes to it? I am looking for some type of hand turn regulator. Nothing fancy.