The V3 will run on any voltage between 6v and 12v. However, the V3's typically leave the shop calibrated for 6v or 12v power sources. Calibration = setting the low battery indicator and auto-shutoff at appropriate voltages for a 6v or 12v SLA power source. So, a V3 calibrated for 6v will function normally on a 6v SLA battery - low battery indicator will work, auto-shutoff feature will work. A "6v" V3 will also run off a 12v battery, but the low battery indicator and auto-shutoff features will not function properly and you'll likely damage your 12v battery by draining it too much. Alternately, a "12v" V3 will not work on a 6v battery because the 6v battery's voltage is below the calibration thresholds for the low battery indicator and auto-shutoff.
All this to say - if it's marked 6v, you can run it off a battery anywhere from 6v to 12v, but the low battery indicator and auto-shutoff features will not function properly. I use a 9v Lithium Ion battery for mine, and don't need the V3's onboard low battery indicator because my battery has one.