I get paid to do all sorts of things with voltage and current, among other things...
Since bulbs are generally sold as a certain power (wattage) for a power supply at a given voltage, I put it in those terms to give a useful answer without a lot of extra stuff (if you want details, pm me...). Worrying about the current isn't that important, because batteries are (at least as far as headlights go) capable of infinite current at whatever voltage they're at, and for all the likely possibilities for substitution, the batteries aren't likely to suffer damage.
I also left out the resistance as a function of temperature. The filaments in incandescent bulbs change resistance as they heat up, so using V=IR to get the resistance of a bulb isn't quite correct if you're going to run it at a different voltage, but it really doesn't matter much in this case.
For halogen lights there's generally very little, if any, circuitry-- a switch, and if you're lucky a fuse. The Nightriders I've taken apart don't have a fuse. When I rebuild them I put one in-- it's much cheaper to buy NiMH cells online and rebuild your batteries yourself than it is to buy them from nightrider.