To charge a battery off of a ac system it first needs to be a system that can output close to or above 12v (car alternators put out around 14.5v).
You then need an regulator & a rectifier. do a google search for these things lots of great info on what they do to ac voltage.
After that all you really need to do is wire the hid into the battery include some type of switch so you can turn the light off when your bike is not on and also please wire an fuse inline (as close to the battery as possible).
As long as your bike is putting out more voltage than the lights on you bike are using up you should not have issue with draining the battery.
If you can find another headlight lens that would benefit from a HID bulbs light output then your light will work much better and other motorists will thank you (I hate those people that toss HIS kits into the Civic's and blind everyone else on the road. An projector style lens with a cutoff will provide the above two things.