Old Click and Clack are "legends in their own minds"! They're all about yamdankee sarcasm, and short on real advice! They ALWAYS go for the smart-donkey remark, and seldom, if ever fully explain their answers.
"Parasitic drain" is any constant current draw from a battery from self-discharge across a moist, dirty battery top to the keep-alive circuits in the memory of an engine control computer, a radio with electronic memory for the selector buttons, etc. One of the BIG culprits is a self-leveling suspension system with a seep leak- - - - -the battery powered air compressor will keep running and trying to maintain the correct ride height. My wife's Explorer will kill the battery in about two days if she parks it in a nose-up attitude and the self-leveler tries to keep raising the rear end. I'm sure there's a seep leak somewhere in the air shock system.
On vehicles without all those electronics, the most common cause of chronic battery drain is an alternator diode that allows a slight, constant drain of battery current back through itself to ground anytime the alternator isn't charging.
Jerry
