Last Updated on October 28, 2007
Parasitic (or ignition key off) drain is the cumulative load produced by electrical devices, for example, emissions computers, clocks, security alarms, radio presets, etc., that operate continuously after the engine is stopped and the ignition key has been switched off. Normal parasitic loads are below 75 milliamps (.075 amps). When the parasitic load is greater than 75 milliamps (.075 amps), batteries will drain more quickly. Glove box, trunk, and under hood lights that do not automatically turn off when the door is closed or shorted diodes in alternators are the most common offenders. Cooling fans, power seat belt retractors, radios and dome lights left on, alarm systems, and electric car antennas have also caused batteries to drain overnight. Leaving your headlights on will generally discharge a fully charged car battery, with 90 minutes of Reserve Capacity (36 amp hours), within a couple of hours.
It is highly recommended, especially if you are using a sealed wet "Maintenance Free" (Ca/Ca) battery, that you allow it to thaw if frozen, fully recharge it in a well ventilated area with an external battery charger, remove the surface charge, and load tested both the battery and the charging system for latent damage from the deep discharge. You could have a damaged or bad battery. If the alternator is warm and the engine is cold, then check for a shorted diode in the alternator.
Below are some methods that are used to test the parasitic load with the engine NOT running, under hood light disconnected, all accessories switched off, and the vehicle doors closed: