I hooked mine to fuse No.1 which is "hot" in positions 1&2 (Acc. & Ign.) of the key. This fuse feeds radio, cigarette lighter and other stuff. It's not really important that it show true battery voltage, just that it tells you if all is normal or if you are above or below normal. You could connect a DMM or accurate, large analog voltmeter to the battery terminals and run the engine at idle, then at ~2000RPM and try turning on various lights and accessories that draw a fair bit of current (blower fan, wipers, rear window demister) and watch both voltmeters. This will tell you what actual battery voltage is under those conditions and what the voltmeter says. It may read 1-volt below...but once you know that, so what? ANY DIY wiring hooked directly to the battery should be fused as close to the battery as possible or you are risking trouble. Easier to take it from the fusebox.