A ballast is a thingy that converts your low battery pack voltage to many volts. Typically they need a number of kV (that's 1000 V) to strike an arc then they throttle that back to run the lamp once it's lit.
Have a look on Mr Knacker's site and you'll see heads with ballasts and battery packs with ballasts. You can't have the cable too long from the ballast to the bulb as if it's too long the voltage drop is too high, IIRC. Generally the bulbs plug straight into the ballasts which is why you can't simply swap a halogen bulb for an HID bulb.
Here you go. You can clearly see the bulb with ballast slug right behind it on KnackWhack's site. He'll be knocking on your door in a minute with a Visa swipe machine with which he will beat you repeatedly until you buy something off him. Don't succumb. Resist. Be strong.
Salvo HID Light heads and ballasts at divingniknaks