Parasitic Draw

📖 YOUCANIC Automotive Glossary

Parasitic draw (also called parasitic drain, dark current, or key-off drain) is abnormal electrical current consumption from the vehicle battery when the ignition is off and all systems should be in sleep mode. Normal parasitic draw for most vehicles is 20-50 milliamps (mA), which the battery can sustain for weeks. Abnormal parasitic draw above 50-75 mA progressively drains the battery, causing dead battery conditions after days or even overnight in severe cases. Common causes include aftermarket accessories that do not sleep properly (alarm systems, stereos, GPS trackers), stuck relays holding circuits active, control modules that fail to enter sleep mode (door module, seat module, BCM), glove box or trunk lights stuck on, and phone chargers left plugged in.

Diagnosing parasitic draw requires a multimeter set to DC amps connected in series between the battery negative cable and negative terminal with all doors closed and the vehicle fully asleep (wait 30-60 minutes for all modules to sleep). Normal reading is 20-50 mA. If draw exceeds 75 mA, systematically pull fuses one at a time while monitoring current — when the draw drops significantly upon pulling a specific fuse, that circuit contains the problem. The YOUCANIC UCAN-II can help by reading module wake-up status to identify which module is not sleeping, and by reading BCM codes that may indicate a module communication issue keeping the network awake.