why does my BMW 3 Series G20/G21 (2019-2024) have a weak battery after replacement

Solution

1. Preparation

• Gather tools: multimeter, socket set, wire brush, battery terminal cleaner, and protective gloves.
• Ensure the vehicle is off and parked on a level surface.

2. Inspect Battery Installation

• Sub-step 1: Open the hood and locate the battery. Ensure the terminals are clean and free of corrosion.
• Sub-step 2: Disconnect the negative terminal first, followed by the positive terminal.
• Sub-step 3: Reconnect the positive terminal first, followed by the negative terminal. Ensure they are tight and secure.

3. Check for Parasitic Draw

• Sub-step 1: Set the multimeter to measure current (amps).
• Sub-step 2: Disconnect the negative battery terminal and connect the multimeter leads to the battery terminal and the cable.
• Sub-step 3: Record the current reading. A draw of more than 50 milliamps indicates a potential issue.
• Sub-step 4: If excessive draw is detected, systematically remove fuses to identify the circuit causing the draw.

4. Test Battery Voltage

• Sub-step 1: With the multimeter, measure the battery voltage with the engine off. It should read around 12.6-12.8 volts.
• Sub-step 2: Start the engine and measure the voltage again. It should increase to between 13.7-14.7 volts.

5. Inspect Alternator

• Sub-step 1: With the engine running, check the alternator belt for proper tension and condition.
• Sub-step 2: Measure the output voltage at the alternator terminals. If it is below 13.5 volts, the alternator may need replacement.