A reading of 12.2V indicates the battery is 50% charged, and anything lower shows the battery is discharged. Turn the ignition key once without starting the engine. That turns on the accessories that draw power from the battery. Read the battery voltage once again. If there is a significant voltage drop, the battery is weak and needs replacement.
This is a video tutorial that show you how to replace key fob battery on BMW 5-Series F10, F11, 3-Series F30, F35, M3 F80, 1- Series F20, 4-Series, 6-Series F12, X5, X6, X4, X3, X1 in 5 simple steps. HERE you can find new CR2450 key fob battery for BMW:
Press the BMW logo on your BMW key fob 3 times while holding down the unlock button. Let go of the unlock button after the third time you’ve pressed the BMW logo. Then, turn on the ignition, and your new key should be paired with your BMW!
Changed batteries a few times but it did not solve the problem. I restarted it by putting the fob under the rear fender near the sensor. We happened to be in Missoula near Big Sky BMW and they checked it and it was showing two different key codes. They cleared them and repaired the fob and all was well. 4. Place the new battery in the PCB battery holes. Hold the PCB by the edges while applying a bit of force down onto the battery. Apply heat to the middle terminal, then the corner terminal and the battery terminals will push through the existing solder into place. 5. Dry fit the PCB into the housing, and test the key. jkZE7.