If you unhook the neg or pos cable and put a voltmeter in series with it you will not get an accurate voltage reading. As you said just 40 milliamps had the voltmeter reading 12 volts. Thats because a voltmeter needs to hooked in paralell in a circuit to read right. When you remove a battery cable and hook a voltmeter to the battery post and the cable you are hooking the voltmeter in series which wont read the correct voltage. Many guys used to use test lights in series with a cable but thats not an accurate way either. Thats why you use the ammeter to check the draw as thats the only way to get accurate reading. Ron