Out of curiosity I did some research. You COULD go a little lower than 12 volts, but again I get scared of not being able to start the car. Here's what I found: http://carbatteryworld.com/car-battery-voltage/

 

Excuse me, but I feel a rant coming on. I did one for @bigrobot last night and the most difficult part was the lack of information. Now it's about, "How long can I run an unknown load on an unknown size of battery in an unknown car model without knowing how old the battery is. C'mon robot. Help us help you. It only takes a few numbers to change this from rough estimates to tight estimates or maybe even hard facts.

[rant/off]

Thank you for your patience.