I got a new Optima installed, after 24 hours of sitting the voltage at the batter car, off just key in ON position, not running is 11.5-11.6, let me guess bad battery?

 

12.6 - 12.8 for a new healthy battery, 12.4 - 12.6 for an OK older battery, 12.2 - 12.4 for a battery with a limited time left. Replace a battery with V less than 12.2 assuming that it's been charged and will not hold a charge over 12.2. It's gonna strand you.

 

try measuring the voltage with the key not in the on position and see what you get. There are couple items in the car that come on when its in the on position. Otherwise the last post is corect.