How To Measure Current Draw On Car Battery. First of all leave the hood open and turn off all current consumers eg a car radio exterior and interior lights then remove the key from the ignition lock and close the door. In order to accurately detect the parasitic draw you need to begin with a fully-charged battery.
The final thing to do is to set the range on your multimeter well above the expected readings. To do this the meter must be placed between the negative battery terminal and the body of the car as you need to measure all of the current in all of the circuits. If your testing is done with the engine turned off I would use a shunt hooked to a logging multimeter with a large internal memory and which has enough resolution and accuracy to cover the whole current range.
Now that we have verified that the battery is experiencing a parasitic draw we can move to exploring the different reasons and parts that can be pulling on your cars battery.
The first test with your multimeter will measure DC voltage indicated with a solid line and a dashed line above a letter V. A DMM with the ability to measure continuous current to 10A will be fine in most cases though one rated 20A would be less prone to blowing a fuse. In order to accurately detect the parasitic draw you need to begin with a fully-charged battery. A parasitic draw test is required when a vehicle battery becomes discharged with the key in the off position.
