If your Passat B8 battery is flat after sitting overnight, a parasitic electrical draw is the likely culprit. This guide explains common causes, how to test for a drain, and whether it’s safe to keep driving.
What’s Causing the Overnight Battery Drain?
A parasitic draw—where electrical components stay active even when the car is off—is a known issue on the Passat B8. The most frequent offenders are the comfort control module (gateway), door lock actuators that fail to enter sleep mode, or an infotainment system that doesn't shut down properly. Aftermarket accessories like dashcams or remote starters wired to constant power can also be culprits. Less common but possible: a faulty alternator diode or a failing battery.
