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 150 milliamps (.150 amps), batteries will drain more quickly. Please note that on newer vehicles, the parasitic drain will vary because the control unit will manage the load and are generally greater on vehicles with larger batteries. 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 common offenders. Cooling fans, power seat belt retractors, radios and convenience lights left on, alarm systems, door locking systems and electric car antennas have also caused batteries to drain overnight. Leaving your headlights on will generally discharge a fully charged car battery within four to five hours.
There is an excellent video from EricTheCarGuy, "How To Perform The Parasitic Draw Test", on https://www.ericthecarguy.com/.
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 when the engine is cold, check for a shorted or leaking diodes in the alternator.
Below are some methods that are used to test the parasitic load with the battery recharged, engine NOT running, under hood light disconnected, all accessories switched off, and the vehicle doors closed or door switch disabled:
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