why is my BMW 3 Series E90/E91 battery dying every night when parked?

Solution

1. Preparation

• Gather tools and parts required.
• Tools: Multimeter, OBD-II scanner, ammeter, basic hand tools (screwdrivers, socket set).
• Safety: Ensure the vehicle is parked on a level surface and the engine is off.

2. Battery Inspection

• Remove the battery from the vehicle if necessary:
  • Disconnect the negative (-) terminal first followed by the positive (+) terminal.
  • Remove any securing brackets or clamps.
• Clean terminals of any corrosion using a mixture of baking soda and water, then dry thoroughly.
• Test the battery:
  • Use a multimeter to check voltage; if below 12.4 volts, consider recharging or replacing the battery.

3. Parasitic Draw Reduction

• Reconnect the battery and perform a parasitic draw test:
  • Disconnect the negative terminal.
  • Connect the ammeter in series with the negative terminal and the battery post.
  • Ensure the draw is below 50 mA; if higher, identify the circuit causing the draw.
• Isolate the circuit:
  • Remove fuses one at a time until the draw decreases significantly, indicating the faulty circuit.
• Repair or replace the faulty component associated with the high draw.

4. Final Reassembly

• Reconnect the battery:
  • Connect the positive (+) terminal first followed by the negative (-).
• Check connections to ensure they are secure.
• Replace any fuses that were removed and ensure proper operation.