Yep 12V car battery. Attach clip on leads to the battery and the volt gage. use a voltmeter on the battery to get its actual voltage, then you can adjust the potentiometer on the volt gage itself to fine tune move the needle to the left or right where you want it set. I adjusted my gage needle with the battery charged at 13.5V on the bench to where the needle is just a tick over half. Or slightly on the charge side of the ammeter face.