On a car you wire it into the wire that is feeding the battery. If you have your engine running and everything is working right it should show a bit of a charge (+) If it shows 'negitive' then you need to switch the connections on the back of the amp gauge. A note: all the 12 volt a car uses/makes passes through the gauge, but Not The Starter's draw; Too Much Juice! There may be a marking on the back of your gauge telling you which side goes the the battery.