Late model BMW vehicles come with an integrated electronic key. This key also houses the keyless entry. When the battery in your BMW key gets low, you will see a light on your instrument cluster (symbol of a battery and a key). Once you see this light, replace the battery immediately. The replacement battery for most models is CR2032. However, you should verify that the new battery is the same model number as the old battery; some models may use different batteries.
Turn your key over--with the BMW emblem facing away from you.
Press the button on the top-right side of the unit to release the integrated mechanical key. Pull the key out of the unit; this will expose the edge of the battery cover.
Flip open the battery cover with your fingernail. Pull the old battery out carefully.
Insert the new battery with the positive symbol (+) facing up. Close the battery cover. Re-insert the integrated mechanical key.