I think all HID ballasts are 12v. You'll need either another 6v battery to hook in series (same AH rating) or a voltage regulator that can upconvert, which will cost more than just buying a new 12v battery.

Offhand I'd say the cheapest way out is to buy a new battery; batteryspace has them with charger for well < $100, or you can get an SLA if you want to slum it. I did for a year.