Well, I'm no auto-electrician so I can't give a definite on that, but it's current (amps) that you need, not volts. As the battery discharges, the voltage will stay around 12v for quite a long time, and then fall away fairly quickly as it becomes depleted - that's why the voltage doesn't necessarily tell you how much charge you have. I think if you're much under 10v you will have trouble starting it.
that a pretty good explanation. thanks.