The charger you have for the batteries... how about making a cord to fit a space in it, and taking that cord down to the bottle? Worth a try, and would save a bit of headache!

I'm kinda off the idea of using a water bottle for my battery. I only have one water bottle mount on my bike, and the Nite Hawk battery takes that up. It's quite annoying. I'd rather use an extra seat bag attached to my head- and down-tubes to hold the battery - which is what I did with my home-brew setup. I'm thinkin' about cracking open the case on my NH battery (I've already screwed the warranty anyway, might as well mess with it until it works the way I want!) and put it into the bag.