Confidence votes 1.7K. Assuming we're talking about the `usual' 12 volt car battery' the resting voltage should be around 11 to 11.5 volts. Under charge it's as high as 15 volts as supplied from the alternator,and most cars won't start if the voltage is under 10.5 to 11.5 volts. The term `12 volt battery' is what's referred to as, `nominal' or `in name only' as a general reference and not meant to be an accurate description.