Parasitic Draw
An electrical drain that continues to draw power from the battery after the vehicle has been turned off and the computers have gone to sleep.
A parasitic draw is an unintended electrical current drain on a vehicle's battery that persists after the ignition key is removed, all accessories are turned off, and the onboard computer modules have entered their low-power sleep modes. While all modern vehicles have a small normal draw (typically 20 to 50 milliamps) to maintain clock and radio memory, a draw exceeding 100 milliamps will drain a battery overnight or within a few days.
Common causes of parasitic draws include sticky electrical relays that fail to open, aftermarket accessories (such as alarms, stereos, or GPS trackers) that are improperly wired, and glove box or trunk lights that stay illuminated. Damaged wiring or corroded modules can also prevent the vehicle's network from going to sleep, keeping modules active and drawing power.
To diagnose a parasitic draw, a technician connects a digital multimeter in series with the battery negative terminal to measure current (amps) or measures millivolts across fuses. They then pull individual fuses one by one until the amp reading drops, pinpointing the circuit experiencing the drain. Once the circuit is identified, the faulty component or wire can be repaired.