Check you voltage at the battery posts themselves, do not use the chassis ground. A fully charged battery should read no more than 12.68 volts. Anything higher is either a surface charge, or can indicate a problem with the battery itself. Try leaving the meter hooked up to the posts and have someone try to crank the car over and watch what the voltage does, if it drops immediately and nothing happens with the car, bad battery. If the voltage doesn't change at all and nothing happens, wiring issue.