what could cause a dead battery in my Volkswagen Golf MK7 (2015-2019) overnight

Solution

1. Preparation

• Gather tools: multimeter, wrench set, battery terminal cleaner, and a digital voltmeter.
• Ensure the vehicle is parked on a flat surface and the ignition is off.
• Disconnect the negative battery terminal to prevent accidental short circuits.

2. Inspect and Clean Battery Terminals

• Use a wire brush or terminal cleaner to remove any corrosion from the battery terminals.
• Reconnect the terminals securely, starting with the positive terminal first, then the negative.

3. Test the Battery

• Reconnect the battery and use a multimeter to measure the voltage; it should read at least 12.6 volts.
• If the voltage is below 12.4 volts, consider charging or replacing the battery.

4. Perform a Parasitic Draw Test

• With the vehicle off, disconnect the negative battery terminal.
• Connect the multimeter in series between the negative terminal and the battery cable.
• Monitor the reading; a draw over 50 milliamps indicates a parasitic draw.
• Begin pulling fuses one at a time to identify which circuit is causing the draw.

5. Test the Alternator

• Start the vehicle and use the multimeter to check the voltage across the battery terminals.
• The reading should be between 13.7 to 14.7 volts; if it's lower, the alternator may be faulty.