how to check the condition of my BMW 3 Series E90/E91 battery?

Solution

1. Preparation

• Gather necessary tools and ensure the vehicle is parked on a flat surface.
• Wear safety gloves and goggles to protect against battery acid.

2. Visual Inspection

• Open the hood and locate the battery.
• Check for any signs of swelling, cracking, or leaks on the battery casing.
• Inspect battery terminals for corrosion and ensure they are securely connected.

3. Voltage Measurement

• Turn off all electrical components (lights, radio).
• Connect the red lead of the multimeter to the positive terminal and the black lead to the negative terminal.
• Read the voltage: a healthy battery should show 12.6 volts or higher.

4. Load Testing

• Disconnect the battery from the vehicle (remove negative terminal first, then positive).
• Connect a load tester to the battery according to the manufacturer's instructions.
• Apply a load equal to half of the battery's cold cranking amps (CCA) for 15 seconds.
• Observe the voltage drop; it should not fall below 9.6 volts during the test.

5. Alternator Output Check

• Reconnect the battery terminals (positive first, then negative).
• Start the engine and measure the voltage at the battery terminals with a multimeter.
• Ensure the reading is between 13.7 and 14.7 volts, indicating proper alternator function.