Parasitic Draw Test Switch Finding electrical problems that drain the battery is getting harder all the time. But using No. with your digital amp meter takes the guesswork out of a complicated job. Attach the test switch between the negative battery cable and the battery, and then road-test the vehicle. Next, connect an amp meter to the switch to determine the parasitic amperage draw, and locate the excessive electrical drain on the battery. Works on any vehicle with side- or top-post batteries. No. – Parasitic draw test switch.