'58 mercury medalist, 312. 12v with a generator. voltage reads the same at idle as it does when i rev it up, no change. if i drive it around a bit the voltage drops significantly. when i turn the car off and let it sit for a few hours the voltage will read almost a full charge, and still read it when i turn it on. wtf? battery is new, i had them test it. generator is a working generator as well. i know when i rev it it's supposed to read higher than 12v but it has no change.