A safe and accurate method of measuring parasitic draw involves a 1.0-ohm, 10-watt resistor. The resistor is placed in series between the negative battery terminal and the negative battery cable to dynamically test parasitic draw. The key-off battery drain passes through the resistor, while battery voltage is allowed to stress the electronic components on the vehicle. Very little current passes through the meter, so there’s no risk of meter damage.

With the ignition off, placing the test leads of a digital voltmeter on either side of the resistor measures the voltage drop across the resistor. Using Ohm’s law, we know the measured voltage drop is a direct reflection of the current flow or parasitic draw that’s present. So a reading of .02 volt on our meter indicates a .02-amp (20mA) key-off battery drain.